Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session V33: Quantum Foundations IFocus Live
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Sponsoring Units: DQI Chair: Todd Brun, Univ of Southern California |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V33.00001: Quantifying the difference between many-body quantum states Davide Girolami The quantum state overlap, i.e. the ``bracket'', is the fundamental measure of the difference between two quantum states. Yet, it can seriously mislead when we study many-body systems. The problem is inherited by widely employed parent quantities, such as the state fidelity and related distance functions. We introduce a new class of information-theoretic measures, the weighted distances, to overcome such limitations. They quantify the difference between quantum states of many particles accounting for the size of the system dimension. The result enables to evaluate limits and precision of complex quantum processes, e.g. the performance of quantum computers. |
Thursday, March 18, 2021 3:12PM - 3:36PM Live |
V33.00002: Einstein's Equivalence principle for superpositions of gravitational fields Invited Speaker: Flaminia Giacomini The Principle of Equivalence, stating that all laws of physics take their special-relativistic form in any local inertial frame, lies at the core of General Relativity. Because of its fundamental status, this principle could be a very powerful guide in formulating physical laws at regimes where both gravitational and quantum effects are relevant. However, its formulation implicitly presupposes that reference frames are abstracted from classical systems (rods and clocks) and that the spacetime background is well defined. Here, we we generalise the Einstein Equivalence Principle to quantum reference frames (QRFs) and to superpositions of spacetimes. We build a unitary transformation to the QRF of a quantum system in curved spacetime, and in a superposition thereof. In both cases, a QRF can be found such that the metric looks locally flat. Hence, one cannot distinguish, with a local measurement, if the spacetime is flat or curved, or in a superposition of such spacetimes. This transformation identifies a Quantum Local Inertial Frame. These results extend the Principle of Equivalence to QRFs in a superposition of gravitational fields. Verifying this principle may pave a fruitful path to establishing solid conceptual grounds for a future theory of quantum gravity. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V33.00003: Hierarchy of Theories with Indefinite Causal Structures: A Second Look at the Causaloid Framework Nitica Sakharwade, Lucien Hardy The Causaloid framework1 is useful to study Theories with Indefinite Causality; since Quantum Gravity is expected to marry the radical aspects of General Relativity (dynamic causality) and Quantum Theory (probabilistic-ness). To operationally study physical theories one finds the minimum set of quantities required to perform any calculation through physical compression. In this framework, there are three levels of compression: 1) Tomographic Compression, 2) Compositional Compression and 3) Meta Compression. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V33.00004: Network Nonlocality via Rigidity of Token-Counting and Color-Matching marc-olivier renou, Salman Beigi arXiv:2011.02769 |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V33.00005: When Is a Non-Markovian Quantum Process Classical? Simon Milz, Dario Egloff, Philip Taranto, Thomas Theurer, Martin Plenio, Andrea Smirne, Susana F. Huelga Quantum mechanics has been developed almost a century ago, due to the need of describing experimental results that were not explicable with classical physics. This lead to a surge of theoretical research aimed at reconciling our intuition, based on experiences of our everyday life, and the new theory. One main result of that research is that some seemingly intuitive assumptions are not reconcilable with the predictions of quantum mechanics and the subsequent experimental tests showed that it is quantum mechanics that prevails. It is this new confidence in the theory which is now leading to new research that aims at making use of these special traits, for building better sensors, quantum simulators and computers. By now, it is known that entanglement, discord and coherence play a central role in the quantum advantage. However, how does it work, exactly and how much is the advantage that we get due to the quantum properties? |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V33.00006: Environment as a witness: the inevitable emergence of classicality Akram Touil, Bin Yan, Davide Girolami, Sebastian Deffner, Wojciech Hubert Zurek Understanding our everyday observations of the classical universe from the underlying quantumness of nature is a major open problem. Therefore, we study quantum Darwinism in the example of a central spin undergoing decoherence in a spin environment. The system-environment interactions are modeled by imperfect CNOT operations. As main results, we derive analytic expressions for discord between the central spin and an arbitrary partition of the environment, where measurements are applied either on the system or environment spins. In the more realistic case of imperfect measurements, we show that we inevitably reach the classical plateau for a large environment size. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V33.00007: Observing a Changing Hilbert-Space Inner Product Salini Karuvade, Abhijeet Alase, Barry Cyril Sanders In quantum mechanics, physical states are represented by rays in Hilbert space, which is a vector space imbued by an inner product. For a given pair of pure states ψ, φ, physical meaning of the inner product between the two arises as the probability amplitude associated to observing the system in φ when the system was originally prepared in ψ. Current quantum theory does not formally address the consequences of a changing inner product during the interval between preparation and measurement of the system. We establish a theoretical framework for such a changing inner product, which we show is consistent with standard quantum mechanics. Furthermore, we show that this change is described by a quantum channel, which is tomographically observable, and we elucidate how our result is strongly related to the exploding topic of PT-symmetric quantum mechanics. Physically, we explain how to realize experimentally a changing inner product for a qubit in terms of a qutrit protocol with a unitary channel. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V33.00008: Black boxes in spacetime: semi-device-independent information processing with spatiotemporal degrees of freedom Markus Müller, Marius Krumm, Andrew Garner Nonlocality, as demonstrated by the violation of Bell inequalities, enables device-independent cryptographic tasks that do not require users to trust their apparatus. In this article, we consider devices whose inputs are spatiotemporal degrees of freedom, e.g. orientations or time durations. Without assuming the validity of quantum theory, we prove that the devices' statistical response must respect their input's symmetries, with profound foundational and technological implications. We exactly characterize the bipartite binary quantum correlations in terms of local symmetries, indicating a fundamental relation between spacetime and quantum theory. For Bell experiments characterized by two input angles, we show that the correlations are accounted for by a local hidden variable model if they contain enough noise, but conversely must be nonlocal if they are pure enough. This allows us to construct a "Bell witness" that certifies nonlocality with fewer measurements than possible without such symmetries, suggesting a new class of semi-device-independent protocols for quantum technologies. |
Thursday, March 18, 2021 4:48PM - 5:24PM Live |
V33.00009: Engineering superpositions over all possible futures through quantum stochastic simulation Invited Speaker: Jayne Thompson Stochastic simulation plays an important role in quantitative science, enabling future predictions based on past observations. However many of the most important systems that we wish to understand and simulate are complex. Predicting and simulating such systems can involve tracking a prohibitive amount of data, evincing a pressing need for more efficient tools in algorithmic modelling and simulation. |
Thursday, March 18, 2021 5:24PM - 5:30PM Live |
V33.00010: How Motion … Created the Universe Manuel S Morales Science and in particular physics are fundamentally an intellectual endeavor. As such, it is founded on the mechanics of how the human mind perceives and expresses the outside world. In other words, how we think (logic) is how we speak. In linguistics, word order typology reveals that the first-order logic code the human brain uses to practice science is conveyed as existence (subject, object) precedes motion (verb). However, the human brain also uses another first-order logic code to express the natural world, i.e., motion (verb) precedes existence (subject, object). The former logic code has got it right to the extent of providing mankind with sufficient knowledge (theories) of the universe. Recently, the later logic code was used in a twelve-year experiment which obtained necessary empirical knowledge thereby establishing absolute internal validity. To test for absolute external validity, one’s own existence can be put to task safely via a thought experiment or in real life (not recommended). What this all means is that the logic used in science and specifically in physics has got it wrong because it is not based on how nature works and in order to advance from a sufficient study of the natural world to a necessary study, a paradigm shift is required thus needs to be explored. |
Thursday, March 18, 2021 5:30PM - 5:42PM Live |
V33.00011: Experimental violation of n-locality in a star quantum network Davide Poderini, Iris Agresti, Guglielmo Marchese, Emanuele Polino, Taira Giordani, Alessia Suprano, Mauro Valeri, Giorgio Milani, Nicolò Spagnolo, Gonzalo Carvacho, Raphael Chaves, Fabio Sciarrino The launch of a satellite capable of distributing entanglement through long distances and the first loophole-free violation of Bell inequalities are milestones indicating a clear path for the establishment of quantum networks. However, non-locality in networks with independent entanglement sources has only been experimentally verified in simple tripartite networks, via the violation of bilocality inequalities. In our work by using a scalable photonic platform, we implement star-shaped quantum networks consisting of up to five distant nodes and four independent entanglement sources. We exploit this platform to violate the chained n-locality inequality and thus witness, in a device-independent way, the emergence of non-local correlations among the nodes of the implemented networks. These results open new perspectives for quantum information processing applications in the relevant regime where the observed correlations are compatible with standard local hidden variable models but are non-classical if the independence of the sources is taken into account. |
Thursday, March 18, 2021 5:42PM - 5:54PM Live |
V33.00012: Why standard entanglement theory is inappropriate for the study of Bell scenarios David Schmid, Thomas C. Fraser, Ravi Kunjwal, Ana Sainz, Elie Wolfe, Robert Spekkens A standard approach to quantifying resources is to determine which operations are freely available and to deduce the partial order over resources induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, that is, entanglement, then it is a near-universal presumption that the appropriate choice of free operations is Local Operations and Classical Communication (LOCC). We here argue that this is not the best choice for quantifying entanglement in one of the most prominent applications of entanglement theory, namely, the study of Bell scenarios. A better choice, we claim, is Local Operations and Shared Randomness (LOSR). We support this thesis by showing that various perverse features of the interplay between entanglement and nonlocality are resolved in the LOSR paradigm. Specifically, focussing on LOSR (i) provides a resolution of the anomaly of nonlocality, (ii) entails a notion of genuine multipartite entanglement that is distinct from the conventional one and which is free of several of its pathological features, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which simplifies and generalizes prior results. |
Thursday, March 18, 2021 5:54PM - 6:00PM Live |
V33.00013: Quantum Darwinism and stability of the pointer states Bin Yan, Akram Touil, Davide Girolami, Sebastian Deffner, Wojciech Hubert Zurek It is a fundamental problem to explain the emergence of the objective classical world from the quantum world. The mechanism of quantum Darwinism asserts that, after the process of natural decoherence, the system becomes correlated with the environment such that each branch of the global wavefunction corresponds to a selected pointer (classical) state of the system. In this talk, we discuss the stability of pointer states upon observing the environment. In particular, we show that the post-measurement state of the system is almost surely one of the pointer states even when the environment is randomly measured, hence, objectivity of the pointer states is further justified. |
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