Papers by Eric Cavalcanti
arXiv (Cornell University), May 24, 2022
In a recent work, Bong et al. [Nature Physics 16, 1199 (2020)] proved a no-go theorem demonstrati... more In a recent work, Bong et al. [Nature Physics 16, 1199 (2020)] proved a no-go theorem demonstrating a contradiction between a set of assumptions called "Local Friendliness" (LF) and certain quantum phenomena on an extended version of the "Wigner's friend" paradox. The LF assumptions can be understood as the conjunction of two independent assumptions: Absoluteness of Observed Events (AOE) requires that events observed by any observer have absolute, rather than relative, values; Local Agency (LA) encodes the assumption that an intervention cannot influence events outside its future light cone. The proof of the LF no-go theorem, however, implicitly assumes the validity of standard probability theory. Here we present a probability-free version of the Local Friendliness theorem, building upon Hardy's no-go theorem for local hidden variables. The argument is phrased in the language of possibilities, which we make formal by using a modal logical approach. It relies on a weaker version of Local Agency, which we call "Possibilistic Local Agency": the assumption that an intervention cannot influence the possibilities of events outside its future light cone.
Why the Quantum? This is an Editorial on "Classi cation of all alternatives to the Born rule in t... more Why the Quantum? This is an Editorial on "Classi cation of all alternatives to the Born rule in terms of informational properties" by Thomas D.
arXiv (Cornell University), May 15, 2023
The Wigner's friend thought experiment has gained a resurgence of interest in recent years thanks... more The Wigner's friend thought experiment has gained a resurgence of interest in recent years thanks to no-go theorems that extend it to Bell-like scenarios. One of these, by us and co-workers, showcased the contradiction that arises between quantum theory and a set of assumptions, weaker than those in Bell's theorem, which we named "local friendliness". Using these assumptions it is possible to arrive at a set of inequalities for a given scenario, and, in general, some of these inequalities will be harder to violate than the Bell inequalities for the same scenario. A crucial feature of the extended Wigner's friend scenario in our aforementioned work was the ability of a superobserver to reverse the unitary evolution that gives rise to their friend's measurement. Here, we present a new scenario where the superobserver can interact with the friend repeatedly in a single experimental instance, either by asking them directly for their result, thus ending that instance, or by reversing their measurement and instructing them to perform a new one. We show that, in these scenarios, the local friendliness inequalities will always be the same as Bell inequalities.
arXiv (Cornell University), May 12, 2023
Lawrence et al. have presented an argument purporting to show that "relative facts do not exist" ... more Lawrence et al. have presented an argument purporting to show that "relative facts do not exist" and, consequently, "Relational Quantum Mechanics is incompatible with quantum mechanics". The argument is based on a GHZ-like contradiction between constraints satisfied by measurement outcomes in an extended Wigner's friend scenario. Here we present a strengthened version of the argument, and show why, contrary to the claim by Lawrence et al., these arguments do not contradict the consistency of a theory of relative facts. Rather, considering this argument helps clarify how one should not think about a theory of relative facts, like RQM.
We address the important issue of how to unambiguously identify signatures of quantum entanglemen... more We address the important issue of how to unambiguously identify signatures of quantum entanglement in the presence of decoherence and loss. We analyse entanglement, EPR and Bell violation experiments, with both state impurity and detector inefficiency effects included. We also find a new class of Bell inequality, applicable to either discrete or continuous measurements. This new Bell inequality has the remarkable property that it is more immune to losses as the photon number and number of parties observing increases.
Physical review, Jul 18, 2016
The fully symmetric Gaussian tripartite entangled pure states will not exhibit two-mode Einstein ... more The fully symmetric Gaussian tripartite entangled pure states will not exhibit two-mode Einstein Podolsky-Rosen (EPR)-steering. This means that any two participants cannot share quantum secrets using the security of one-sided device independent quantum key distribution (1SDI-QKD) without involving the third. They are restricted at most to standard quantum key distribution (S-QKD), which is less secure. Here we demonstrate an asymmetric tripartite system that can exhibit bipartite EPR-steering, so that two of the participants can use 1SDI-QKD without involving the other. This is possible because the promiscuity relations of continuous-variable tripartite entanglement are different from those of discrete-variable systems. We analyse these properties for two different systems, showing that the asymmetric system exhibits practical properties not found in the symmetric one.
Quantum Information Processing, Dec 11, 2011
In post-Stalin Soviet Union, dissidents used underground networks to distribute censored material... more In post-Stalin Soviet Union, dissidents used underground networks to distribute censored material. This activity, which was called samizdat (translates to "self-published") put its practitioners at grave risk. Though certainly not nearly as risky, it requires substantial courage for scientists to "come out" and make public their philosophical thoughts. It likewise requires a large dose of chutzpah for one to publish one's own emails as a book. In "Coming of Age with Quantum Information" Christopher Fuchs-who is a Senior Researcher at the Perimeter Institute for Theoretical Physics, in Waterloo, Canada-does both of these things. The book grew out of his correspondence with friends and colleagues between 1995 and 2001. and which he started to distribute-also electronically-as his "samizdat". Among the "subversives" that were-knowingly or not-dragged into Chris' samizdat bv corresponding with him are many leading quantum information scientists:
Physical Review Letters, Oct 23, 2019
In a measurement-device-independent or quantum-refereed protocol, a referee can verify whether tw... more In a measurement-device-independent or quantum-refereed protocol, a referee can verify whether two parties share entanglement or Einstein-Podolsky-Rosen (EPR) steering without the need to trust either of the parties or their devices. The need for trusting a party is substituted by a quantum channel between the referee and that party, through which the referee encodes the measurements to be performed on that party's subsystem in a set of nonorthogonal quantum states. In this Letter, an EPR-steering inequality is adapted as a quantum-refereed EPR-steering witness, and the trust-free experimental verification of higher dimensional quantum steering is reported via preparing a class of entangled photonic qutrits. Further, with two measurement settings, we extract 1.106 ± 0.023 bits of private randomness per every photon pair from our observed data, which surpasses the one-bit limit for projective measurements performed on qubit systems. Our results advance research on quantum information processing tasks beyond qubits.
Physical Review Letters, Jun 25, 2014
According to a recent no-go theorem (M. Pusey, J. Barrett and T. Rudolph, Nature Physics 8 475 (2... more According to a recent no-go theorem (M. Pusey, J. Barrett and T. Rudolph, Nature Physics 8 475 (2012)), models in which quantum states correspond to probability distributions over the values of some underlying physical variables must have the following feature: the distributions corresponding to distinct quantum states do not overlap. This is significant because if the distributions do not overlap, then the quantum state itself is encoded by the physical variables. In such a model, it cannot coherently be maintained that the quantum state merely encodes information about underlying physical variables. The theorem, however, considers only models in which the physical variables corresponding to independently prepared systems are independent. This work considers models that are defined for a single quantum system of dimension d, such that the independence condition does not arise. We prove a result in a similar spirit to the original no-go theorem, in the form of an upper bound on the extent to which the probability distributions can overlap, consistently with reproducing quantum predictions. In particular, models in which the quantum overlap between pure states is equal to the classical overlap between the corresponding probability distributions cannot reproduce the quantum predictions in any dimension d ≥ 3. The result is noise tolerant, and an experiment is motivated to distinguish the class of models ruled out from quantum theory.
Nature Physics, Feb 2, 2015
Quantum mechanics is an outstandingly successful description of nature, underpinning fields from ... more Quantum mechanics is an outstandingly successful description of nature, underpinning fields from biology through chemistry to physics. At its heart is the quantum wavefunction, the central tool for describing quantum systems. Yet it is still unclear what the wavefunction actually is: does it merely represent our limited knowledge of a system, or is it an element of reality? Recent no-go theorems[11-16] argued that if there was any underlying reality to start with, the wavefunction must be real. However, that conclusion relied on debatable assumptions, without which a partial knowledge interpretation can be maintained to some extent[15, 18]. A different approach is to impose bounds on the degree to which knowledge interpretations can explain quantum phenomena, such as why we cannot perfectly distinguish non-orthogonal quantum states[19-21]. Here we experimentally test this approach with single photons. We find that no knowledge interpretation can fully explain the indistinguishability of non-orthogonal quantum states in three and four dimensions. Assuming that some underlying reality exists, our results strengthen the view that the entire wavefunction should be real. The only alternative is to adopt more unorthodox concepts such as backwards-intime causation, or to completely abandon any notion of objective reality.
arXiv (Cornell University), Feb 23, 2023
We introduce a formalism for the evaluation of counterfactual queries in the framework of quantum... more We introduce a formalism for the evaluation of counterfactual queries in the framework of quantum causal models, by generalising the three-step procedure of abduction, action, and prediction in Pearl's classical formalism of counterfactuals [1]. To this end, we define a suitable extension of Pearl's notion of a 'classical structural causal model', which we denote analogously by 'quantum structural causal model'. We show that every classical (probabilistic) structural causal model can be extended to a quantum structural causal model, and prove that counterfactual queries that can be formulated within a classical structural causal model agree with their corresponding queries in the quantum extension-but the latter is more expressive. Counterfactuals in quantum causal models come in different forms: we distinguish between active and passive counterfactual queries, depending on whether or not an intervention is to be performed in the action step. This is in contrast to the classical case, where counterfactuals are always interpreted in the active sense. As a consequence of this distinction, we observe that quantum causal models break the connection between causal and counterfactual dependence that exists in the classical case: (passive) quantum counterfactuals allow counterfactual dependence without causal dependence. This illuminates an important distinction between classical and quantum causal models, which underlies the fact that the latter can reproduce quantum correlations that violate Bell inequalities while being faithful to the relativistic causal structure.
arXiv (Cornell University), Nov 29, 2022
We address various aspects of a widely used tool in quantum information theory: the Choi-Jamio lk... more We address various aspects of a widely used tool in quantum information theory: the Choi-Jamio lkowski isomorphism [A. Jamio lkowski, Rep. Math. Phys., 3, 275 (1972)]. We review different versions of the isomorphism, their properties and propose a unified description that combines them all. To this end, we identify the physical reason for the appearance of the (basis-dependent) operation of transposition in the isomorphism as used in Choi's theorem [M.-D. Choi, Lin. Alg. Appl., 10, 285 (1975)]. This requires a careful distinction between Jordan algebras and the different C *-algebras they arise from, which are distinguished by their order of composition. Physically, the latter encodes a choice of time orientation in the respective algebras, which relates to a number of recent results, including a characterisation of quantum from more general nonsignalling bipartite correlations [M. Frembs and A. Döring, arXiv:2204.11471] and a classification of bipartite entanglement [M. Frembs, arXiv:2207.00024].
Physical Review E, 2020
The standard formulation of thermostatistics, being based on the Boltzmann-Gibbs distribution and... more The standard formulation of thermostatistics, being based on the Boltzmann-Gibbs distribution and logarithmic Shannon entropy, describes idealized uncorrelated systems with extensive energies and short-range interactions. In this letter, we use the fundamental principles of ergodicity (via Liouville's theorem), the self-similarity of correlations, and the existence of the thermodynamic limit to derive generalized forms of the equilibrium distribution for long-range-interacting systems. Significantly, our formalism provides a justification for the well-studied nonextensive thermostatistics characterized by the Tsallis distribution, which it includes as a special case. We also give the complementary maximum entropy derivation of the same distributions by constrained maximization of the Boltzmann-Gibbs-Shannon entropy. The consistency between the ergodic and maximum entropy approaches clarifies the use of the latter in the study of correlations and nonextensive thermodynamics.
arXiv (Cornell University), Sep 18, 2022
Conference on Lasers and Electro-Optics
The Wigner’s friend paradox illuminates the quantum measurement problem. We derive—and study, in ... more The Wigner’s friend paradox illuminates the quantum measurement problem. We derive—and study, in a series of entangled-photon experiments—a new and robust no-go theorem based on the paradox, with stronger constraints than Bell's theorem.
Xiang Zhan, Eric G. Cavalcanti, Jian Li, Zhihao Bian, Yongsheng Zhang, 4, ∗ Howard M. Wiseman, 5,... more Xiang Zhan, Eric G. Cavalcanti, Jian Li, Zhihao Bian, Yongsheng Zhang, 4, ∗ Howard M. Wiseman, 5, † and Peng Xue 6, ‡ Department of Physics, Southeast University, Nanjing 211189, China Centre for Quantum Dynamics, Griffith University, Gold Coast, QLD 4222, Australia Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei 230026, China Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China Centre for Quantum Computation and Communication Technology (Australian Research Council), Griffith University, Brisbane 4111, Australia State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
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Papers by Eric Cavalcanti