Fall 2024 Speaker Schedule:
To be announced...
Fall 2024 Speaker Schedule:
To be announced...
Wednesday, May 22nd PST in KC 149 or Join us on Zoom!
Abstract: Despite decades of effort, we have neither solved the measurement problem in quantum mechanics, nor the unification of quantum and gravitational physics. Are these problems related? Could it be that the underlying difficulty lies in the essential use that the concept of infinity plays in the continuum Hilbert (Foch) state spaces of quantum (field) theory. Here I describe a particular discretisation of Hilbert Space (generating what I call Rational Quantum Mechanics - RaQM) and show, using elementary results from number theory, it accounts for the Uncertainty Principle, wave-particle duality and quantum non-commutativity, whilst at the same time allowing (something impossible in quantum mechanics) a finite deterministic ensemble-based underpinning of the wavefunction in which Born’s rule is automatically satisfied. In this model quantum mechanics is a singular limit of RaQM as the discretisation goes to zero. The violation of counterfactual definiteness in discrete RaQM implies it is not Bell-nonlocal, determinism notwithstanding.
Palmer, T.N., Superdeterminism without Conspiracy. Universe 2024, 1, 47.
https://doi.org/10.3390/
Thursday, May 9th @ 10:00am join us on Zoom!
Wednesday, May 8th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: Quantum systems exhibit complex dynamics marked by information scrambling, leading to phenomena such as long-range entanglement and thermalization. Local measurements can significantly alter these dynamics, which can freeze local degrees of freedom and give rise to new non-equilibrium phases and associated Measurement-induced Phase Transitions (MiPTs). Unlike the stochastic nature of monitored quantum trajectories, post-selected detector readouts yield deterministic dynamics governed by a non-Hermitian Hamiltonian, with MiPTs exhibiting distinct universal characteristics.
Wednesday, May 1st @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: Atom interferometers form the basis for extremely high precision measurements of quantities of both fundamental and applied interest. As an example, after the demonstration of the first atom interferometers, scientists were able to put tighter bounds on the so-called “alpha-dot” parameter using atom interferometers. Nowadays, technology has matured to the point where these devices can be used in more applied settings, such as providing acceleration and rotation measurements to aid in navigation in GPS-denied environments. In this talk, I will first discuss the physics underlying an atom interferometer, beginning with the use of light pulses [1] to form the atom optics. Next, I will highlight the origins of the scaling of the sensitivity of an atom interferometer to T^{2}, where T is the time between light pulses. I will take a brief excursion to describe an atom interferometer we are currently developing which is focused on inertial navigation applications. Most of the talk will focus on my group’s efforts to develop a novel atom interferometer with high scaling (T^{3}) [2] [3]. The higher scaling leads to a greater sensitivity without increasing the physical size of the atom interferometer. Finally, I will describe a project currently under construction at NPS to build a 30-meter-tall atom fountain. I will motivate the project and our vision for making the apparatus available as a user facility. I will then describe, somewhat humorously, the planned first experiment to use the device to measure the period of a Foucault pendulum. Finally, I will describe the status of the construction of the apparatus.
Wednesday, April 24th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: Understanding the emergence of the rules of statistical mechanics for an isolated many-body system from an underlying quantum-mechanical microdynamics is a longstanding problem of fundamental physics. Concepts from the theory of quantized classically chaotic systems can be used to resolve at least some of the key issues, and lead to the notion of “eigenstate thermalization”: individual energy eigenstates of the system as a whole appear to be states of thermal equilibrium when probed by local (few-body) observables.
Wednesday, April 17th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: Interactions between atoms or atom-like particles and electromagnetic fields are at the heart of nearly all quantum optical phenomena and quantum information applications. In nanoscale quantum systems, quantum fluctuation phenomena such as Casimir-Polder forces, surface-modified dissipation and decoherence become an inevitable element of consideration. I will present an overview of fluctuation-induced phenomena in nanoscale quantum optical systems, focusing on Brownian motion and decoherence of levitated nanoparticles from quantum fluctuations.
Monday, April 15th @ 2:00pm PST in KC 149 or Join us on Zoom!
Abstract: In quantum mechanics, the wavefunction serves as a mathematical expression that characterises the quantum state of a system. Nonetheless, a debate persists regarding whether the wavefunction encompasses all necessary features or whether there exists a requirement for hidden variables, either local or non-local. Photons, particles of light, and electrons, particles of charge, can possess wavefunctions labelled by various quantum numbers, such as frequency and energy, polarisation and spin, and spatial and temporal modes. The ability to generate, manipulate, and detect a quantum wavefunction with specific states is essential in quantum information processing. For example, it is crucial to accurately generate and detect photon states when performing secret key sharing via quantum key distribution or complex computations, or electron states when using them to detect objects in a quantum imaging/sensing setup. These fields of research can be categorised as the study of structured waves.
In my talk, I will provide an overview of how to engineer the quantum states of photons and electrons and show examples of how structured waves can help us understand fundamental questions in science. I will also briefly discuss their applications in quantum key distribution, quantum simulators, and quantum microscopy.
Wednesday, April 3rd @ 10:00am in KC 149 or Join us on Zoom!
Abstract: A key question in the thermodynamics of open quantum systems is how to partition thermodynamic quantities such as entropy, work, and internal energy between the system and its environment. We show that the only partition under which entropy is non-singular is based on a partition of Hilbert-space, which assigns half the system-environment coupling to the system and half to the environment. However, quantum work partitions non-trivially under Hilbert-space partition, and we derive a Work Sum Rule that accounts for quantum work at a distance. All state functions of the system are shown to be path independent once this nonlocal quantum work is properly accounted for. The thermodynamics of two classes of quasi-statically driven open quantum systems is analyzed: systems with a finite environment in the grand canonical ensemble, and systems with an unbounded environment. Our results are illustrated with applications to a time-dependent two-level system and the driven resonant-level model.
Wednesday, March 27th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: The energy absorbed by a conductor from a non-equilibrium environment can be rectified to generate finite electrical power. Typically, this depends on tiny energy-dependent asymmetries of the device, formed by e.g. a quantum dot [1]. We show that larger currents are expected in hybrid systems, where a superconductor hybridizes the even-parity states in the quantum dot [2]. We consider the environment to consist on a quantum dot Coulomb-coupled to the conductor and tunnel-coupled to a hot reservoir. Two main mechanisms contribute to the generation of power. On one hand, the non-equilibrium charge fluctuations in the second dot correlate with the Andreev processes, hence injecting Cooper pairs in the superconductor. This provides the necessary symmetry breaking energy transfer. On the other hand, this mechanism competes with quasiparticle contributions, which benefit from the sharp features of the superconducting density of states, and is able to increase the engine performance [3].
Tuesday, March 19th @ 2:30 PM in KC 149 or Join us on Zoom!
Abstract: Quantum technologies promise to revolutionize our capabilities in areas ranging from precision measurement to secure communications to high-performance computing to pharmaceutical & battery design – governments and companies around the world are investing billions of dollars into developing these applications. This second quantum revolution grew, however, out of 60+ earlier years of research into deep, foundational questions, which for most of that time were seen by the mainstream as distractions from the real day-to-day business of physics. If not for the workers who followed those distractions, we would not be where we are today. In the excitement and the hype, it would be easy to imagine that the deep questions have been resolved and there remains nothing more than engineering, but this could not be further from the truth. My group’s research agenda focuses on identifying the open questions lying in plain sight at the heart of quantum physics. We have done this mostly by studying the problem of “quantum measurement” – what are the limits on the information one can extract about a quantum system, and the best ways to extract it? What are the side-effects – good or bad – of observation on a system? What can present observation teach us about the past, in a quantum world? I will give an overview of three example projects we have underway.
Wednesday, March 13th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: In 1927, Madelung was the first to interpret the Schrodinger equation as describing the flow of a conserved (probability) fluid, and the mathematical formalism he developed was the foundation for Bohm's interpretation of quantum mechanics, although it was no longer a fluid picture, and the physics occurs in a high-dimensional configuration space, rather than in 3-space. The recent development of the local many worlds interpretation allows all of the physics to be put back into Minkowski space, where everything can be understood in a nearly classical fluid picture, with many noninteracting fluids occupying the same space. During a local interaction between different quantum systems, the different fluids of each system may be mixed (interference), matched, or divided up into more non-interacting branches, and the local interaction potential acts as a boundary condition between the pre-interaction fluids and the post-interaction fluids. The Born rule describes a frequentist probability that follows directly from the proportion of fluid assigned to each outcome of a measurement. The purpose of this talk is to focus on the fluid mechanics in 3-space, starting with the single-particle Schrodinger equation. Momentum and energy densities are derived for the fluid and interpreted. For each fluid, there is an unusual imaginary term in the momentum density, which is unrelated to the bulk flow, and so is interpreted as a distribution of fluid particle momenta whose vector sum is zero at each event in spacetime, called symmetric momentum. Related to that is an unusual energy density term, which comes from the quantum kinetic energy, but is interpreted as a new kind of internal potential energy carried by the fluid, called the quantum potential. With this interpretation, the average flow of the fluid looks almost entirely classical, and the new type of energy naturally explains situations where fluid is in a classically forbidden region, like quantum tunneling. We examine the local exchanges of energy between the different types as the fluid moves, and conclude that in order for energy to be locally conserved, the quantum potential energy must flow between particles in the fluid, rather than only being carried by them (somewhat like heat flowing through a rod). Finally, there is an alternative version of this picture which comes from separating the quantum potential into a symmetric kinetic energy and a reduced quantum potential, which may provide a more physical explanation of the kinetic energy in energy eigenstates. The two picture offer different insights. The mathematical formalism of the fluid pictures will be presented, and time permitting, I will discuss relativistic generalizations, and the ubiquitous role of superoscillations.
Wednesday, February 28th @ 10am PST in KC 149 or Join us on Zoom!
Abstract: Feynman said that quantum interference experiments contain the “only mystery” of quantum mechanics. Contra Feynman, in [1], we showed that those aspects of quantum Mach-Zehnder interferometry that are Traditionally Regarded As Problematic (TRAP), such as the Elitzur-Vaidman bomb tester and the quantum eraser, can be accounted for by a classical model. This opens the question of whether there are other aspects that are ``genuinely nonclassical’’. Adopting Spekkens noncontextuality as our definition of ``classical’’, we show that the uncertainty tradeoff for qubit observables can only occur in a contextual model [2], and we apply this to show the same for Greenberger-Yasin wave-particle duality relation for a Mach-Zehnder interferometer [3]. Finally, we show that the quantum Zeno effect, in the form required for asymptotically perfect interaction free measurement, also requires contextuality. We conclude that quantum interference effects do illustrate one of the fundamental mysteries of quantum mechanics, but not in the way that Feynman envisaged.
Monday, February 19th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: Einstein’s General Relativity applied to black holes appears to lead to Information loss, thus violating one of the fundamental tenets of Quantum Mechanics. Recent Quantum-Information-Theory-based arguments imply that information loss can only be avoided if at the scale of the black hole horizon there exists a structure (commonly called fuzzball or firewall) that allows information to escape. I will discussed the highly-unusual properties that this structure must have and how these properties emerge in the realization of this structure in String Theory via branes, fluxes, topology and enhanced quantum tunneling.
Wednesday, February 14th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: Superconducting circuits have become a leading platform in the pursuit of analog quantum simulation and fault-tolerant quantum information processing. However, realizing large-scale quantum computing using existing architectures poses significant challenges. In this talk, I will describe the recent advances in constructing superconducting qubits that are more robust against environmental noise, and how to tailor extensible interactions between them. In particular, I will report the latest results in empowering the operations of a high-dimensional quantum processor and the progress toward realizing noise-protected qubits. These advances herald transformative pathways to construct future superconducting architectures, motivating a shift in quantum information processing using this platform.
Wednesday, February 7th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: In this talk I will use the existing literature on the panpsychist combination problem as a starting point to think about how to address a structurally similar combination problem in relational quantum mechanics. I note some similarities and differences between the two problems, and I consider various proposed solutions to the panpsychist problem, assessing the prospects for a similar solution in the context of RQM. I argue that overall the prospects for solving RQM's combination problem look better for RQM with cross-perspective links than for orthodox versions of RQM.
Wednesday, December 6th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: Wolfgang Pauli called solid-state physics "the physics of dirt effects", and this name might appear well-deserved at first sight since transport properties are more often than not set by extrinsic properties, like impurities. In this talk, I will present solid-state systems in which electrons behave like a hydrodynamic fluid, and for which transport properties are instead set by intrinsic properties, like the viscosity. This new regime of transport opens the way for a “viscous electronics”, and provides a new angle to study how quantum mechanics can constrain and/or enrich hydrodynamics.
Friday, December 1st @ 1:00pm in KC 149 or join us on Zoom!
Wednesday, November 29th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: The emerging field of quantum-enhanced telescopy may allow us to observe stellar objects impossible to see now by implementing solutions known from quantum theory to observational astronomy. We consider the improvements to long-baseline interferometry working in the weak thermal light regime, where the stellar source of interest sends low number of photons to the observatories. We will consider the Clock Game – a task formulated in the language of quantum information science that can be used to improve the existing schemes of quantum-enhanced telescopy. The problem of learning when a stellar photon reaches a telescope is translated into an abstract game. We propose a winning strategy that involves performing a quantum non-demolition measurement that verifies which stellar spatio-temporal modes are occupied by a photon without disturbing the phase information.
Wednesday, November 15th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: The burgeoning field of quantum metrology seeks to find “quantum advantages” over existing classical measurement schemes. Owing to its importance in gyroscopes and gravitational wave detection as well as its fundamental nature in all branches of interferometry, phase estimation beyond the standard quantum limit has been the prototypical example [1–3]. Pragmatically, due to loss, quantum phase estimation techniques have, so far, only offered a few percent improvement over the standard quantum limit in the few-photon regime [4] or a few dB improvement in the high power regime [5]. However, what if phase estimation for a class of experiments is suboptimal? Depending on the measurement apparatus, phase estimation may have different fundamental limits than frequency estimation [6]. I will discuss a new type of gyroscope that relies on an ultra-steep, frequency-dependent gain measurement rather than performing phase estimation in a passive gyroscope. With this technique we can achieve orders of magnitude improvement below the phase estimation standard quantum limit of a single-loop Sagnac interferometer of the same size. I will discuss important insights into a long-debated open question about the role of Doppler shifts in the Sagnac effect.
Wednesday, October 23rd @10:00am in KC 194 or Join us on Zoom!
Abstract: The talk will give an introduction to Schrödinger Bridges in a way that highlights connections and parallels to the problem of Optimal Mass Transport. Our exposition will highlight key ideas behind particular non-commutative formulations of both problems that will follow and be discussed in the talk.
Bio: Tryphon T. Georgiou was educated at the National Technical University of Athens, Greece (Diploma 1979) and the University of Florida, Gainesville (PhD. 1983). He is a Distinguished Professor at the Department of Mechanical and Aerospace Engineering, University of California, Irvine, and Professor Emeritus at the University of Minnesota. He is a Fellow of IEEE, SIAM, IFAC, AAAS and a Foreign Member of the Royal Swedish Academy of Engineering Sciences (IVA)
Wednesday, October 11th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: I will discuss perspectives on some key problems in quantum gravity using recent results from a pilot-wave based approach to the Ashtekar formulation of the Wheeler-de Witt equation (based on joint work with Justin Dressel and Stephon Alexander). I will first illustrate several key ideas using pilot-wave account of a holomorphic representation of quantum harmonic oscillator, and then apply these to the interacting fermionic-gravitational system in (Phys. Rev. D 106.10 (2022): 106012). I will show how the notion of a definite configuration in pilot-wave allows us to define a real global time variable and render the total Hamiltonian constraint to the form of a Schrodinger equation without semiclassical approximations. I will then discuss the guidance equation and the reality conditions for the Ashtekar connection in our approach. Lastly, I will generalize the notion of unitarity to non-normalizable states from pilot-wave perspective, and show the existence of unitary states in minisuperspace.
Monday, October 9th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: Most of the exotic hadrons discovered over the last 20 years fit nicely into the quark model as normal mesons and baryons if the existence of a seventh flavor of quark is hypothesized. For the quark to reproduce the mass, spin, parity, production and decay modes of exotic hadrons, it would have to have a mass of ~2.8 GeV, a charge of -1/3, and a W-boson-mediated interaction with the right-chiral component of the charm quark. In addition to presenting the mapping, the talk addresses cancellation of gauge anomalies as well as experimental data that seemingly rule out the possibility of an additional light quark.
Wednesday, October 4th @ 10:00am PST in KC 149 or Join us on Zoom!
Abstract: A bandlimited function is said to be superoscillatory when it locally oscillates at a rate higher than its fastest Fourier component. Superoscillatory optical fields are routinely used for far-field superresolution imaging, i.e., for achieving resolution beyond the Rayleigh limit. The presence of very intense sidelobes limits the effectiveness of such fields in experimental implementations. The appearance of these sidelobes leads to poor imaging quality and unrealistic constraints on the dynamic ranges of the detectors. Supergrowth, a recently introduced concept [Jordan, Quantum Stud.: Math. Found. 7, 285-292(2020)], is a promising candidate for mitigating such issues. Supergrowth is defined as the local amplitude growth rate of a function being higher than its fastest Fourier component. In my talk, I will show how supergrowing and superoscillating optical fields can help us reconstruct subwavelength objects. We will also see that supergrowing regions can have intensities exponentially higher than superoscillating regions, alleviating the effects of sidelobes. In the second part of the talk, I will show how to generate supergrowing/superoscillating functions robustly. In the final part, I will describe the first-ever experimental synthesis of supergrowing fields.
Wednesday, September 27th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: I will show how to time-optimally drive a quantum state along arbitrary trajectories under energy constraints and identify which trajectories are achievable under such constraints. This is equivalent to solving the Schrödinger equation's inverse problem, i.e., given a solution of the Schrödinger equation, determining the Hamiltonian that generates such a solution. Next, I will demonstrate how solving the Schrödinger equation's inverse problem allows us to describe the wave function collapse process via a stochastic, time- and state-dependent Hamiltonian. On the other hand, generalizing these quantum state driving techniques in the non-unitary regime yields a probabilistic unitary formulation of open quantum system dynamics. This new formulation of open quantum system dynamics provides us with a simple, intuitive, and unified framework for describing open quantum system dynamics exactly under all regimes, whether Markovian or non-Markovian. Alongside the Schrödinger equation, it reveals that all quantum processes, whether open or closed, continuous or discontinuous, finite or infinite-dimensional, can be categorized as either unitary or probabilistic unitary, i.e., probabilistic combinations of unitary evolutions.
Wednesday, September 20th @ 10:00AM PST in KC 149 or Join us on Zoom!
Abstract: The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental evidence of the quantization of gravity.
Reference: Tobar, Germain, Sreenath K. Manikandan, Thomas Beitel, and Igor Pikovski. "Detecting single gravitons with quantum sensing." arXiv preprint arXiv:2308.15440 (2023).
Wednesday, September 13th @ 10:00am in KC 149 or Join us on Zoom!
Abstract: There has been a proliferation of different interpretations of quantum mechanics, some of which are vague, incomplete, or internally inconsistent. There is also a group of operational theories based only on regularities in empirical data (laws), which explicitly avoid specifying any ontology, and yet their proponents make claims about them, like nonlocality or superdeterminism, that can only be true or false given an ontology, which is a fundamental logic error. We present the generative programs framework (GPF), which encompasses any and all physical theories that explain the empirical data for a given scenario or model, as a standard for describing physical theories, whether ontological or operational, which prevents ambiguity, incompleteness, and inconsistency. A generative program is a set of instructions that starts from nothing and generates all of the relevant empirical data. The instructions are executed in a logical order independent of space and time, and may include the creation of intermediate entities, like functions and their inputs, which participate in the generation of the data. An operationalist will interpret such a program as a useful summary of the data and some regular laws that it obeys, but will ascribe it no ontological significance. A realist will interpret a program as describing true ontological machinery that creates the empirical data, such that what is `physical' is defined as what appears in the instructions of the program, rendering all other facts about the data incidental. Yes/no questions about ontology like, "is this theory local?", or "is this theory superdeterministic?" can only be answered for programs which are considered to be ontologically real, since these are questions about the physical machinery of the ontology. Laws that appear in empirical data describe correlations, from which causality can never be inferred, meaning that the existence of causality is another yes/no question about an ontological program, and causality is thus independent of realism. The information flow in a generative program can be represented as a directed-acyclic graph (DAG) of ontological priority describing the logical order in which entities are generated in the process of the generation of the empirical data. Parts of the ontological priority DAG may also represent causality for an ontologically real program, but this is not required. The GPF is a unifying framework for evaluating and comparing physical theories, in that, a) no physical theory can be taken seriously unless its program can be specified, and b) given the programs for various theories, we can rigorously analyze their ontological properties and then apply our many subjective heuristics (locality is good, superdeterminism is bad, fine-tuning is bad, causality is good, Occam's razor, identity of indiscernibles, etc.) to argue why one should be preferred over another.
Wednesday, May 17th @ 10:00am in KC 149 or Join us on Zoom
Abstract: Readout for superconducting transmon qubits involves the dispersive coupling of their energy levels to a detuned resonator, which is then probed with a resonant microwave tone. To extract qubit information from this microwave field, the field is amplified and demodulated to yield a pair of heterodyne signals that must be inverted to infer the corresponding measurement-conditioned qubit evolution in the form of stochastic quantum trajectories. This talk details the theoretical subtleties about this inversion process as well as recent experimental progress in using modern machine learning methods to infer the conditioned qubit evolution.
Wednesday, April 19th @ 10:00am in KC 149 or Join us on Zoom
Abstract: Any N-qubit quantum state can be built from modular 1- and 2-qubit gates, and (for large N) a full description of such a quantum circuit is usually far simpler than the information encoded in the resulting state (formally requiring the specification of 2^N complex numbers). This motivates a hidden-variable approach to quantum circuits, where the actual state space grows only linearly with the number of qubits, while the specification of those hidden variables requires an all-at-once analysis of the connections in the full quantum circuit. In order to eliminate the traditional 2^N-sized mathematical structures, the necessary price is hidden retrocausality, because altering a future gate will alter the full circuit geometry, thereby corresponding to different hidden variables throughout the circuit. On the positive side, such a model would be comprised of localizable single-qubit states which only interact where they are physically brought together, rescuing our traditional notions of locality and reality. The mathematical formalism and preliminary successes of such a framework will be presented here for the first time.
Wednesday, April 5th @ 10:00am in KC 149 or join us on Zoom
Abstract: By the quantum-classical correspondence of light propagation, the optical implementation of the weak measurement inside quantum measurement can be considered in the linear optics regime. By the usage of the specific initial probe state, the weak value can be extracted as the two-dimensional image. This is called a weak-value imaging. By the combination of the post-selection, the quantum state tomography of the polarized state is demonstrated. Then, in the case of the entangled polarized state, this weak-value imaging technique can be extended. In this talk, we propose two methods on quantum state tomography of the entangled polarized state. One is that the joint weak value consists of the product of the weak value with the different post selection. The other is to utilize the deep learning technique with the weak value to estimate the concurrence of the target state.
Friday, March 3rd @ 11:00am in KC 149 or join us on Zoom
Abstract: We propose and theoretically investigate the behavior of a ballistic Aharonov-Bohm (AB) ring when embedded in a N-S two-terminal setup, consisting of a normal metal (N) and superconducting (S) leads. This device is based on available current technologies, and we show in this work that it constitutes a promising hybrid quantum thermal device, as quantum heat engine and quantum thermal rectifier. Remarkably, we evidence the interplay of single-particle quantum interferences in the AB ring and of the superconducting properties of the structure to achieve the hybrid operating mode for this quantum device. Its efficiency as a quantum heat engine reaches 55% of the Carnot efficiency, and we predict thermal rectification factor attaining 350%. These predictions make this device highly promising for future phase-coherent caloritronic nanodevices.
Wednesday, March 1st @ 10:00am in KC 149 or join us on Zoom
Abstract: Encoding quantum information in the higher energy levels of the transmon circuit provides a resource efficient way to control large Hilbert spaces and to execute quantum algorithms at reduced circuit complexity. In this talk, I will discuss our recent realization of a ‘qutrit quantum processor’ that manipulates quantum information in three-level systems (qutrits). At UC Berkeley, we used this device to verify the scrambling of quantum information using teleportation [1] and benchmark its gate fidelity using a qutrit randomized benchmarking protocol [2]. Finally, I will discuss our ongoing work at the University of Rochester where we are exploring new qutrit gates and the encodings beyond three-level systems towards a genuine qudit processor.
Thursday, February 23^{rd} @ 11:00am in SC 404 or join us on Zoom
Wednesday, February 15th @ 10:00am in KC 149 or Join us on Zoom
Abstract: Quantum batteries are energy-storing devices, governed by quantum mechanics, that promise high charging performance thanks to collective effects. Due to its experimental feasibility, the Dicke battery - which comprises N two-level systems coupled to a common photon mode - is one of the most promising designs for quantum batteries. In this talk, I will show how reinforcement learning can be used to optimize the charging process of a Dicke battery, showing that both the extractable energy (ergotropy) and quantum mechanical energy fluctuations (charging precision) can be greatly improved with respect to standard charging strategies.
Wednesday, February 8th @ 11:00am in KC 149 of Join us on Zoom
Abstract: Originally formulated for macroscopic machines, the laws of thermodynamics were recently shown to hold for quantum systems coupled to ideal sources of work (external classical fields) and heat (systems at equilibrium). Ongoing efforts have been focusing on extending the validity of thermodynamic laws to more realistic, non-ideal energy sources. Here, we go beyond these extensions and show that energy exchanges between arbitrary quantum systems are structured by the laws of thermodynamics. We first generalize the second law and identify the associated work and heat exchanges. After recovering known results from ideal work and heat sources, we analyze some consequences of hybrid work and heat sources. We illustrate our general laws with microscopic machines realizing thermodynamic tasks in which the roles of heat and work sources are simultaneously played by elementary quantum systems. Our results open perspectives to understand and optimize the energetic performances of realistic quantum devices, at any scale.
Tuesday, January 24th @ 11:00am in Keck Center 149 or Join us on Zoom
Abstract: The world of superconducting qubits has been dominated by the transmon for a while. Over the course of more than a decade, much effort has been devoted to enhancing this circuit's coherence times. Despite the remarkable success, we should ask: is the transmon the best we can do, and will it ultimately suffice for implementing quantum error correction and leaving the NISQ era behind? In this talk, I will discuss interesting circuit alternatives with enhanced intrinsic protection from noise that may well play a decisive role in the future. I will give a tour of some of our recent work on noise-protected qubits and illustrate how our open-source "scqubits" package is making it straightforward to explore the world of superconducting qubits.
Wednesday, January 18th 2023
Abstract: Hypertwined analysis is a refinement of general hypercomplex theories of differential operators. In the assumption that the configuration space has a hypertwined structure, I will discuss several (re)interpretations of notions and results in Topological Quantum Field Theory (TQFT), such as supersymmetry, path–integral quantization, cancellation of anomalies, et al. Particular TQFTs of interest in this study are supersymmetric (SUSY) Yang–Mills theories on four–manifolds, which are deeply related with Donaldson and Seiberg–Witten invariants.
Wednesday, 14 December 2022
Abstract: It has been more than 20 years since Deutsch and Hayden demonstrated that quantum systems could be completely described locally within the Heisenberg picture. More recently, Raymond-Robichaud proposed two other approaches to the same conclusion. After being proven all equivalent, the cost of such local descriptions is quantified by the dimensionality of their space. The dimension of a single qubit's description grows exponentially with the size of the total system considered, in sharp contrast with the mere three-dimensionality of the reduced density matrix. However, the apparently unreasonable cost is shown to be expected of any local and complete description of quantum systems. Such a local approach to quantum theory enhances the universal wave function with more structures, therefore questioning the sense in which the Schrödinger and the Heisenberg pictures are equivalent.
Tuesday, 13 December 2022
Abstract: The Quantum Information Revolution is in full swing, and entanglement — the spooky nonclassical, nonlocal connection that can be shared by quantum particles — is the key ingredient. In this talk we’ll discuss how to create (photon) entanglement, and several applications for secure communication and quantum-enhanced sensing. Time permitting, we’ll include a lesson in quantum cooking.
Wednesday, 9 November 2022
Abstract: Superconducting materials are known to be good thermal insulators at sufficiently low temperatures thanks to the presence of the energy gap in their density of states (DOS). Yet, the proximity effect allows to tune the local DOS of a metallic wire by controlling the phase biasing imposed across it. As a result, the wire thermal resistance can be largely tuned by phase manipulation. In this talk I will show the experimental implementation of efficient control of thermal current by phase tuning the superconducting proximity effect. This is achieved by using the magnetic flux driven modulation of the DOS of a quasi one-dimensional aluminum nanowire forming a weak-link embedded in a superconducting loop [1]. Moreover, phase-slip events occurring in the nanowire are able to induce a hysteretic dependence of its local DOS on the direction of the applied magnetic field. Thus, we also demonstrate the operation of the nanovalve as a phase-tunable thermal memory [1, 2], thereby encoding information in the local temperature of a metallic electrode nearby connected. Besides quantum physics, our results are relevant for the design of innovative phase-coherent caloritronics devices such as thermal valves and temperature amplifiers, which are promising nanostructures for the realization of heat logic architectures.
Wednesday, 2 November 2022
Abstract: Despite the development of increasingly capable quantum computers, an experimental demonstration of a provable algorithmic quantum speedup employing today's non-fault-tolerant devices has remained elusive. In this talk, I will report on the first demonstration of such a speedup, quantified in terms of the scaling of time-to-solution with problem size. The demonstration is based on the single-shot Bernstein-Vazirani algorithm which efficiently solves the problem of identifying a hidden bitstring that changes after every oracle query. We implemented this algorithm utilizing two different 27-qubit IBM Quantum superconducting processors. The speedup is observed when the computation is protected by dynamical decoupling — an open-loop quantum control protocol designed to suppress noise due to the environment — but not without decoupling. In contrast with recent quantum supremacy demonstrations, the quantum speedup reported here does not rely on complexity-theoretic conjectures.
Tuesday, 1 November 2022
Abstract: Owing to their wave-like nature, quantum systems can never be truly at rest. Indeed, the value of some observables—those which do not commute with the Hamiltonian—fluctuate, even when the system is in its ground state. Following major advances in the manipulation and control of quantum systems, the prospect of extracting useful work out of these ubiquitous fluctuations becomes increasingly tangible.
"In this talk, I will present a new design for a quantum engine where local measurements on a many-body system are used to extract work from the ground state fluctuations of local observables. Indeed, performing local energy measurements on a system whose ground state is entangled can reveal excited states from which work can be extracted via local feedback operations. The engine is powered by the energy gap between the local ground state and the entangled many-body ground state. This quantum resource is essentially “free” in the setup at stake as it can simply be generated by coupling the interacting many-body system to a cold reservoir.
I will illustrate this proposal with two different systems: a chain of coupled qubits and a network of coupled harmonic oscillators. These models respectively correspond to fermionic and bosonic excitations."
Wednesday, 10 October 2022
Abstract: Graphene is perhaps the most prominent "Dirac material,” a class of systems whose lattice structure gives rise to charge carriers that behave as relativistic massless fermions. This emergence of relativistic behavior at laboratory scale energies makes graphene an ideal environment for probing the thermodynamics of relativistic quantum systems. For multilayer graphene, consisting of several stacked crystal sheets, subject to an external magnetic field the scaling of the energy spectrum with the magnetic field depends strongly on the number of layers. We examine the performance of a finite-time, endoreversible Otto cycle with multilayer graphene as its working medium. We show that there exists a simple relationship between the engine efficiency and the number of layers, and that the efficiency at maximum power can exceed that of a classical working medium.
Wednesday, 21 September 2022 @ 10 AM in 149 Keck Center, or join us on Zoom.
Abstract: Is physical reality local, or does what we do here and now have an immediate influence on events elsewhere? Do we have freedom of choice, or are our decisions predetermined? In this talk, I will briefly discuss how we understand these concepts and how Bell’s theorem undermines our most cherished intuitions about cause-and-effect on the fundamental level. I will also show how to quantitatively compare the assumptions of locality and free choice, with a view to better appreciate their role and weight for causal (or realist) explanations of observed correlations.
Wednesday, 14 September 2022 @ 10 AM in 149 Keck Center, or join us on Zoom.
Abstract: We will discuss the thermodynamic aspects of a single qubit-based and coupled qubit-based devices, powered by weak quantum measurements, and feedback controlled by a quantum Maxwell's demon. Single qubit-based device: We will discuss both discrete and time-continuous operation of the measurement-based device at finite temperature of the reservoir. In the discrete example where a demon acquires information via discrete weak measurements, we will show that the thermodynamic variables including the heat exchanged, extractable work, and the entropy produced are completely determined by an information theoretic measure of the demon's perceived arrow of time. In the time-continuous limit, we will present the exact finite-time statistics of work, heat and entropy changes along individual quantum trajectories of the quantum measurement process and relate them to the demon's arrow of time. Coupled qubit-based devices: we will study the discrete case for the coupled qubit-based system and study the relation between thermodynamic quantities and the arrow of time. We will also present the case for obtaining feedback based quantum thermal machines when the reservoirs are not detachable. Finally, we will give a brief outline for future directions we are pursuing.
Wednesday, 7 September 2022 @ 10 AM in 149 Keck Center, or join us on Zoom.
Abstract:I will show how to generalize superoscillations to arbitrary observables in quantum mechanics. Super phenomena of total angular momentum and energy will be described. Using the example of a sequence of harmonic oscillators, I will demonstrate that high energy approximate solutions can be created from asymptotically zero energy solutions that converges everywhere on the real line in a certain mathematical limit.
Wednesday, 27 July 2022 @ 12 PM in 149 Keck Center, or join us on Zoom.
Abstract: Several tasks in quantum-information processing involve quantum learning. For example, quantum sensing, quantum machine learning and quantum-computer calibration involve learning and estimating unknown parameters from measurements of many copies of a quantum state that depends on those parameters. This type of metrological information is described by the quantum Fisher information matrix, which bounds the average amount of information learnt about the parameters per measurement of the state. In this talk, I will show that the quantum Fisher information about parameters encoded in N copies of the state can be compressed into M copies of a related state, where M << N. I will show that M/N can be made arbitrarily small, and that the compression can happen without loss of information. The resource behind this compression ability is non-classical entries in the Kirkwood-Dirac distribution (a quantum extension of a probability distribution). By studying the Kirkwood-Dirac distribution, we show how to construct filters that perform this unbounded and lossless information compression. Our results are not only theoretically interesting, but also practically. In several technologies, it is advantageous to compress information in as few states as possible, for example, to avoid detector saturation and/or to reduce post-processing costs. Our filters can reduce arbitrarily the quantum-state intensity on experimental detectors, while retaining all initial information. Thus, we extend pre- and post-selected techniques of weak-value amplification to the growing fields of quantum machine learning and quantum multiparameter metrology.
Thursday, 30 June 2022 @ 12 PM in 149 Keck Center, or join us on Zoom.
Abstract: Quantum Theory requires a background causal structure for its formulation, with temporal and spatial correlations treated in a very asymmetric way. However, spacetime might lose its classical properties when Quantum Theory combines with General Relativity. This motivates a more general framework where causal relations are not set a priori. A further motivation comes from distributed networks, where the causal order of events might not be known in advance. I will present a formalism that posits the local validity of quantum mechanics locally but does not assume a global causal structure. Besides reproducing all standard quantum scenarios, the formalism predicts novel, “indefinite” causal relations. Such causal relations can be realised in thought experiments involving superpositions of spacetime metrics, but also describe laboratory experiments with events delocalised in time, opening the way to novel resources for computation and communication. Finally, the same formalism provides a natural framework to describe non-Markovian quantum processes, resolving an open problem in the study of open quantum systems.
Wednesday, 25 May 2022 @ 12 PM. Join us on Zoom.
Abstract: In this talk, Sandu will discuss a type of dynamic effect, a Dynamic Cheshire Cat effect, that is at the core of the so called “counterfactual computation” and especially “counterfactual communication” quantum effects that have generated a lot of interest recently. The basic feature of these counterfactual setups is the fact that particles seem to be affected by actions that take place in locations where they never (more precisely, only with infinitesimally small probability) enter.
Thursday, 12 May 2022 @ 4 PM. Join us on Zoom.
Abstract: In its usual formulation, Quantum Theory presents apparently unavoidable difficulties when applied to Cosmology. Hence, either it is assumed that the Quantum Theory is not wide enough to apply to the physics of the Universe, nowadays successfully tested by numerous sophisticated observations, or consistent alternative formulations must be sought. In this talk, I will apply one of these alternatives to Cosmology, the de Broglie-Bohm quantum theory, which not only contributes to a better understanding of unsolved riddles concerning the early Universe, like the quantum origin of the seeds which originated the large scale structures in the Cosmo, but also implies in possible testable observational consequences, and the completion of the Standard Cosmological Model by solving the cosmological singularity problem.
Friday, 6 May 2022 @ 9 AM. Join us on Zoom.
Abstract: Weak value amplification and other postselection-based metrological protocols can enhance precision while estimating small parameters, outperforming postselection-free protocols. In general, these enhancements are largely constrained because the protocols yielding higher precision are rarely obtained due to a lower probability of successful postselection. It is shown that this precision can further be improved with the help of quantum resources like entanglement and negativity in the quasiprobability distribution. However, these quantum advantages in attaining considerable success probability with large precision are bounded irrespective of any accessible quantum resources. Here we derive a bound of these advantages in postselected metrology, establishing a connection with weak value optimization where the latter can be understood in terms of geometric phase. We introduce a scheme that saturates the bound, yielding anomalously large precision. Usually, negative quasiprobabilities are considered essential in enabling postselection to increase precision beyond standard optimized values. In contrast, we prove that these advantages can indeed be achieved with positive quasiprobability distribution. We also provide an optimal metrological scheme using a three level non-degenerate quantum system.
Wednesday, 27 April @ 12PM in 149 Keck Center, or join us on Zoom.
Abstract: The book The Subtle Art of Not Giving a F*** has made the New York Times bestseller list for months by disparaging self-help books that glorify positivity. Positivity doesn’t live up to expectations in metrology, either. Common metrological protocols include parameter estimation. In some parameter estimations, the final measurement is the most costly step. In these cases, postselecting—discarding some trials before they finish—can raise the average information obtained per unit cost. Such experiments, I will show, are usefully described by a particular quasiprobability distribution. Quasiprobabilities resemble probaiblities but can assume negative and nonreal, or “nonclassical,” values. These values arise only if relevant operators fail to commute with each other. Only if the distribution contains negative quasiprobabilities does the information-cost rate exceed the rate achievable with commuting operators. Hence this distribution serves as a mathematical tool for pinpointing operator noncommutation—a hallmark of quantum physics—as a metrological advantage’s source. I will discuss this theory and a recent photonic test of it. The experiment uncovered a proportionality between the metrological advantage and a measure of the distribution’s nonclassicality.
Friday, 22 April 2022 @ 4 PM in 149 Keck Center, or join us on Zoom.
Abstract: A quantum decaying system can reveal its nonclassical behavior by being noninvasively measured. Correlations of weak measurements in the noninvasive limit violate the classical bound for a universal class of systems. The violation is related to incompatibility between exponential decay and unitary evolution, and as such are closely related to the quantum Zeno effect (QZE). The phenomenon can be observed together with QZE by a continuous weak measurement. Starting with the correlations mentioned above one could also estimate the coherence time of decay process.
Thursday, 14 April 2022 @ 4 PM in 149 Keck Center, or join us on Zoom.
Abstract: After a pedagogical introduction of Axion and the Strong CP problem, I will show how the axion induces an additional spin-precession in the Schrodinger equation. We will together explore the possibility of using ideas in weak measurements to tease out the axion.
Wednesday, 13 April 2022 @ 12PM in 149 Keck Center, or join us on Zoom.
Abstract: Relationalism posits that one must be able to describe the universe as a whole without reference to any external structure (clocks, rods, frame of reference, etc.) and should consequently also not obey any external arrow of time. Clocks, rods, reference frames etc. are defined through (suitably isolated) processes within the universe itself. I suggest that the arrow of time should be an analogous derived property as well and suggest to derive it from “direction of growth of local records.” This begs two questions: Do all these local arrows of time point in the same direction? and: Why do they? I give an answer by considering the relational description of the Newtonian N-body problem, which possesses a unique past point (“Janus point”) which splits almost all solutions into two halves. The solutions exhibit an emergent gravitational arrow of time that points away from this past point. One finds that the gravitational arrow of time pushes all local arrows of time in its direction and that entropic arrows of time as well as collapse arrows of time follow.
Abstract:
In this talk we will discuss the phenomenon of Steering — that which puzzled Einstein, Podolsky, Rosen, and Schroedinger back in the 1930’s. We will see the similarities and differences between Bell and steering experiments, and notice hence other aspects in which quantum systems can behave non-classically. We will also take a step further into the realm of post-quantumness: we will discuss how an analogue of the PR-box can be constructed in steering scenarios, and pin down some particular insights on quantum foundations that we can only draw from studying steering. In this talk I’ll also briefly touch upon resource theories, generalised probabilistic theories, and 'quantum from principles', all regarding steering assemblages.
Full title: Can a qubit be your friend? Why experimental metaphysics needs a quantum computer
Abstract:
Experimental metaphysics is the study of how empirical results can reveal indisputable facts about the fundamental nature of the world, independent of any theory. It is a field born from Bell’s 1964 theorem, and the experiments it inspired, proving the world cannot be both local and deterministic. However, there is an implicit assumption in Bell’s theorem, that the observed result of any measurement is absolutely real (it has some value that is not real only to the observer who made it, or only in the ‘branch’ in which it appears). This assumption is called into question when one thinks of the observer as a quantum system (the “Wigner’s Friend” scenario), which has recently been the subject of renewed interest. In [1], I and co-workers derived a theorem, in experimental metaphysics, for this scenario. It is similar to Bell’s 1964 theorem but dispenses with the assumption of determinism. We show that the remaining assumptions, which we collectively call "local friendliness", are still predicted, by most approaches to quantum mechanics, to be violable. We illustrate this in an experiment in which the “friend” system is a single photonic qubit. In [2], I and other co-workers argue that a truly convincing experiment could be realised if that system were a sufficiently advanced artificial intelligence software running on a very large quantum computer, so that it could be regarded genuinely as a friend. We formulate a new version of the theorem for that situation, using six assumptions, each of which is violated in at least one approach to quantum theory. The popular attitude that “quantum theory needs no interpretation” is untenable because it does not indicate that any of the assumptions are invalid.
[1] Bong et al., “A strong no-go theorem on the Wigner’s friend paradox”, Nature Physics 16, 1199 (2020).
[2] Wiseman, Cavalcanti, and Rieffel, “A ‘thoughtful’ Local Friendliness no-go theorem”, in preparation.
Wednesday, 16 March 2022 @ 12PM in 149 Keck Center, or join us on Zoom.
Abstract: In this talk, we are interested in techniques to recover the moment of magnetization given measurements of the magnetic field. We introduce the notion of inverse problems and we write the question of moment recovery as an inverse problem. The inverse problem can be resolved using a class of approximation problems called bounded extremal problems. We give a mathematical approach to build an approximation of the moment of the magnetization. It is a joint work with J. Leblond (INRIA Sophia Antipolis, France).
Wednesday, March 9th, 2022, 12 PM, Keck Center 149
Abstract: Spontaneous collapse models models and Bohmian mechanics are two different solutions to the measurement problem plaguing orthodox quantum mechanics. They have a priori nothing in common. At a formal level, collapse models add a non-linear noise term to the Schrödinger equation, and extract definite measurement outcomes either from the wave function (e.g. mass density ontology) or the noise itself (flash ontology). Bohmian mechanics keeps the Schrödinger equation intact but uses the wave function to guide particles, which comprise the primitive ontology. Collapse models modify the predictions, whilst Bohmian mechanics keeps the empirical content intact. However, it turns out that (non-Markovian) collapse models and their primitive ontology can be exactly recast as Bohmian theories.
Thursday, March 3rd, 2022, 11 AM, Keck Center 149
Abstract: The classical action has been intertwined with quantum theory for over a century, but the connection between the two continues to be unclear. Is the action meaningful in its own right, or just a "trick" for generating dynamical equations of motion? Are the all-at-once aspects of action principles "retrocausal", if taken literally? Can classical action principles really account for quantum phenomena like entanglement? Does the path integral have any realistic implications for what is happening between measurements? And what exactly is the action, anyway? These and other related questions will all be addressed, and maybe even answered.
Monday, December 13th, 2021, 11 AM, Keck Center 149
Abstract: All matter, visible and dark, had to originate from some mysterious event in the early universe-baryogenesis and dark-genesis. This necessary physics goes beyond our standard cosmology and the standard model of particle physics. In this talk, I provide a pedagogical introduction to cosmic inflation, baryogenesis and argue the necessity of Chern-Simons theory in collaboration with the quantum dynamics of cosmic inflation (but not limited to it) to explain the coincidence between the density of dark and visible matter. A surprise regarding quantum coherence in halos awaits as a result.
Monday, 29 November, 2021 @ 12 PM in Room 149 of the Keck Center
or on Zoom
Abstract:For the last few years, quantum dots have served as building blocks for many theoretical and experimental works in quantum thermodynamics as a consequence of their straightforward energy-filtering properties to which thermoelectricity is tightly linked. Making use of the physics of resonant tunneling, we design a quantum absorption refrigerator where a hot electronic cavity is coupled to two colder electron reservoirs via single-level quantum dots. By appropriately positioning the dots’ resonant levels, one can extract heat from the hot cavity and use it as a resource to refrigerate the coldest electron reservoir. The refrigerator’s performance can be optimized by fine-tuning the dot levels’ positions and widths. Further, we associate arbitrary number of such refrigerators in series, and demonstrate how to achieve precise thermal control across the chain: This is accomplished by positioning the dots' energy levels such that a predetermined distribution of heat currents is realized across the chain in the steady state. Finally, the rich physics of single-level quantum dots strongly coupled to structured electron reservoirs will be discussed. For reservoirs with band gaps, we witness an abrupt transition, linked to the appearance of an infinite-lifetime bound state, as the dot-reservoir coupling is increased. A signature of this transition can be observed in the dot’s transmission function, with dramatic effects on the dot’s thermoelectric properties.
Monday, 22 November, 2021 @ 12 PM in Room 149 of the Keck Center
or on Zoom
Abstract: We will study the phenomena of heat transport and dissipation in open quantum systems. In particular, we will investigate the phenomena of absorption refrigeration, where refrigeration is achieved by heating instead of work, in two different setups: a minimal set up based on coupled qubits, and two non-linearly coupled resonators. Considering ZZ interaction between the two qubits, we will outline the basic ingredients required to achieve cooling. We will compare the cooling effect obtained in the qubit case with that of non-linearly coupled resonators (multi-level system) where the ZZ interaction translates to a Kerr-type non-linearity. Using Keldysh non-equilibrium Green's function formalism, we will go beyond first order sequential tunneling processes and study the effect of higher order processes on refrigeration. We find reduced cooling effect compared to the master equation calculations.
In the next half of the presentation, we will present a unified approach to study continuous measurement based quantum thermal machines in static as well as adiabatically driven systems. In the adiabatically driven case, we will show how measurement based thermodynamic quantities can be attributed geometric characteristics. We will also provide the appropriate definition for heat transfer and dissipation owing to continuous measurement in the presence and absence of adiabatic driving. We will illustrate the aforementioned ideas and study the phenomena of refrigeration in two different paradigmatic examples: a coupled quantum dot and a coupled qubit system, both undergoing continuous measurement and slow driving. We will observe that quantum measurement can provide significant boost to the power of adiabatic quantum refrigerators.
Monday, 15 November, 2021 @ 12 PM in Room 149 of the Keck Center
or on Zoom
Monday, 8 November, 2021 @ 12 PM in Room 149 of the Keck Center
or on Zoom
Abstract: While complex numbers are essential in mathematics, they are not needed to describe physical experiments, expressed in terms of probabilities, hence real numbers. Physics however aims to explain, rather than describe, experiments through theories. Although most theories of physics are based on real numbers, quantum theory was the first to be formulated in terms of operators acting on complex Hilbert spaces. This has puzzled countless physicists, including the fathers of the theory, for whom a real version of quantum theory, with real operators and states, seemed much more natural. In fact, previous works showed that such a "real quantum theory" can reproduce the outcomes of any multipartite experiment, as long as the parts share arbitrary real quantum states. In this talk I will show that real and complex quantum theory make different predictions in network scenarios comprising independent states and measurements. This allows us to devise a Bell-like experiment whose successful realization would disprove real quantum theory, just as standard Bell experiments disproved classical physics.
Wednesday, 3 November, 2021 @ 4 PM in Room 149 of the Keck Center
or on Zoom
Abstract: Physical advantages to building a quantum computer out of optical frequency photons include: they suffer negligible environment decoherence even at room temperature, there is no cross talk, they network easily into arbitrary geometries, the relevant physics is not heuristic and is often both efficiently simulatable and verifiable with classical light, and measurements – the critical element for entropy reduction to achieve fault tolerance – are sharp and extremely fast. However these pale in comparison to the engineering advantages: all parts of the machine can be built in a tier-1 foundry, and packaged in the same back-end-of-line processes used to build laptops and cellphones. Thus with photons we can realistically stare down the sorts of numbers (~1 million qubits) which capture the size of machine required to do useful quantum computation.
So what is the catch? The primary obstacle is that the specific type of entanglement we need between different photons can only be created probabilistically, and is difficult to create in the presence of loss. In this talk I will overview an architecture Fusion Based Quantum Computing (FBQC) that sits somewhere between the extremes of matter (circuit) based and one-way (cluster state) quantum computing. It requires the production of only fixed size entangled states regardless of the size of computation being performed, and these states can have high probability of loss (or failure to be produced at all). This allows us to attempt creation of the desired entangled states multiple times in parallel, and then to select out successful events.
Monday, 25 October, 2021 @ 12PM in Room 149 of the Keck Center
or on Zoom
Abstract: Quantum walks (QWs) are unitary analogues of classical random walks, and quantum cellular automata (QCAs) are unitary analogues of classical cellular automata. The QW on the 3D body-centered cubic lattice gives rise to solutions of the Dirac equation in the long-wavelength limit, both in 1D and 3D; in 1D, a two-dimensional internal space is required, and in 3D a four-dimensional internal space. QWs can be treated as the one-particle sector of a QCA, so it is natural to seek QCAs that give rise to quantum field theories in a similar limit. This can be done fairly straightforwardly in one spatial dimension, with the QCA being naturally described in terms of creation and annihilation operators that create or destroy particle locally, evolve simply under the QCA unitary, and obey the usual anticommutation relations. However, generalizing this construction to two or more spatial dimensions fails: the requirements of anticommuting creation and annihilation operators are inconsistent with a local QCA. For a QCA to give rise to a fermionic quantum field theory in the long-wavelength limit, one must give up at least one desired property of the QCA. To evade this no-go theorem, one can let the local subsystems become high-dimensional, and restrict to the completely antisymmetric subspace of a larger space. Bosonic QCAs can also be constructed; these do not have the same problem with anticommutation, but also require high-dimensional local subsystems. Taking these constructions as a model of particles propagating in discrete spacetime, the discreteness could be detected using non-parallel matter interferometers. Finally, we consider the problem of adding fermion-boson interactions, and progress towards constructing a fully interacting QCA model, and the potential for using QCAs to simulate quantum field theory.
Monday, 18 October, 2021 @ 12PM in 370 Keck Center
or on Zoom
Abstract: Traditional uncertainty relations dictate a minimal amount of noise in incompatible projective quantum measurements. The noise is often thought to come from the measurements' failure to commute. However, not all measurements are projective. In particular, weak measurements are minimally invasive tools for obtaining partial state information without projection.
In this talk, I'll describe an experiment in which such measurements can reconcile two incompatible (non-commuting) strong measurements. The weak measurements' slight back action on the state accounts for a majority of the reconciliation. The measurements obey an entropic uncertainty relation based on generalized measurement operators. In this relation a weak value appears, lowering the uncertainty bound.
Monday, 11 October, 2021 @ 12PM in 370 Keck Center
or on Zoom
Abstract: Uncertainty relations play a crucial role in quantum mechanics. Well-defined methods exist for the derivation of such uncertainties for pairs of observables. Specific methods also lead to time-energy uncertainty relations. However, in these cases, different approaches are associated with different meanings and interpretations. In this talk, the time-energy uncertainty relation of interest revolves around the idea of whether quantum mechanics inherently imposes a fundamental minimum duration for energy measurements with a certain precision. Within the Page and Wootters timeless framework, it will be discussed how energy measurements modify the relative ``flow of time'' between internal and external clocks. This provides a unified framework for discussing the subject, allowing the recovery of previous results and derivation of new ones. In particular, it will be shown that the duration of an energy measurement carried out by an external system cannot be performed arbitrarily fast from the perspective of the internal clock. Moreover, it will be demonstrated that the evolution given by the internal clock during any energy measurement is non-unitary. Finally, if time allows, new developments associating non-unitarity to non-inertial clocks will be presented.