» IQS Seminar Series

Who’s troubled by Bell inequalities, or what is the weight of locality and free choice? by Pawel Blasiak

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. 

Quantum Feedback Thermal Machines by Bibek Bhandari

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.

Generalized super-phenomena for arbitrary quantum observables by Andrew Jordan

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.

Quantum information about unknown parameters can be compressed unboundedly without loss by David Shukur

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.

Quantum processes with quantum causal structure by Fabio Costa

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.

A dynamical quantum Cheshire Cat effect and implications for counterfactual communication by Sandu Popescu

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.

Post-selection and Quantum Energetics by Spencer Rogers

Tuesday, 24 May 2022 @ 2 PM in 149 Keck Center, or join us on Zoom.
Abstract: We investigate the anomalous energy change of the measurement apparatus when a qubit is measured in bases that do not commute with energy. We model two possible measurement implementations: one is a quantum clock model with a completely time-independent Hamiltonian, while the other is a Jaynes-Cummings model which is time-dependent but conserves the total excitation number. We look at the mean energy change of the measurement apparatus in both models, con- ditioned on the qubit post-selection, and find that this change can be much greater than the level spacing of the qubit, like an anomalous weak value. In the clock model, the expression for the apparatus energy shift explicitly contains the weak value of the qubit Hamiltonian. Our two models give different results, which we explain to be a consequence of the non-degenerate spectrum of the Jaynes-Cummings model.

"Quantum Theory faces Cosmology, and vice-versa" by Nelson Pinto-Neto

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.

"Bounding quantum advantages in Postselected Metrology" by Subhrajit Modak

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.

"Why negativity can be useful (in metrology)" by Nicole Yunger Halpern

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. 

"Weak measurements and quantum Zeno effect" by Stanislaw Soltan

stanislaw

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.

"Detecting Axions from electron-spin precession" by Stephon Alexander

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.

"The arrow of time in a relational universe" by Tim Koslowski

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.

"No-signalling assamblages beyond quantum mechanics: when quantum theory can generate extremal points in a post-quantum framewor" by Paweł Horodecki

Wednesday, 6 April 2022 @ 12PM in 149 Keck Center, or join us on Zoom.
Abstract: It is well known that in the language of  no-signaling boxes  – consistent families of probabilities representing measurement correlations –  the following three sets: local deterministic, quantum and no-signalling constitute the increasing sequence with strict inclusions. Moreover the outer set of no-signalling boxes has no nontrivial (ie. not local deterministic) extremal points that are generated by measurement on quantum states. This fact significantly complicates all quantum information security  protocols potentially robust against attacks of more powerful,  post-quantum adversaries.
      In the case of no-signalling assamblages which attracted attention recently, the strict  inclusion of the three analogous sets is also true except bipartite case. However the issue of extremality, potentially vital to information security,  has been open for a long time.
     We present results that fill gaps in the above picture. First, even in the sequential measurement scenario  no nontrivial quantumly generated extremal point can exist in  the set of no-signalling boxes. Second, somewhat surprisingly,  the analog of this no-go theorem fails for no-signalling assamblages. This is for the first time when quantum mechanics is observed to produce points that are extremal in some post-quantum framework. The new concept of the inflexibility of assamblages with pure quantum elements plays here an important role.
     We also study the boundary of the no-signalling assamblages. In analogy to quantum entanglement theory we define and study the edge of the set of assamblages.  We found that here quantum mechanics can produce edge assamblages  via measurements on at most rank-two 3-qubit states  as opposed to rank five for the edge of the 3-qubit quantum entanglement set.
    Future perspectives of the presented research will be discussed.

"Steering: a modern perspective on this fundamental puzzle" by Ana Belen Sainz

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.

"Can a qubit be your friend?" by Howard Wiseman

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.

"How to recover a moment of a magnetization vector from a magnetic field?" by Elodie Pozzi

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).

"Non-Markovian wave-function collapse models are Bohmian theories in disguise" by Antoine Tilloy

Wednesday, March 9th, 2022, 12 PM, Keck Center 149

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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.

"Implications of action principles for quantum foundations" by Ken Wharton

Thursday, March 3rd, 2022, 11 AM, Keck Center 149

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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.

"Quantum Galaxies" by Stephon Alexander

Monday, December 13th, 2021, 11 AM, Keck Center 149

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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.

"Thermoelectric transport across single-level quantum dots" by Étienne Jussiau

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.

"Refrigeration in quantum systems: Quantum absorption refrigerators and measurement boosted adiabatic quantum refrigerators" by Bibek Bhandari

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.

"Revealing the entanglement in the history of a multipartite open quantum system: Universal causal separability criterion" by Gleb Skorobagatko

Monday, 15 November, 2021 @ 12 PM in Room 149 of the Keck Center

or on Zoom

Abstract: "Quantum nature implies fundamental quantum uncertainties in the temporal evolution of arbitrary open multi-partite quantum systems. This concerns uncontrollable effects of quantum state decoherence during quantum state preparation\measurement in system's history as well as quantum fluctuations of observables being measured throughout such generalized temporal evolution and also unavoidable effects of interactions between different subsystems of a given quantum system and its environment. All such a history of a  multipartite quantum system  is encoded in its density matrix elements defined for each instant of time. One may ask, having in mind all the above-mentioned quantum uncertainties, whether it is possible at least to deduce whether any sort of entanglement took place between any parties of a given quantum system during their common temporal evolution?  - The most general answer on this question has not yet been given because all related quantum separability criteria including famous Peres-Horodecki ppt-criterion have remained very limited (such as e.g. the ppt- )  first of all due to lack of general enough physical background behind them which would allow to generalize any such approach to the case of arbitrary dimensionality of system's Hilbert space. Here the first example of a general separability problem solution is demonstrated for quantum density matrices of arbitrary dimensionality including the  infinite-dimensional case as well. The approach proposed is based on the heuristic causal considerations analyzing different causal symmetries in the temporal evolution of separable and entangled quantum states. The latter basic idea is also proposed here for the first time. As the result, general analytical formulas for the causal separability criterion are derived  in the most general form for arbitrary density matrices. Then entanglement predictions given by these new formulas have become tested on different physically relevant examples of one-parametric quantum density matrices, encoding e.g. arrays of qubits, any numbers of interacting and non-interacting quantum oscillators, uniformly interacting many-body quantum systems, etc. This way a number of specific predictions about the entanglement\separability existence has been made for the parameter spaces of all these types of one-parametric multipartite quantum systems."

The world is not real by Miguel Navascués (IQOQI, Vienna)

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.

Quantum Computing at the Speed of Light by Terry Rudolph

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.

Quantum field theories in discrete spacetimes by Todd Brun

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.

Weak Measurements Reconcile Incompatible Observables by Jonathan Monroe

Monday, 18 October, 2021 @ 12PM in 370 Keck Center

or on Zoom

AbstractTraditional 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.

Energy measurements with quantum clocks and non-inertial clock frames by Ismael De Paiva

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.