Quantum Physics · Cross-listed from nucl-th
Entanglement and accidental symmetries in the nucleon-nucleon system
Alma L. Cavallin, Oliver Thim and Christian Forssén
We study the connection between accidental
symmetries in the
nuclear interaction and
spin entanglement in two-nucleon scattering. Specifically, we incorporate different levels of Wigner $SU(4)$ and Serber
symmetries into leading-order potentials derived from
chiral effective field theory. We conduct a quantitative analysis by computing the full $S$
matrix, demonstrating that the
neutron-
proton spin entanglement can be related to the
symmetry properties of the interaction and the presence of certain operators and partial waves. Furthermore, we study the order-by-order
evolution of the
spin entanglement, up to next-to-next-to-leading order in Weinberg power counting, for both
neutron-
proton and
neutron-
neutron scattering.
Entanglement suppression is not observed in
neutron-
neutron scattering, which can be attributed to the Pauli principle and the absence of accidental
symmetries in this system. We conclude that
entanglement is a useful guide for studying the power counting and
symmetries in
nuclear interactions derived from effective field theories.
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Quantum Physics
Fast CZ Gate via Energy-Level Engineering in Superconducting Qubits with a Tunable Coupler
Benzheng Yuan, Chaojie Zhang, Chuanbing Han et al. (+8 more)
In superconducting
quantum circuits, decoherence errors in
qubits constitute a critical factor limiting
quantum gate performance. To mitigate decoherence-induced gate infidelity, rapid implementation of
quantum gates is essential. Here we propose a scheme for rapid controlled-Z (CZ) gate implementation through energy-level engineering, which leverages Rabi oscillations between the $\left|11\right\rangle$ state and the non-computational state in a tunable-coupler architecture. Numerical simulations achieved a $\mathrm{22~ns}$ nonadiabatic CZ gate with fidelity over $99.99\%$. We further investigated the performance of the CZ gate in the presence of anharmonicity offsets. The results demonstrate that a high-fidelity CZ gate with an error rate below $10^{-4}$ remains achievable even with finite anharmonicity variations. Furthermore, the detrimental impact of spectator
qubits in different
quantum states on the fidelity of CZ gate is effectively suppressed by incorporating a tunable coupler. This scheme exhibits potential for extending the
circuit execution depth constrained by coherence time limitations.
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All authors Benzheng Yuan, Chaojie Zhang, Chuanbing Han, Shuya Wang, Peng Xu, Huihui Sun, Qing Mu, Lixin Wang, Bo Zhao, Weilong Wang, Zheng Shan
Quantum Physics
Nonlocal Games Through Communication Complexity and Quantum Cryptography
Pierre Botteron
This thesis explores foundational aspects of
quantum information theory and
quantum cryptography. First, we investigate
quantum correlations in interactive settings, including the CHSH and graph isomorphism games. We aim to distinguish
quantum correlations from non-signaling correlations by leveraging the principle of communication complexity. To this end, we employ techniques such as distributed computation, majority-function-based
distillation protocols, the algebraic and
geometric properties of nonlocal box wirings, and variations of some graph properties such as isomorphism, transitivity, and equitable partitions. This inquiry advances our understanding of non-physical correlations. Second, we address a key open problem in cryptography: the feasibility of unclonable encryption. We aim to construct an encryption scheme that prevents two distant parties from simultaneously obtaining information about a shared encrypted message. We introduce a candidate for unclonable encryption in the plain model, i.e. without assumptions, in working towards an unconditional proof. Our protocol is based on Clifford
algebra, utilizing complex Hermitian
unitary matrices that anti-commute. For small key sizes, we rigorously prove security using sum-of-squares methods, while for larger key sizes, we provide strong numerical evidence via the NPA hierarchy.
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Quantum Physics
Quantum Channel Masking
Anna Honeycutt, Hailey Murray and Eric Chitambar
Quantum masking is a special type of secret sharing in which some information gets reversibly distributed into a multipartite system, leaving the original information inaccessible to each subsystem. This paper proposes a dynamical extension of
quantum masking to the level of
quantum channels. In channel masking, the identity of a channel becomes locally hidden but still globally accessible after its output is sent through a bipartite broadcasting channel. We first characterize all families of d-dimensional
unitaries that can be isometrically masked, a condition that holds even in the presence of depolarizing noise. For the case of
qubits, we identify which families of Pauli channels can be masked, and we prove that a
qubit channel can be masked against the identity if and only if it is unital and has a pure-state fixed point. Masking against the identity describes a scenario in which channel noise becomes completely delocalized through a broadcast map and undetectable through subsystem dynamics alone.
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Quantum Physics
A Decoy-like Protocol for Quantum Key Distribution: Enhancing the Performance with Imperfect Single Photon Sources
Chanaprom Cholsuk, Furkan Ağlarc{\i}, Daniel K. L. Oi et al. (+2 more)
Quantum key distribution (QKD) relies on single
photon sources (SPSs), e.g. from solid-state systems, as flying
qubits, where security strongly requires sub-Poissonian
photon statistics with low second-order correlation values (\$g^{(2)}(0)\$). However, achieving such low \$g^{(2)}(0)\$ remains experimentally challenging. We therefore propose a decoy-like QKD protocol that relaxes this constraint while maintaining security. This enables the use of many SPSs with \$g^{(2)}(0) > \$0.1, routinely achieved in experiments but rarely considered viable for QKD. Monte Carlo simulations and our experiment from defects in hexagonal boron nitride show that, under linear loss, \$g^{(2)}(0)\$ remains constant, whereas
photon-number-splitting (PNS) attacks introduce nonlinear effects that modify the measured \$g^{(2)}(0)\$ statistics. Exploiting this \$g^{(2)}(0)\$ variation as a diagnostic tool, our protocol detects PNS attacks analogously to decoy-state methods. Both single- and two-
photon pulses consequently securely contribute to the secret key rate. Our protocol outperforms the Gottesman–Lo–Lutkenhaus–Preskill (GLLP) framework under high channel loss across various solid-state SPSs and is applicable to the satellite-based communication. Since \$g^{(2)}(0)\$ can be extracted from standard QKD experiments, no additional hardware is required. The relaxed \$g^{(2)}(0)\$ requirement simplifies the
laser system for SPS generation. This establishes a practical route toward high-performance QKD without the need for ultra-pure SPSs.
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All authors Chanaprom Cholsuk, Furkan Ağlarc{\i}, Daniel K. L. Oi, Serkan Ateş, Tobias Vogl
Quantum Physics
Coherent control of ionization via stabilization by resonant pulse pairs
Edvin Olofsson, Evan Lovelle Fulton, Rezvan Tahouri et al. (+3 more)
We study the nonlinear and resonant process of two-
photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the
quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain
laser parameters, the control over the dressed state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such “bound states in the continuum” are sensitive to the degeneracy of the coupled continuum.
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All authors Edvin Olofsson, Evan Lovelle Fulton, Rezvan Tahouri, Mattias Bertolino, Jean Marcel Ngoko Djiokap, Jan Marcus Dahlström
Quantum Physics · Cross-listed from physics.atom-ph
Magnetotactic bacterial populations studied with a Pound-Drever-Hall atomic magnetometer
María Hernández Ruiz, Christopher Kiehl, Vito Giovanni Lucivero et al. (+1 more)
We demonstrate an optically pumped magnetometer that monitors
spin polarization using Pound Drever
Hall (PDH) technique. The instrument exhibits a noise floor of 22.2 pT/sqrt(Hz) limited by optical
photon shot noise, short-term instability of 30.8 pT/sqrt(Hz)/sqrt({\tau}) for averaging times {\tau} < 0.2 s , instability below 70 pT for 0.2 s < {\tau} < 20 s and a minimum instability of 47 pT at {\tau} = 6 s. We apply the OPM to investigate the ability of magnetotactic bacteria (Magnetospirillum gryphiswaldense, MSR-1) to orient in externally applied
magnetic fields. Observing an opaque, concentrated suspension, we detect deviations from exponential relaxation dynamics on second time-scales, which give information about the dispersion of bacterial
magnetic moment and rotational damping coefficient. These parameters are observed to evolve as the population further concentrates due to evaporation and settling. To our knowledge, this is the first time such
magnetic inhomogeneities and long-term relaxation deviations have been directly observed. This study showcases both the sensitivity and stability of our OPM and its potential for probing biophysical processes.
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All authors María Hernández Ruiz, Christopher Kiehl, Vito Giovanni Lucivero, Morgan W. Mitchell
Quantum Physics
Two-Photon Induced Coherence without Induced Emission
Dong-Gil Im, Seung-Yeun Yoo, Chung-Hyun Lee et al. (+2 more)
At the heart of recent breakthroughs in
quantum imaging and spectroscopy utilizing undetected
photons lies the
quantum optical effect known as induced coherence without induced emission. This fundamental
quantum interference effect has unlocked new possibilities in accessing challenging wavelength regimes for advanced imaging and spectroscopic analysis. Despite these advancements, the full spectrum of
quantum metrology's capabilities, particularly the enhanced phase sensitivity offered by
quantum optical Fock states or N00N states, has yet to be realized. This is due to the fact that, until now, the exploration of induced coherence has been confined to phenomena involving single
photons. In this study, we present the observation of two-
photon induced coherence without induced emission. This advancement hinges on a two-
photon Fock state that creates
quantum coherence between pairs of two-
photon spontaneous emission amplitudes. The result is a doubling of the interferometric phase modulation compared to what is observed with single
photons. Specifically, we show that a phase change $\phi$ applied to undetected 1016 nm near-infrared
photons leads to $2\phi$ modulation in the detection of the 632 nm visible
photons, verifying two-
photon induced coherence without induced emission. These findings pave the way for innovative high-resolution
quantum metrological applications leveraging multi-
photon induced coherence without induced emission.
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All authors Dong-Gil Im, Seung-Yeun Yoo, Chung-Hyun Lee, Jongheon Suh, Yoon-Ho Kim
Quantum Physics
Structure of quantum measurements implementable with one round of classical communication
Arthur C. R. Dutra, Ties-A. Ohst, Hai-Chau Nguyen et al. (+1 more)
Measurements that can be implemented via local operations and classical communication (LOCC) constitute a class of operations that is available in future
quantum networks in which parties share entangled resource states. We characterise the different classes of measurements implementable with LOCC, where communication is restricted to a single round with a fixed direction. In particular, using the framework of constrained separability problems, we provide a complete characterisation of the class of LOCC measurements that require one round of classical communication with a limit on the transmitted information. Furthermore, we show how to distinguish between adaptive and non-adaptive measurements strategies. Using our techniques we present examples where the success probability of state discrimination depends on the direction of communication as well as on the message size. We also discuss explicit instances of state ensembles where non-projective measurements provide an advantage and where adaptive measurement strategies lead to improved success rates when compared to all non-adaptive strategies.
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All authors Arthur C. R. Dutra, Ties-A. Ohst, Hai-Chau Nguyen, Otfried Gühne
Quantum Physics
A Framework for Distributed Resource Allocation in Quantum Networks
Nitish K. Panigrahy, Leonardo Bacciottini, C. V. Hollot et al. (+5 more)
We introduce a distributed resource allocation framework for the
Quantum Internet that relies on feedback-based, fully decentralized coordination to serve multiple co-existing applications. We develop
quantum network control
algorithms under the mathematical framework of
Quantum Network Utility Maximization (QNUM), where utility functions quantify network performance by mapping
entanglement rate and quality into a joint optimization objective. We then introduce QPrimal-Dual, a decentralized, scalable
algorithm that solves QNUM by strategically placing network controllers that operate using local state information and limited classical message exchange. We prove global asymptotic stability for concave, separable utility functions, and provide sufficient conditions for local stability for broader non-concave cases. To reduce control overhead and account for
quantum memory decoherence, we also propose schemes that locally approximate global quantities and prevent congestion in the network. We evaluate the performance of our approach via simulations in realistic
quantum network architectures. Results show that QPrimalDual significantly outperforms baseline allocation strategies, scales with network size, and is robust to latency and decoherence. Our observations suggest that QPrimalDual could be a practical, high-performance foundation for fully distributed resource allocation in
quantum networks.
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All authors Nitish K. Panigrahy, Leonardo Bacciottini, C. V. Hollot, Emily A. Van Milligen, Matheus Guedes de Andrade, Nageswara S. V. Rao, Gayane Vardoyan, Don Towsley
Quantum Physics · Cross-listed from math.AP
Metaplectic time-frequency representations
Gianluca Giacchi
Time-frequency representations stemmed in 1932 with the introduction of the Wigner distribution. For most of the 20th century, research in this area primarily focused on defining joint probability distributions for position and momentum in
quantum mechanics. Applications to electrical engineering were soon established with the seminal works of Gabor and the researchers at Bell Labs. In 2012, Bai, Li and Cheng used for the first time metaplectic operators, defined in the middle of 20th century by Van Hove, to generalize the Wigner distribution and unify effectively the most used time-frequency representations under a common framework. This work serves as a comprehensive up-to-date survey on time-frequency representations defined by means of metaplectic operators, with particular emphasis on the recent contributions by Cordero and Rodino, who exploited metaplectic operators to their limits to generalize the Wigner distributions. Their idea provides a fruitful framework where properties of time-frequency representations can be explained naturally by the structure of the
symplectic group.
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Quantum Physics
Fiber-optic power limiter device based on carbon nanotubes
Ekaterina Borisova, Anastasiya Ponosova, Natalia Arutyunyan et al. (+4 more)
We experimentally demonstrate a power limiter based on single-walled carbon nanotubes dispersed in a
polymer matrix. This simple fiber-optic device permanently increases its attenuation when subjected to 50-mW or higher cw illumination at 1550 nm and initiates a fiber-fuse effect at 1 to 5 W. It may be used for protecting
quantum key distribution equipment from light-injection attacks. We demonstrate its compatibility with phase- and polarisation-encoding
quantum key distribution systems.
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All authors Ekaterina Borisova, Anastasiya Ponosova, Natalia Arutyunyan, Alexey Shilko, Elena Obraztsova, Boris Galagan, Vadim Makarov
Quantum Physics
Entangled Photon Pair Generator via Biexciton-Exciton Cascade in Semiconductor Quantum Dots and its Simulation
Simon Sekavčnik, Paul Kohl and Janis Nötzel
The generation of entangled
photon pairs is highly useful for many types of
quantum technologies. In this work an entangled
photon pair generator that utilises the biexciton-
exciton cascade in
semiconductor quantum dots is described on a physical, mathematical, and software level. The system is implemented and simulated as a self-contained component in a framework for bigger
quantum optical experiments. Thus, it is a description to further the holistic understanding of the system for interdisciplinary audiences in a hopefully simple yet sufficient manner. It is described from the condensed matter physics fundamentals, over the most important
quantum optical properties, to a mathematical description of the used model, and finally a software description and simulation, making it an executable description of such a system. We provide a compact description in the Kraus operator formalism to seamlessly incorporate such an entangled
photon pair generator simulation component into bigger simulations consisting of multiple components at a reasonable computational cost. The simulation accommodates a wide range of parameter regimes and makes it possible to simulate many different excitation strategies. This includes resonant two-
photon excitation, adiabatic rapid passage chirped excitation, and dichromatic pulsed excitation.
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Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Strained Donor-Bound Excitons in $^{28}$Si
David A. Vogl, Noah L. Braitsch, Başak Ç. Özcan et al. (+3 more)
We present a comprehensive experimental study of the neutral donor to donor-bound
exciton transition (D$^0$$\rightarrow\,$D$^0$X) in isotopically enriched $^{28}$Si, focusing on the group-V donors P, As, and Sb under finely tuned uniaxial stress along the [100] and [110]
crystal axes and
magnetic fields from 3.5 mT to 1.7 T. From these measurements, donor-specific deformation potentials are extracted. The uniaxial electron deformation potential $\Xi_\mathrm{u}$ is found to be significantly larger than values reported for other states or transitions in silicon and shows a clear dependence on the donor species, indicating an increased sensitivity of the D$^0$X state to strain and central-
cell effects. We also observe a
magnetic field dependence of the hole shear deformation potential $d$, suggesting a more complex strain coupling mechanism than captured by standard theory. Diamagnetic shift parameters determined from Zeeman spectra show good agreement with earlier measurements. Our results provide a refined parameter set critical for the design of silicon
quantum devices based on D$^0$X transitions.
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All authors David A. Vogl, Noah L. Braitsch, Başak Ç. Özcan, Niklas S. Vart, M. L. W. Thewalt, Martin S. Brandt
Quantum Physics · Cross-listed from physics.optics
A quantum-coherent photon–emitter interface in the original telecom band
Marcus Albrechtsen, Severin Krüger, Juan Loredo et al. (+10 more)
Quantum dots stand out as the most advanced and versatile light-matter interface available today. Their ability to deliver high-quality, high-rate, and pure
photons has set
benchmarks that far surpass other emitters. Yet, a critical frontier has remained elusive: achieving these exceptional capabilities at telecom wavelengths, bridging the gap to fiber-optic infrastructure and scalable silicon photonics. Overcoming this challenge demands high quality
quantum materials and devices which, despite extensive efforts, have not been realized yet. Here, we demonstrate waveguide-integrated
quantum dots and realize a fully
quantum-coherent
photon-emitter interface operating in the original telecommunication band. The quality is assessed by recording transform-limited linewidths only 8 % broader than the inverse lifetime and bright 41.7 MHz emission rate under 80 MHz $\pi$-pulse excitation, unlocking the full potential of
quantum dots for scalable
quantum networks.
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All authors Marcus Albrechtsen, Severin Krüger, Juan Loredo, Lucio Stefan, Zhe Liu, Yu Meng, Lukas L. Niekamp, Bianca F. Seyschab, Nikolai Spitzer, Richard J. Warburton, Peter Lodahl, Arne Ludwig, Leonardo Midolo
Quantum Physics · Cross-listed from cond-mat.mes-hall
Quantum-Limited Acoustoelectric Amplification in a Piezoelectric-2DEG Heterostructure
Eric Chatterjee, Daniel Soh and Matt Eichenfield
We provide a
quantum mechanical description of
phonon amplification in a
heterostructure consisting of a two-dimensional electron gas (2DEG) stacked on top of a piezoelectric material. An applied drift voltage effectively creates a population inversion in the momentum states of the 2DEG electrons, giving rise to spontaneous emission of
phonons. Once an acoustic wave is launched, the pumped electrons release
phonons via stimulated emission, returning to depleted ground states before being pumped back to the excited states. We show that whereas efficient amplification using a 1D electron gas requires the acoustic wavelength to roughly equal the average electron-electron spacing, a 2DEG enables efficient amplification for any wavelength greater than the average electron-electron spacing. We derive the imaginary and real parts of the 2DEG first-order acoustic susceptibility as functions of electronic drift velocity in specific limits and derive the gain per unit length for the signal and the
quantum noise, with the gain matching the classical result in the short-electronic-lifetime (low-mobility) regime. Moreover, we analyze the gain clamping due to pump depletion and calculate the maximum achievable intensity. Our results provide a framework for designing novel acoustic devices including a
quantum phononic
laser and phase-insensitive
quantum phononic amplifiers.
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Quantum Physics
QBism and relational interpretation of quantum mechanics from the point of view of a contextual quantum realism (CQR)
Francois-Igor Pris
A realist interpretation of
quantum mechanics is proposed - Contextual
Quantum Realism (CQR) - according to which there exists a categorical distinction between the ideal (theory, observation instrument) and the real (
quantum physical systems, properties), and, consequently,
quantum ontology is context-sensitive. CQR is compared with QBism and Relational
Quantum Mechanics (RQM), both of which also claim to offer realist interpretations. However, both approaches conflate the ideal and the real, thereby introducing an anti-realist dimension. RQM is an objectivist (physicalist) interpretation that naturalizes epistemic concepts, whereas QBism is subjectivist (phenomenological). RQM and QBism share a common presupposition of (post-)Modern philosophy - namely, the reduction of reality to objectivity - which CQR explicitly rejects.
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Quantum Physics
Partial Self-Correction in Layer Codes
Dominic J. Williamson
The storage of large-scale
quantum information at finite temperature requires an autonomous and reliable
quantum hard drive, also known as a self-correcting
quantum memory. It is a long-standing open problem to find a self-correcting
quantum memory in three dimensions. The recently introduced Layer Codes achieve the best possible scaling of code parameters and logical energy barrier in three dimensions, these are tantalizing features for the purposes of self-correction. In this work we show that a family of Layer Codes, based on good
Quantum Tanner Codes, exhibit partial self-correction. Their
memory time grows exponentially with linear system size, up to a length scale that is exponential in the inverse temperature. At this length scale, the
memory time scales as a double exponential of inverse temperature. To establish this result we introduce a concatenated matching decoder that combines three rounds of parallelized minimum-weight perfect-matching with a decoder for good
Quantum Tanner Codes. We show that our decoder corrects errors up to a constant fraction of the energy barrier, and a constant fraction of the code distance, for a family of Layer Codes. Our results position Layer Codes as the leading candidate for a partially self-correcting
memory in three dimensions. While they fall short of achieving strict self-correction in the thermodynamic limit, our work highlights the potential of these local codes in three dimensions, with fast distance and logical
qubit growth, fast decoders, and a long
memory time over a wide range of parameters.
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Quantum Physics
Scaling Enhancement in Quantum Metrology via Indefinite-Time-Direction Encoding
Binke Xia, Jingzheng Huang, Yuxiang Yang et al. (+1 more)
The precision limit in
quantum metrology, quantified by the root-mean-square error of parameter estimation, is conventionally expected to improve at most linearly with the total interrogation time T and with the number N of queried
quantum gates. Although several metrological schemes have been shown to achieve precision scaling faster than linear in T and N, they typically rely on unbounded probe-side information resources, usually qualified by an increasingly large variance of the parameter generator. This requirement complicates the interpretation of the resulting scaling advantage and poses substantial technical challenges. In this work, we employ an indefinite-time-direction encoding process to achieve a nonlinear-scaling enhancement of the precision limit. Rather than relying on increasingly informative probe states, our method converts controllable noncommuting encoding operations into metrological gain. Experimentally, we implement this protocol for angular-rotation measurement in a
quantum optical system and demonstrate a nonlinear-scaling improvement in practical precision without using probe-side information resources. These results establish a practical framework for surpassing conventional linear-scaling precision limits in
quantum metrology and provide new insights into precision enhancement in realistic
quantum metrological and sensing applications.
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All authors Binke Xia, Jingzheng Huang, Yuxiang Yang, Guihua Zeng
Quantum Physics
Breakdown of Non-Bloch Bulk-Boundary Correspondence and Emergent Topology in Floquet Non-Hermitian Systems
Hong Wu, Xue-Min Yang and Hui Liu
Topological edge states in gaps of non-Hermitian systems are robust due to topological protection. Using the non-Hermitian Floquet Su-Schrieffer-Heeger model, we show that this robustness can break down: edge states may be suppressed by infinitesimal perturbations that preserve sublattice
symmetry. We identify this fragility to the instability of the quasienergy spectrum in finite-size systems, leading to a breakdown of the non-Bloch bulk-boundary correspondence defined on the generalized Brillouin zone. To resolve this, we establish a correspondence between the number of stable zero-mode singular states and the topologically protected edge states in the thermodynamic limit. Our results formulate a bulk-boundary correspondence for Floquet non-Hermitian systems, where
topology arises intrinsically from the driven non-Hermitian systems, even without
symmetries. Our results provide a promising new avenue for exploring novel non-Hermitian topological phases.
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Quantum Physics
The foundational value of quantum computing for classical fluids
Sauro Succi, Claudio Sanavio and Peter Love
Quantum algorithms for classical physics problems expose new patterns of
quantum information flow as compared to the many-body Schrödinger equation. As a result, besides their potential practical applications, they also offer a valuable theoretical and computational framework to elucidate the foundations of
quantum mechanics, particularly the validity of the many-body Schrödinger equation in the limit of large number of particles, on the order of the Avogadro number. This idea is illustrated by means of a concrete example, the Block-Encoded Carleman
embedding of the
Lattice Boltzmann formulation of fluid dynamics (CLB).
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Quantum Physics
High-Fidelity Single-Shot Readout and Selective Nuclear Spin Control for a Spin-1/2 Quantum Register in Diamond
Prithvi Gundlapalli, Philipp J. Vetter, Genko Genov et al. (+5 more)
Quantum networks offer a way to overcome the size and complexity limitations of single
quantum devices by linking multiple nodes into a scalable architecture. Group-IV color centers in diamond, paired with long-lived
nuclear spins, have emerged as promising building blocks demonstrating proof-of-concept experiments such as blind
quantum computing and
quantum-enhanced sensing. However, realizing a large-scale electro-
nuclear register remains a major challenge. Here we establish the germanium-vacancy (GeV) center as a viable platform for such network nodes. Using correlation spectroscopy, we identify single
nuclear spins within a convoluted
spin environment, overcoming limitations imposed by the color center's
spin-$1/2$ nature and thereby enabling indirect control of these
nuclear spins. We further demonstrate high-fidelity single-shot readout of both the GeV center ($95.8\,\%$) and a neighboring ${}^{13}\text{C}$
nuclear spin ($93.7\,\%$), a key tool for feed-forward error correction. These critical advances position the GeV center as a compelling candidate for next-generation
quantum network nodes.
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All authors Prithvi Gundlapalli, Philipp J. Vetter, Genko Genov, Michael Olney-Fraser, Peng Wang, Matthias M. Müller, Katharina Senkalla, Fedor Jelezko
Quantum Physics · Cross-listed from cond-mat.mes-hall
Chern insulators and topological flat bands in cavity-embedded kagome systems
Hikaru Goto, Ryo Okugawa and Takami Tohyama
We investigate topological band structures of a kagome system coupled to a circularly polarized cavity mode, using a model based on a muffin-tin potential and
quantum light-matter interaction. We show that Chern insulating phases emerge in the cavity-embedded kagome system due to the light-matter interaction that breaks time-reversal
symmetry. We also find that a nearly flat band can be topologically nontrivial with a nonzero Chern number. By varying the light-matter interaction, we also reveal that topological phase transitions occur between different Chern insulating phases in the ultrastrong coupling regime. The phase transitions change the sign of the Chern number, switching the direction of the edge current. We demonstrate the existence of topological edge modes in the cavity-embedded kagome Chern
insulators by constructing a low-energy effective tight-binding model.
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Quantum Physics
A one-world interpretation of quantum mechanics
Isaac Layton, Jonathan Oppenheim and Zachary Weller-Davies
The measurement problem is the issue of explaining how the objective classical world emerges from a
quantum one. Here we take a different approach. We assume that there is an objective classical system, and then ask that the standard rules of probability theory apply to it when it interacts with a
quantum system. Under mild assumptions, we recover the
unitary dynamics, collapse and Born rule postulates from
quantum theory. Nonetheless, there is no decoherence, because the
quantum state remains pure conditioned on the classical trajectory. This results in one world, rather than many-worlds. Our main technical tool is to exploit a change of measure on the space of classical paths, the functional form of which is shown to characterise the
quantum dynamics and Born rules of a class of
quantum-like theories.
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Quantum Physics
Accessibility of Global Properties from Internal Quantum Reference Frame Perspectives
Anne-Catherine de la Hamette, Viktoria Kabel and Časlav Brukner
A fundamental question in the field of
quantum reference frames concerns what global properties of a system can be determined by observers operating entirely from within that system. We investigate this question by extending both the perspectival and perspective-neutral approaches beyond the commonly studied zero total momentum case to arbitrary fixed charge sectors. When the entire system, including the reference frames, moves at a fixed total momentum $P$ relative to an external frame, this global charge becomes encoded in the
quantum states and transformations between reference frames. Our extension leads to modified relative states and observables with QRF transformations that induce an additional $P$-dependent phase, treating all charge sectors as equally fundamental. By granting the internal observers successively more access and resources, we identify under which conditions they can infer the total momentum. These results clarify the relationship between major QRF approaches – perspectival, perspective-neutral, operational, and extra-particle – showing how their differing conclusions stem from different assumptions about which observables are deemed accessible from within. Our findings cast light on the relation between local and global perspectives and raise fundamental questions about scenarios where no global perspective exists, contributing to a deeper understanding of relationality and the role of perspectives in
quantum theory.
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Quantum Physics
Exploiting higher-order correlation functions for photon-statistics-based characterization and reconstruction of arbitrary Gaussian states
Philip Heinzel and René Sondenheimer
Gaussian states are an essential building block for various applications in
quantum optics and
quantum information science, yet the precise relation between their second- and third-order correlation functions remains not fully explored. We discuss connections between these correlation functions by constructing an explicit decomposition formula for arbitrary sixth-order moments of ladder operators for general Gaussian states and demonstrate how the derived relations enable state classification from correlation data alone. Whereas violating these relations certifies non-Gaussianity, satisfying them provides evidence for a Gaussian-state description and allows a direct distinction among non-displaced, non-squeezed, and displaced-squeezed sectors of the Gaussian state space. Further, we show that it is not possible to uniquely extract state parameters solely from correlation-function measurements without prior assumptions about the Gaussian state. Resolving this ambiguity requires additional loss-sensitive information, e.g., measuring the mean intensity or the
vacuum overlap of each mode. In particular, we show under which circumstances these measurements can be used to reconstruct a generic Gaussian state.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Imaging of Gate-Controlled Suppression of Superconductivity via the Meissner Effect
P. J. Scheidegger, K. J. Knapp, U. Ognjanovic et al. (+5 more)
It was recently discovered that supercurrents flowing through thin superconducting nanowires can be quenched by a gate voltage. This gate control of supercurrents, known as the GCS effect, could enable superconducting transistor logic. Here, we report that the GCS also manifests in a suppression of Meissner screening, establishing the phenomenon as a genuine feature of superconductivity that is not restricted to transport. Using a scanning nitrogen-vacancy magnetometer at sub-Kelvin temperatures, we image the nanoscale spatial region of GCS suppression in micron-size niobium islands. Our observations are compatible with a microscopic hot-spot model of quasiparticle generation and
diffusion, and in conflict with other candidate mechanisms such as Joule heating or an electric field effect. Our work introduces an alternative means for studying quasiparticle dynamics in superconducting nanostructures, and showcases the power of local imaging techniques for understanding emergent condensed matter phenomena.
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All authors P. J. Scheidegger, K. J. Knapp, U. Ognjanovic, L. Ruf, S. Diesch, E. Scheer, A. Di Bernardo, C. L. Degen
Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Atomistic origin of low thermal conductivity in quaternary chalcogenides Cu(Cd, Zn)$_2$InTe$_4$
Nirmalya Jana, Amit Agarwal and Koushik Pal
Crystalline
semiconductors with intrinsically low
lattice thermal conductivity ($\mathcal{K}$) are vital for device applications such as barrier coatings and thermoelectrics. Quaternary chalcogenide
semiconductors such as CuCd$_2$InTe$_4$ and CuZn$_2$InTe$_4$ are experimentally shown to exhibit low $\mathcal{K}$, yet its microscopic origin remains poorly understood. Here, we analyse their thermal transport mechanisms using a unified first-principles framework that captures both the Peierls (particle-like propagation, $\mathcal{K}_P$) and coherence (wave-like tunneling, $\mathcal{K}_C$) mechanisms of
phonon transport. We show that extended antibonding states below the
Fermi level lead to enhanced
phonon anharmonicity and strong scattering of heat-carrying
phonon modes, suppressing $\mathcal{K}$ in these chalcogenides. We show that $\mathcal{K}_P$ dominates the total thermal conductivity, while $\mathcal{K}_C$ remains negligible even under strong anharmonicity of the
phonon modes. The heavier Cd ions in CuCd$_2$InTe$_4$ induce greater acoustic-optical
phonon overlap and scattering compared to CuZn$_2$InTe$_4$, further lowering thermal conductivity of the former. Additionally, grain boundary scattering in realistic samples contributes to further suppression of thermal transport. Our findings establish the atomistic origins of low $\mathcal{K}$ in quaternary chalcogenides and offer guiding principles for designing low-thermal-conductivity
semiconductors.
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Quantum Physics
A Davydov Ansatz approach to accurate system-bath dynamics in the presence of multiple baths with distinct temperatures
Chenlin Ma, Fulu Zheng, Kewei Sun et al. (+2 more)
We perform
benchmark simulations using the time-dependent variational approach with the multiple Davydov Ansatz (mDA) to study realtime nonequilibrium dynamics in a single
qubit model coupled to two thermal baths with distinct temperatures. A broad region of the parameter space has been investigated, accompanied by a detailed analysis of the convergence behavior of the mDA method. In addition, we have compared our mD2 results to those from two widely adopted, numerically "exact" techniques: the methods of hierarchical equations of motion (HEOM) and the quasi-adiabatic path integral (QUAPI). It is found that the mDA approach in combination with thermal field dynamics yields numerically accurate, convergent results in nearly all regions of the parameter space examined, including those that pose serious challenges for QUAPI and HEOM. Our results reveal that mDA offers a highly adaptable framework capable of capturing long-time dynamics, even in challenging regimes where other methods face limitations. These findings underscore the potential of mDA as a versatile tool for exploring
quantum thermodynamics, energy transfer processes, and non-
equilibrium quantum systems.
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All authors Chenlin Ma, Fulu Zheng, Kewei Sun, Lu Wang, Yang Zhao
Quantum Physics
Beyond the Carnot limit: work extraction via an entropy battery
Liam Judd McClelland
Heat is a physical manifestation of
entropy, where the removal of
entropy from a thermal energy reservoir permits the conversion of heat into work. This
entropy transfer is facilitated by the cold thermal energy reservoir in typical heat engines. Recent developments in
quantum heat engines that operate between thermal energy and
spin angular momentum reservoirs show that it is possible to transfer
entropy out of energy and into a different conserved quantity. The implications of this type of
entropy transfer have not been fully explored, especially on the work extractable using an ensemble with multiple conserved quantities. Using the aforementioned heat engines, we show that such an ensemble will transform heat into work beyond the Carnot efficiency limit while operating at maximum power. This result is obtained without induced
quantum coherence, a technique commonly used in the field of
quantum heat engines to achieve the same outcome. Without loss of generality, we also show that thermal
spin reservoirs behave as thermodynamic baths with well-defined temperatures, heat capacities, and fluctuation-dissipation relations. Finally, our analysis of
entropy capacity suggests that particle indistinguishability is necessary for inter-particle interactions, and for
entropy to transfer between canonical ensembles. These results establish a foundation for
entropy-based
quantum devices that extract work from a hot thermal energy reservoir more efficiently than possible with a cold thermal energy reservoir. These devices also act as high energy-density batteries and efficient heat storage systems. Our results will have implications for
quantum heat engines, spinor condenstates, spintronics, and
quantum batteries.
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