Spacelike Separated Research Articles

Certified Randomness in Tight Space

Reliable randomness is a core ingredient in algorithms and applications ranging from numerical simulations to statistical sampling and cryptography. The outcomes of measurements on entangled quantum states can violate Bell inequalities, thus guaranteeing their intrinsic randomness. This constitutes the basis for certified randomness generation. However, this certification requires spacelike separated devices, making it unfit for a compact apparatus. Here we provide a general method for certified randomness generation on a small-scale application-ready device and perform an integrated photonic demonstration combining a solid-state emitter and a glass chip. In contrast to most existing certification protocols, which in the absence of spacelike separation are vulnerable to loopholes inherent to realistic devices, the protocol we implement accounts for information leakage and is thus compatible with emerging compact scalable devices. We demonstrate a two-qubit photonic device that achieves the highest standard in randomness, yet is cut out for real-world applications. The full 94.5-h-long stabilized process harnesses a bright and stable single-photon quantum-dot-based source, feeding into a reconfigurable photonic chip, with stability in the milliradian range on the implemented phases and consistent indistinguishability of the entangled photons above 93%. Using the contextuality framework, we certify private randomness generation and achieve a rate compatible with randomness expansion secure against quantum adversaries. Published by the American Physical Society 2024

Two Quantum Algorithms for Communication between Spacelike Separated Locations

Two Quantum Algorithms for Communication between Spacelike Separated Locations

Type-Independent Characterization of Spacelike Separated Resources.

Quantum theory describes multipartite objects of various types: quantum states, nonlocal boxes, steering assemblages, teleportages, distributed measurements, channels, and so on. Such objects describe, for example, the resources shared in quantum networks. Not all such objects are useful, however. In the context of spacelike separated parties, devices which can be simulated using local operations and shared randomness are useless, and it is of paramount importance to be able to practically distinguish useful from useless quantum resources. Accordingly, a body of literature has arisen to provide tools for witnessing and quantifying the nonclassicality of objects of each specific type. In the present Letter, we provide a framework which subsumes and generalizes all of these resources, as well as the tools for witnessing and quantifying their nonclassicality.

Probing the non-classicality of temporal correlations

Correlations between spacelike separated measurements on entangled quantum systems are stronger than any classical correlations and are at the heart of numerous quantum technologies. In practice, however, spacelike separation is often not guaranteed and we typically face situations where measurements have an underlying time order. Here we aim to provide a fair comparison of classical and quantum models of temporal correlations on a single particle, as well as timelike-separated correlations on multiple particles. We use a causal modeling approach to show, in theory and experiment, that quantum correlations outperform their classical counterpart when allowed equal, but limited communication resources. This provides a clearer picture of the role of quantum correlations in timelike separated scenarios, which play an important role in foundational and practical aspects of quantum information processing.

Strong Loophole-Free Test of Local Realism.

We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p values as small as 5.9×10^{-9} for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p value is 2.3×10^{-7}. We therefore reject the hypothesis that local realism governs our experiment.

Quantum interactions with closed timelike curves and superluminal signaling

There is now a significant body of results on quantum interactions with closed timelike curves (CTCs) in the quantum information literature, for both the Deutsch model of CTC interactions (D-CTCs) and the projective model (P-CTCs). As a consequence, there is a prima facie argument exploiting entanglement that CTC interactions would enable superluminal and, indeed, effectively instantaneous signaling. In cases of spacelike separation between the sender of a signal and the receiver, whether a receiver measures the local part of an entangled state or a disentangled state to access the signal can depend on the reference frame. We propose a consistency condition that gives priority to either an entangled perspective or a disentangled perspective in spacelike separated scenarios. For D-CTC interactions, the consistency condition gives priority to frames of reference in which the state is disentangled, while for P-CTC interactions the condition selects the entangled state. Using the consistency condition, we show that there is a procedure that allows Alice to signal to Bob in the past via relayed superluminal communications between spacelike separated Alice and Clio, and spacelike separated Clio and Bob. This opens the door to time travel paradoxes in the classical domain. Ralph (arXiv:1107.4675) first pointed this out for P-CTCs, but we show that Ralph's procedure for a 'radio to the past' is flawed. Since both D-CTCs and P-CTCs allow classical information to be sent around a spacetime loop, it follows from a result by Aaronson and Watrous (Proc.Roy.Soc.A, 465:631-647 (2009)) for CTC-enhanced classical computation that a quantum computer with access to P-CTCs would have the power of PSPACE, equivalent to a D-CTC-enhanced quantum computer.

Time-reversal asymmetry and state-vector collapse

A sentence in the Search and Discovery story “Time-reversal asymmetry in particle physics has finally been clearly seen” (Physics Today, November 2012, page 16) has me somewhat puzzled. Bertram Schwarzschild writes, “If the first decay of a daughter reveals it to have been in a specific flavor or CP eigenstate, her still undecayed sister must—at that instant—be in the opposite state.” What is implicit in this sentence is that state-vector collapse takes place at the decay of the first B meson, so that the second meson is instantaneously projected onto the opposite state.I have two objections to this sentence. First, it is not relativistically invariant: Because the two events—decay of the first meson and “transformation” of the second—have spacelike separation, their time ordering is not defined. Second, the decay is governed by a relativistic version of the Schrödinger equation, which is reversible, whereas state-vector collapse is, by nature, irreversible.It is instructive to make a comparison with the analysis of a Bell-type experiment, in which a two-particle observable is measured via two detectors that are far apart, with each measuring the spin of one of the two particles. The quantum mechanical result is given by applying Born’s rule to the two-particle observable.The two experimentalists, Alice and Bob, then compare their results by exchanging them at a speed less than that of light, and they notice the results are correlated, in conformity with the theoretical predictions of nonlocal correlations. The same theoretical results may be derived from state-vector collapse: When Alice measures the first spin, Bob’s spin is projected instantaneously onto a well-defined polarization state; correlations may be easily computed and are, of course, in agreement with those derived from Born’s rule.State-vector collapse is strictly equivalent to Born’s rule in a nonrelativistic context, but I think it is safer to use Born’s rule when special relativity comes into play. Indeed, to the best of my knowledge, there is no satisfactory relativistic generalization of state-vector collapse, which is understandable since simultaneity is not defined for two spacelike separated events. Therefore, a full justification of the argument used in the Physics Today report should begin with state-vector collapse taking place in a well-defined reference frame at the instant when the decay products of the B mesons reach the detectors. Then, from the correlations observed between the results of the detectors, it should be possible to reconstruct the decays and show that they can be interpreted as an effective state-vector collapse.Such an analysis would likely vindicate that of the BaBar experiment, but as the use of Einstein-Podolsky-Rosen correlations goes far beyond the usual one, it should be fully justified.© 2013 American Institute of Physics.

Quantum Steering and Spacelike Separation

In nonrelativistic quantum mechanics, measurements performed by separate observers are modeled via tensor products. In algebraic quantum field theory, though, local observables corresponding to spacelike separated parties are just required to commute. The problem of determining whether these two definitions of separation lead to the same set of bipartite correlations is known in nonlocality as Tsirelson's problem. In this article, we prove that the analog of Tsirelson's problem in steering scenarios is false. That is, there exists a steering inequality that can or cannot be violated depending on how we define spacelike separation at the operator level.

Causality implies formal state collapse

A physical theory of experiments carried out in a space-time region can accommodate a detector localized in another space-like separated region, in three, not necessarily exclusive, ways: 1) the detector formally collapses physical states across space-like separations, 2) the detector enables superluminal signals, and 3) the theory becomes logically inconsistent. If such a theory admits autonomous evolving states, the space-like collapse must be instantaneous. Time-like separation does not allow such conclusions. We also prove some simple results on structural stability: within the set of all possible theories, under a weak empirical topology, the set of all theories with superluminal signals and the set of all theories with retrograde signals are both open and dense.