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1. Building Multiple Access Channels with a Single Particle

   https://doi.org/10.22331/q-2022-02-16-653  

   Zhang, Yujie; Chen, Xinan; Chitambar, Eric (February 2022, Quantum)

   A multiple access channel describes a situation in which multiple senders are trying to forward messages to a single receiver using some physical medium. In this paper we consider scenarios in which this medium consists of just a single classical or quantum particle. In the quantum case, the particle can be prepared in a superposition state thereby allowing for a richer family of encoding strategies. To make the comparison between quantum and classical channels precise, we introduce an operational framework in which all possible encoding strategies consume no more than a single particle. We apply this framework to an $N$-port interferometer experiment in which each party controls a path the particle can traverse. When used for the purpose of communication, this setup embodies a multiple access channel (MAC) built with a single particle.We provide a full characterization of the $N$-party classical MACs that can be built from a single particle, and we show that every non-classical particle can generate a MAC outside the classical set. To further distinguish the capabilities of a single classical and quantum particle, we relax the locality constraint and allow for joint encodings by subsets of $1<K\le N$parties. This generates a richer family of classical MACs whose polytope dimension we compute. We identify a generalized fingerprinting inequality'' as a valid facet for this polytope, and we verify that a quantum particle distributed among $N$separated parties can violate this inequality even when $K=N-1$. Connections are drawn between the single-particle framework and multi-level coherence theory. We show that every pure state with $K$-level coherence can be detected in a semi-device independent manner, with the only assumption being conservation of particle number. 

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2. Gravitationally induced entanglement in a harmonic trap

   https://doi.org/10.1103/PhysRevD.107.106018  

   Yant, Jackson; Blencowe, Miles (May 2023, Physical Review D)

   Recent work has shown that it may be possible to detect gravitationally induced entanglement in tabletop experiments in the not-too-distant future. However, there are at present no thoroughly developed models for this type of experiment where the entangled particles are treated more fundamentally as excitations of a relativistic quantum field, and with the measurements modeled using expectation values of field observables. Here we propose a thought experiment where two particles are initially prepared in a superposition of coherent states within a common three-dimensional (3D) harmonic trap. The particles then develop entanglement through their mutual gravitational interaction, which can be probed through particle position detection probabilities. The present work gives a nonrelativistic quantum mechanical analysis of the gravitationally induced entanglement of this system, which we term the “gravitational harmonium” due to its similarity to the harmonium model of approximate electron interactions in a helium atom; the entanglement is operationally determined through the matter wave interference visibility. The present work serves as the basis for a subsequent investigation, which models this system using quantum field theory, providing further insights into the quantum nature of gravitationally induced entanglement through relativistic corrections, together with an operational procedure to quantify the entanglement. 

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3. Localization of overdamped bosonic modes and transport in strange metals

   https://doi.org/10.1073/pnas.2402052121  

   Patel, Aavishkar A; Lunts, Peter; Sachdev, Subir (April 2024, Proceedings of the National Academy of Sciences)

   The strange metal phase of correlated electrons materials was described in a recent theory by a model of a Fermi surface coupled a two-dimensional quantum critical bosonic field with a spatially random Yukawa coupling. With the assumption of self-averaging randomness, similar to that in the Sachdev–Ye–Kitaev model, numerous observed properties of a strange metal were obtained for a wide range of intermediate temperatures, including the linear in temperature resistivity. The Harris criterion implies that spatial fluctuations in the local position of the critical point must dominate at lower temperatures. For an $M$-component boson with $M\ge 2$, we use multiple graphics processing units (GPUs) to compute the real frequency spectrum of the boson propagator in a self-consistent mean-field treatment of the boson self-interactions, but an exact treatment of multiple realizations of the spatial randomness from the random boson mass. We find that Landau damping from the fermions leads to the emergence of the physics of the random transverse-field Ising model at low temperatures, as has been proposed by Hoyos, Kotabage, and Vojta. This regime is controlled by localized overdamped eigenmodes of the bosonic scalar field, also has a resistivity which is nearly linear-in-temperature, and extends into a “quantum critical phase” away from the quantum critical point, as observed in several cuprates. For the $M=1$Ising scalar, the mean-field treatment is not applicable, and so we use Hybrid Monte Carlo simulations running on multiple GPUs; we find a rounded transition and localization physics, with strange metal behavior in an extended region around the transition. 

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4. Nonlocal subpicosecond delay metrology using spectral quantum interference

   https://doi.org/10.1364/OPTICA.458565  

   Seshadri, Suparna; Lingaraju, Navin; Lu, Hsuan-Hao; Imany, Poolad; Leaird, Daniel_E; Weiner, Andrew_M (November 2022, Optica)

   Precise knowledge of position and timing information is critical to support elementary protocols such as entanglement swapping on quantum networks. While approaches have been devised to use quantum light for such metrology, they largely rely on time-of-flight (ToF) measurements with single-photon detectors and, therefore, are limited to picosecond-scale resolution owing to detector jitter. In this work, we demonstrate an approach to distributed sensing that leverages phase modulation to map changes in the spectral phase to coincidence probability, thereby overcoming the limits imposed by single-photon detection. By extracting information about the joint biphoton phase, we measure a generalized delay—the difference in signal–idler arrival, relative to local radio frequency (RF) phase modulation. For nonlocal ranging measurements, we achieve ( $2\mathrm{\sigma <\#comment/>}$) precision of $\pm <\#comment/>0.04\mathrm{p}\mathrm{s}$and for measurements of the relative RF phase, ( $2\mathrm{\sigma <\#comment/>}$) precision of $\pm <\#comment/>{0.7}^{\circ <\#comment/>}$. We complement this fine timing information with ToF data from single-photon time-tagging to demonstrate absolute measurement of time delay. By relying on off-the-shelf telecommunications equipment and standard quantum resources, this approach has the potential to reduce overhead in practical quantum networks. 

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5. Quantum computation from dynamic automorphism codes

   https://doi.org/10.22331/q-2024-08-27-1448  

   Davydova, Margarita; Tantivasadakarn, Nathanan; Balasubramanian, Shankar; Aasen, David (August 2024, Quantum)

   We propose a new model of quantum computation comprised of low-weight measurement sequences that simultaneously encode logical information, enable error correction, and apply logical gates. These measurement sequences constitute a new class of quantum error-correcting codes generalizing Floquet codes, which we call dynamic automorphism (DA) codes. We construct an explicit example, the DA color code, which is assembled from short measurement sequences that can realize all 72 automorphisms of the 2D color code. On a stack of $N$triangular patches, the DA color code encodes $N$logical qubits and can implement the full logical Clifford group by a sequence of two- and, more rarely, three-qubit Pauli measurements. We also make the first step towards universal quantum computation with DA codes by introducing a 3D DA color code and showing that a non-Clifford logical gate can be realized by adaptive two-qubit measurements. 

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