arXiv subjects

Brian R. La Cour

Publications and source records attributed to Brian R. La Cour.

At least 19 recordsLinked to original sources

arXiv2025

Wigner function negativity in a classical model of quantum light

The presence of negative values in the Wigner quasiprobability distribution is deemed one of the hallmarks of nonclassical phenomena in quantum systems. Here we demonstrate a classical model of squeezed light that, when combined with post-selection on amplitude threshold-crossing detection events, is capable of reproducing observed behavior of single-photon added coherent states. In particular, a classical model of balanced homodyne detection and standard tomographic techniques are used to infer the density matrix in the Fock basis. The resulting Wigner functions exhibit negatively for photon-added vacuum and weak coherent states.

quant-ph↗

arXiv2025

An Adversarial Quantum Key Distribution Project

Quantum key distribution (QKD) is a popular introduction to quantum technologies used in education and public outreach, as very little background in quantum theory is needed and the practical applications are easily understood. There is considerably less exposure to the many real-world considerations of practical QKD, as access to the necessary hardware is quite limited. Here we describe a simple, simulation-based QKD project that can be implemented with only a minimal background in quantum concepts and programming. Students are assembled in small groups to develop an ``Alice and Bob'' protocol for securely distributing symmetric keys in a simulated noisy channel. Their protocol is then shared anonymously with another group who plays the role of Eve and attempts to steal as much secret key as possible. The adversarial aspect is popular with students, and the project itself provides a deeper understanding and appreciation for practical QKD.

physics.ed-ph↗

arXiv2025

Definitive Proof of the Classical Multiverse!

Recent astonishing experiments with quantum computers have demonstrated unambiguously the existence of a quantum multiverse, where calculations of mind-boggling complexity are effortlessly computed in just a few minutes. Here, we investigate whether a similar computation on a digital computer can demonstrate the existence of a classical multiverse. To this end we describe a classical algorithm for efficiently sampling from a $d^n$-dimensional discrete probability distribution representing $n$ digits of $d$ possible values with strong statistical dependence. Although the full distribution for large $n$ quickly becomes intractable, probabilities for given samples can be computed quite efficiently. This allows us to compute exact empirical linear cross-entropy benchmark (XEB) values. Results on a low-end laptop for $d=2$ show excellent agreement with the true XEB for $n \le 30$ and large positive values of the exact empirical XEB for $n \le 1023$ computed over one million samples. We conclude that classical, as well as quantum, computation occurs in many parallel universes.

quant-ph↗

arXiv2025

Pinching operators for approximating multiphoton entangled states

We introduce the pinching operator, which extends the theory of squeezing operators to non-Gaussian operators, and use it to approximate $n$-photon entangled states using a pinched vacuum state and pinching tensor of rank $n$. A simple recursion relation is derived for generating the Bogoliubov transformed creation and annihilation operators, which may be used to express the pinched state as a statistically equivalent set of nonlinearly transformed complex Gaussian random variables. Using this representation, we compare low-order approximations of the pinched state to entangled multiphoton Fock states, such as Greenberger-Horne-Zeilinger (GHZ) and W states. Using post-selection and a threshold detector model to represent non-Gaussian measurements, we find that this model is capable of producing states with a fidelity comparable to that of experimentally prepared multiphoton entangled states. Our results show that it is possible to classically simulate large multiphoton entangled states to high fidelity within the constraints of finite detection efficiency.

quant-ph↗

arXiv2025

Classical Attack on Bell Inequalities

Representing multi-mode squeezed light with a Gaussian random vector, our locally deterministic detection model challenges the CHSH game, achieving fidelities exceeding 96\%. Squeezing strength, detector threshold, and efficiency influence the security of the quantum bound.

quant-ph↗

arXiv2024

Photon number resolving detectors as evidence for the corpuscular nature of light

We consider the question of whether photon-number-resolving (PNR) detectors provide compelling evidence for the discrete nature of light; i.e., whether they indicate the prior presence of a certain number of discrete photons. To answer this question, we reveal the insufficient signal-to-noise ratio (SNR) of existing PNR detectors, and propose an alternative interpretation for the analysis of PNR detector output that is consistent with a wave picture of light and does not rely on the presumption of light particles. This interpretation is based on the aggregation of correlated or accidentally coincident detections within a given detector coincidence window. Our interpretation accounts for the arbitrary character of detector coincidence windows and includes connections to established treatments of intensity interferometers. To validate our interpretation, we performed an experiment on a multiplexed PNR detector and examined the dependence of photon number on the coincidence window via post-processing. These observations were then compared to a fully classical wave model based on amplitude threshold detection, and the results were found to be in excellent agreement. We find that results from low SNR PNR detectors, such as those existing in the literature, are able to be described by classical descriptions, and therefore do not demonstrate evidence for the discrete nature of light.

quant-ph↗

arXiv2023

Generalized Noncoherent Space-Time Block Codes from Quantum Error Correction

This paper generalizes results in noncoherent space-time block code (STBC) design based on quantum error correction (QEC) to new antenna configurations. Previous work proposed QEC-inspired STBCs for antenna geometries where the number of transmit and receive antennas were equal and a power of two. In this work we extend these results by providing QEC-inspired STBCs applicable to all square antenna geometries and some rectangular geometries where the number of receive antennas is greater than the number of transmit antennas. We derive the maximum-likelihood decoding rule for this family of codes for the special case of Rayleigh fading with additive white Gaussian noise. We present Monte Carlo simulations of the performance of the codes in this environment for a three-antenna square geometry and a three-by-six rectangular geometry. We demonstrate competitive performance for these codes with respect to a popular noncoherent differential code.

eess.SP↗

arXiv2022

Classical model of quantum interferometry tests of macrorealism

Macrorealism is a characteristic feature of many, but not all, classical systems. It is known, for example, that classical light can violate a Leggett-Garg inequality and, hence, reject a macrorealist interpretation. A recent experiment has used entangled light and negative measurements to demonstrate a loophole-free test of macrorealism [PRX Quantum $\mathbf{3}$, 010307 (2022)]. This paper shows that such an experiment, while soundly rejecting macrorealism, may nevertheless be open to a classical interpretation. This is done by offering an explicit classical model of heralded photon detection in an optical interferometer with beam blockers. A numerical analysis of the model shows good agreement with experimental observations and consistency with both local realism and a rejection of macrorealism.

quant-ph↗

arXiv2022

Quantum Games and Interactive Tools for Quantum Technologies Outreach and Education

In this article, we provide an extensive overview of a wide range of quantum games and interactive tools that have been employed by the community in recent years. The paper presents selected tools, as described by their developers. The list includes Hello Quantum, Hello Qiskit, Particle in a Box, Psi and Delta, QPlayLearn, Virtual Lab by Quantum Flytrap, Quantum Odyssey, ScienceAtHome, and The Virtual Quantum Optics Laboratory. Additionally, we present events for quantum game development: hackathons, game jams, and semester projects. Furthermore, we discuss the Quantum Technologies Education for Everyone (QUTE4E) pilot project, which illustrates an effective integration of these interactive tools with quantum outreach and education activities. Finally, we aim at providing guidelines for incorporating quantum games and interactive tools in pedagogic materials to make quantum technologies more accessible for a wider population.

physics.ed-ph↗

arXiv2021

Piloting a full-year, optics-based high school course on quantum computing

Quantum computing was once regarded as a mere theoretical possibility, but recent advances in engineering and materials science have brought practical quantum computers closer to reality. Currently, representatives from industry, academia, and governments across the world are working to build the educational structures needed to produce the quantum workforce of the future. Less attention has been paid to growing quantum computing capacity at the high school level. This article details work at The University of Texas at Austin to develop and pilot the first full-year high school quantum computing class. Over the course of two years, researchers and practitioners involved with the project learned several pedagogical and practical lessons that can be helpful for quantum computing course design and implementation at the secondary level. In particular, we find that the use of classical optics provides a clear and accessible avenue for representing quantum states and gate operators and facilitates both learning and the transfer of knowledge to other Science, Technology, and Engineering (STEM) skills. Furthermore, students found that exploring quantum optical phenomena prior to the introduction of mathematical models helped in the understanding and mastery of the material.

physics.ed-ph↗

arXiv2021

A Quantum-Inspired Classical Solver for Boolean k-Satisfiability Problems

In this paper we detail a classical algorithmic approach to the k-satisfiability (k-SAT) problem that is inspired by the quantum amplitude amplification algorithm. This work falls under the emerging field of quantum-inspired classical algorithms. To propose our modification, we adopt an existing problem model for k-SAT known as Universal SAT (UniSAT), which casts the Boolean satisfiability problem as a non-convex global optimization over a real-valued space. The quantum-inspired modification to UniSAT is to apply a conditioning operation to the objective function that has the effect of "amplifying" the function value at points corresponding to optimal solutions. We describe the algorithm for achieving this amplification, termed "AmplifySAT," which follows a familiar two-step process of applying an oracle-like operation followed by a reflection about the average. We then discuss opportunities for meaningfully leveraging this processing in a classical digital or analog computing setting, attempting to identify the strengths and limitations of AmplifySAT in the context of existing non-convex optimization strategies like simulated annealing and gradient descent.

quant-ph↗

arXiv2021

The Virtual Quantum Optics Laboratory

We present a web-based software tool, the Virtual Quantum Optics Laboratory (VQOL), that may be used for designing and executing realistic simulations of quantum optics experiments. A graphical user interface allows one to rapidly build and configure a variety of different optical experiments, while the runtime environment provides unique capabilities for visualization and analysis. All standard linear optical components are available as well as sources of thermal, coherent, and entangled Gaussian states. A unique aspect of VQOL is the introduction of non-Gaussian measurements using detectors modeled as deterministic devices that "click" when the amplitude of the light falls above a given threshold. We describe the underlying theoretical models and provide several illustrative examples. We find that VQOL provides a a faithful representation of many experimental quantum optics phenomena and may serve as both a useful instructional tool for students as well as a valuable research tool for practitioners.

quant-ph↗

arXiv2021

Quantum contextuality in the Mermin-Peres square: A hidden variable perspective

The question of a hidden variable interpretation of quantum contextuality in the Mermin-Peres square is considered. The Kochen-Specker theorem implies that quantum mechanics may be interpreted as a contextual hidden variable theory. It is shown that such a hidden variable description can be viewed as either contextual in the random variables mapping hidden states to observable outcomes or in the probability measure on the hidden state space. The latter view suggests that this apparent contextuality may be interpreted as a simple consequence of measurement disturbance, wherein the initial hidden state is altered through interaction with the measuring device, thereby giving rise to a possibly different final hidden variable state from which the measurement outcome is obtained. In light of this observation, a less restrictive and, arguably, more reasonable definition of noncontextuality is suggested. To prove that such a description is possible, an explicit and, in this sense, noncontextual hidden variable model is constructed which reproduces all quantum theoretic predictions for the Mermin-Peres square. A critical analysis of some recent and proposed experimental tests of contextuality is also provided. Although the discussion is restricted to a four-dimensional Hilbert space, the approach and conclusions are expected to generalize to any Hilbert space.

quant-ph↗

arXiv2021

Quantum decoherence emulated in a classical device

We demonstrate that a classical emulation of quantum gate operations, here represented by an actual analog electronic device, can be modeled accurately as a quantum operation in terms of a universal set of Pauli operators. This observation raises the possibility that quantum error correction methods may be applied to classical systems to improve fault tolerance.

quant-ph↗

arXiv2021

Classical Simulated Annealing Using Quantum Analogues

In this paper we consider the use of certain classical analogues to quantum tunneling behavior to improve the performance of simulated annealing on a discrete spin system of the general Ising form. Specifically, we consider the use of multiple simultaneous spin flips at each annealing step as an analogue to quantum spin coherence as well as modifications of the Boltzmann acceptance probability to mimic quantum tunneling. We find that the use of multiple spin flips can indeed be advantageous under certain annealing schedules, but only for long anneal times.

quant-ph↗

arXiv2021

A local hidden-variable model for experimental tests of the GHZ puzzle

The Greenberger-Horne-Zeilinger (GHZ) puzzle has been used to study quantum nonlocality and provide an all-or-nothing, no-go theorem for local hidden-variable models. Recent experiments using coincident-detected entangled photons prepared in a three-particle GHZ state have been used to test quantum nonlocality, but fail to rule out local realism due to a reliance on the fair-sampling hypothesis and insufficient detection efficiency. This paper describes a physically motivated local hidden-variable model based on amplitude-threshold detection that is capable of producing similar results. Detection efficiencies for the model are within the bounds permitted for local realism and, interestingly, exhibit statistical correlations between detectors, even when the detection events are spacelike separated. Increasing the detection threshold improves agreement with the ideal quantum predictions at the cost of decreased detection efficiency. A curious emergent feature of the model is that detection efficiencies may depend upon which observables are chosen for measurement.

quant-ph↗

arXiv2021

Derivation of the Onsager Principle from Large Deviation Theory

The Onsager linear relations between macroscopic flows and thermodynamics forces are derived from the point of view of large deviation theory. For a given set of macroscopic variables, we consider the short-time evolution of near-equilibrium fluctuations, represented as the limit of finite-size conditional expectations. The resulting asymptotic conditional expectation is taken to represent the typical macrostate of the system and is used in place of the usual time-averaged macrostate of traditional approaches. By expanding in the short-time, near-equilibrium limit and equating the large deviation rate function with the thermodynamic entropy, a linear relation is obtained between the time rate of change of the macrostate and the conjugate initial macrostate. A Green-Kubo formula for the Onsager matrix is derived and shown to be positive semi-definite, while the Onsager reciprocity relations readily follow from time reversal invariance. Although the initial tendency of a macroscopic variable is to evolve towards equilibrium, we find that this evolution need not be monotonic. The example of an ideal Knundsen gas is considered as an illustration.

cond-mat.stat-mech↗

arXiv2021

Macroscopic Determinism in Interacting Systems Using Large Deviation Theory

We consider the quasi-deterministic behavior of systems with a large number, $n$, of deterministically interacting constituents. This work extends the results of a previous paper [J. Stat. Phys. 99:1225-1249 (2000)] to include vector-valued observables on interacting systems. The approach used here, however, differs markedly in that a level-1 large deviation principle (LDP) on joint observables, rather than a level-2 LDP on empirical distributions, is employed. As before, we seek a mapping $ψ_{t}$ on the set of (possibly vector-valued) macrostates such that, when the macrostate is given to be $a_0$ at time zero, the macrostate at time $t$ is $ψ_{t}(a_0)$ with a probability approaching one as $n$ tends to infinity. We show that such a map exists and derives from a generalized dynamic free energy function, provided the latter is everywhere well defined, finite, and differentiable. We discuss some general properties of $ψ_{t}$ relevant to issues of irreversibility and end with an example of a simple interacting lattice, for which an exact macroscopic solution is obtained.

cond-mat.stat-mech↗