Quantum Physics · Cross-listed from cond-mat.quant-gas
Equal-spin and opposite-spin density-density correlations in the BCS-BEC crossover: Gauge Symmetry, Pauli Exclusion Principle, Wick's Theorem and Experiments
Nikolai Kaschewski, Axel Pelster and Carlos A. R. Sá de Melo
We develop a general theory of
spin-dependent density-density correlations, that is valid for any temperature, interactions, dimensions and mass or population status of
Fermi gases with two internal states. We use gauge invariance and the Pauli principle to establish constraints on the
spin-dependent density-density correlations that are consistent with the fluctuation-dissipation and Wick's
theorem. As an example, we study the
spin-dependent density-density correlations from the BCS to the Bose regime in two dimensions at zero temperature, inspired by experiments in 6Li. We show that two-particle irreducible contributions involving collective excitations, many-particle scattering and vertex corrections, are essential to describe experiments. In particular they turn out to be responsible for the emergence of an experimentally observed minimum in the opposite-
spin density-density correlations.
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Quantum Physics
A quantum feasibility preserving modeling for the min cut problem
Ali Abbassi, Yann Dujardin, Eric Gourdin et al. (+2 more)
We study the minimum cut problem in weighted undirected graphs using variational
quantum algorithms in which only feasible cut configurations are explored. Although minimum cut admits efficient classical solutions, it is a fundamental component of more complex network optimization problems such as multicut and network interdiction. Our objective is to examine
quantum models in which feasibility is preserved by the mixer dynamics, without introducing penalty terms in the cost
Hamiltonian. We employ a ring structured XY mixer that restricts the
quantum evolution to the subspace of valid cut configurations, ensuring that all sampled states correspond to feasible solutions. To address scalability limitations, we suggest an iterative metaheuristic strategy that decomposes large instances into smaller subproblems solved sequentially using the same
quantum model. The results obtained using the mixer indicate that the initial probability distribution can be systematically controlled, thereby enabling the development of warm start techniques within variational
quantum based
algorithms.
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All authors Ali Abbassi, Yann Dujardin, Eric Gourdin, Philippe Lacomme, Caroline Prodhon
Quantum Physics
Coherence squeezing in optical interference
Martti Hanhisalo, Atri Halder, Tero Setälä et al. (+1 more)
We introduce the concept of optical coherence squeezing in double-slit interference. We construct Hermitian operators that characterize the coherence at the slits, leading to coherence uncertainty relations and a corresponding squeezing condition. We also analyze states exhibiting such squeezing and show its manifestations in the uncertainty of the magnitudes and positions of the intensity fringes. Our work identifies coherence as a fundamental degree of freedom for squeezing, complementing phase, amplitude, and polarization, which could benefit
quantum-enhanced interferometry.
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All authors Martti Hanhisalo, Atri Halder, Tero Setälä, Andreas Norrman
Quantum Physics
Aygul Azatovna Galimova
Hyperbolic Floquet codes use only weight-2 measurements and can be implemented directly on hardware with native pair measurements. We construct
hyperbolic and semi-
hyperbolic Floquet codes from $\{8,3\}$, $\{10,3\}$, and $\{12,3\}$ tessellations via the Wythoff kaleidoscopic construction with the Low-Index Normal Subgroups (LINS)
algorithm. The $\{10,3\}$ and $\{12,3\}$ families are new to
hyperbolic Floquet codes. We evaluate these codes under four noise models. Under ancilla-based Entangling Measurement (EM3) noise, all three families achieve a threshold of ${\sim}1.5\%$. With a native pair-measurement depolarizing model (SDEM3), thresholds are ${\sim}1.0$–$1.2\%$. For heralded
photon loss, the $\{8,3\}$ family achieves ${\sim}8.5$–$9\%$, exceeding the planar honeycomb threshold of ${\sim}6.3\%$. In the multi-parameter SPOQC-2 noise model, the $\{8,3\}$ codes achieve a 2D fault-tolerant area $2.2\times$ that of the surface code compiled to pair measurements. We present the first
photon loss and SPOQC-2 thresholds for
hyperbolic Floquet codes.
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Quantum Physics
Information and coherence as resources for work extraction from unknown quantum state and providing quantum advantages
Tanmoy Biswas
The amount of extractable work from a physical system is fundamentally connected to the information available about its state, as illustrated by Maxwell's demon and the Gibbs paradox. In standard thermodynamic protocols involving system–bath interactions, the maximum work is given by the free-energy difference between the initial state and the corresponding Gibbs state at the bath temperature. This motivates a natural question: does information also limit work extraction in closed
quantum systems that do not involve a heat bath and where work is obtained through
unitary operations generated by a time-dependent
Hamiltonian? While ergotropy quantifies the maximum work extractable via
unitary operations, it assumes complete knowledge of the
quantum state, typically requiring full state
tomography. In realistic scenarios, however, only partial information is accessible. In this case, the relevant figure of merit is observational ergotropy, which depends explicitly on the measurement used to probe the system. We show that observational ergotropy decreases under classical post-processing of measurement outcomes, implying that fine-grained measurements allow greater work extraction than coarse-grained ones. Moreover, maximizing observational ergotropy over all possible measurements recovers standard ergotropy, which decomposes into incoherent (classical) and coherent (
quantum) contributions. Our results demonstrate that coherence in the measurement projectors constitutes the key resource, enabling work extraction beyond the incoherent limit and establishing coherence as the origin of
quantum advantage in observational ergotropy extraction.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Metastable confinement in Rydberg lattice gauge theories
Yaohua Li, Devendra Singh Bhakuni, Yong-Chun Liu et al. (+1 more)
Confinement and string breaking are two fundamental phenomena in gauge theories. Signatures of both are currently pursued in
quantum-simulator experiments, opening a new angle on strongly interacting dynamics of gauge fields out of
equilibrium, complementary to traditional particle-physics settings. In this work, we report the emergence of metastable confinement dynamics in a U(1)
lattice gauge theory, originating from the competition between string tension and four-
Fermi coupling - a competition that naturally arises in Rydberg atom arrays. We show that the initial string state can be resonantly melted through controlled energy matching, a phenomenon we identify as resonant string breaking. We demonstrate this mechanism for both static and Floquet-driven systems, where periodic modulation generates a spectrum of tunable sideband resonances. Our work provides new insights into the mechanisms of confinement and string breaking driven by long-range interactions and time-dependent fields, which are available in current
quantum simulators on a variety of platforms.
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All authors Yaohua Li, Devendra Singh Bhakuni, Yong-Chun Liu, Marcello Dalmonte
Quantum Physics
Spin-Cat Qubit with Biased Noise in an Optical Tweezer Array
Toshi Kusano, Kosuke Shibata, Chih-Han Yeh et al. (+8 more)
Bias-tailored
quantum error correcting codes (QECCs) offer a higher error threshold than standard QECCs and have the potential to achieve lower logical errors with less space overhead. The
spin-cat
qubit, encoded in a large
nuclear spin-$F$ system, is a promising candidate for bias-tailored QECCs. Yet its feasibility is hindered by the difficulty of performing fast covariant SU(2) rotation with arbitrary rotation angles for
nuclear spins and by a lack of noise characterization for gate operations in neutral atom platforms. Here we demonstrate single-
qubit controls of ${}^{173}\mathrm{Yb}$
spin-cat
qubits with
nuclear spin $I=5/2$ in an optical tweezer array. We implement a covariant SU(2) rotation and non-linear rotations by optical beams and achieve an averaged single-Clifford gate fidelity of $0.961_{-5}^{+5}$. The measurement of the coherence time and
spin relaxation time shows that the idling error becomes increasingly biased toward dephasing errors as the magnitude of the encoded sublevel $|m_F|$ increases. Furthermore, we
benchmark the noise bias of rank-preserving gates on
spin-cat
qubits, demonstrating a finite bias of $18_{-11}^{+132}$, in contrast to the case of the two-level system in ${}^{171}\mathrm{Yb}$, which shows no bias within the experimental uncertainty. Our work demonstrates the feasibility of
spin-cat
qubits for realizing bias-tailored QECCs, paving the way for achieving hardware-efficient
quantum error correction.
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All authors Toshi Kusano, Kosuke Shibata, Chih-Han Yeh, Keito Saito, Yuma Nakamura, Rei Yokoyama, Takumi Kashimoto, Tetsushi Takano, Yosuke Takasu, Ryuji Takagi, Yoshiro Takahashi
Quantum Physics
Experimental demonstration of the absence of noise-induced barren plateaus using information content landscape analysis
Sebastian Schmitt, Linus Ekstrøm, Alberto Bottarelli et al. (+1 more)
Variational
quantum algorithms are promising candidates for near-term
quantum computing but can be hindered by barren plateaus, where gradients vanish exponentially and optimization becomes intractable. Noise-Induced Barren Plateaus (NIBP) are particularly concerning because they are predicted to arise generically from noise accumulation, independent of system size,
circuit structure, and observable locality. We experimentally investigate NIBP on IBM
quantum hardware. Using Information Content Landscape Analysis (ICLA), we efficiently estimate gradient norms for variational
circuits ranging from 8 to 102
qubits, up to hundreds of parameters and
circuit runtimes of hundreds of microseconds. Contrary to NIBP expectations, we observe that gradient magnitudes saturate beyond a characteristic runtime rather than decaying exponentially. Classical simulations of the 8-
qubit case under noiseless, depolarizing, amplitude-damping, and dephasing noise models support this behavior. Consistent with recent theory, our results show that $T_1$-dominated non-unital noise inhibits the emergence of NIBP. Our analysis suggest that average calibration metrics may be insufficient to predict variational
algorithm performance.
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All authors Sebastian Schmitt, Linus Ekstrøm, Alberto Bottarelli, Xavier Bonet-Monroig
Quantum Physics
A robust method to reach the motional quantum regime of (anti-)protons in cryogenic multi-Penning traps
Nikita Poljakov, Jan Schaper, Julia-Aileen Coenders et al. (+5 more)
Sympathetic
laser cooling is a key concept in precision spectroscopy and
quantum state control of charged particles. Significant challenges arise in the metrologically relevant case where the effective interaction between the particles is weak and the particle to be cooled exhibits significant initial motional energy. Here we specifically address the most generally applicable case where the
laser-cooled ion and the particle of interest are confined to two spatially separate potential wells with equal motional frequency for resonant enhancement of the cooling dynamics. We analyze the latter through numerical simulations and find that anharmonicities of the potential wells can prevent maintaining the resonance condition throughout the cooling process and thus inhibit a significant reduction in motional energy. We propose a cooling scheme that sweeps the trapping frequency of the potential wells. We show that this scheme enables efficient cooling from cryogenic temperatures all the way to the
quantum regime of motion. As a specific application scenario, we analyze the sympathetic cooling of (anti-)
protons into the
quantum regime of motion for
quantum-logic-spectroscopy-based tests of CPT invariance at the
quantum limit in Penning traps. Nevertheless, our results and cooling strategies are generally applicable to other
laser-inaccessible ion species.
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All authors Nikita Poljakov, Jan Schaper, Julia-Aileen Coenders, Philipp Luca Hoffmann, Juan Manuel Cornejo, Klemens Hammerer, Stefan Ulmer, Christian Ospelkaus
Quantum Physics
Control of Multipartite Entanglement through Anisotropy against Thermal Noise
Samudra Sur and Saikat Sur
Preserving multipartite
entanglement in open many-body
quantum systems is fundamentally limited by unavoidable environmental noise. We study the open-system dynamics of multipartite
entanglement in an anisotropic XXZ
spin chain interacting with a thermal
spin bath, focusing on two states with distinct types of multipartite
entanglement: the generalized GHZ and the generalized W state. Using a master-equation approach combined with the Bethe ansatz technique, we show analytically that robustness of multipartite
entanglement at low temperatures can be enhanced by suitably tuning the anisotropy of the system. Our results highlight interaction-induced spectral control as a mechanism for stabilizing multipartite
entanglement in
quantum computing platforms.
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Quantum Physics · Cross-listed from physics.optics
Experimental investigation of the effect of dispersion on squeezing generation in a synchronously pumped optical parametric oscillator
Edoardo Suerra, Samuele Altilia, Stefano Olivares et al. (+4 more)
An experimental investigation of intracavity dispersion effects in a synchronously pumped optical parametric oscillator (SPOPO) is presented. A flexible setup combining spectral and phase shaping of both pump and local oscillator fields with frequency-resolved balanced homodyne detection is employed to examine how intracavity dispersion influences squeezing. Different cavity configurations with varying finesse and dispersion conditions are studied, and the squeezing is measured as a function of pump power and local oscillator bandwidth. Contrary to expectations based on existing theoretical models, the measured squeezing levels remain essentially unchanged as dispersion varies. To account for these observations, a modeling approach is introduced in which intracavity dispersion is described as an effective spectral filtering occurring at the stage of SPOPO supermode generation. Within this framework, the filtering is incorporated directly into the interaction
Hamiltonian of the nonlinear process. This perspective establishes a consistent experimental
benchmark for the study of dispersion in SPOPOs and underscores the importance of spectral filtering in the interpretation of multimode squeezing experiments.
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All authors Edoardo Suerra, Samuele Altilia, Stefano Olivares, Alessandro Ferraro, Sebastiano Corli, Enrico Prati, Simone Cialdi
Quantum Physics · Cross-listed from physics.atom-ph
Full Single-Quantum Control of Particles in Penning Traps for Symmetry Tests at the Quantum Limit
J.M. Cornejo, J.-A. Coenders, A. Lissel et al. (+9 more)
The BASE collaboration aims to measure antimatter systems with the highest precision in order to perform a rigorous test of CPT
symmetry and search for physics beyond the Standard Model. As part of the BASE collaboration, we pursue the development of
quantum logic inspired cooling and detection techniques for g-factor measurements of (anti-)
protons. Implementing these methods requires full
quantum-level control of individual antimatter particles confined in cryogenic Penning traps. By mapping the (anti-)
proton's internal state onto a co-trapped 9Be+ "logic" ion via free Coulomb coupling in a double-well potential, we can accelerate measurement cycles and push g-factor precision measurements on (anti-)
protons toward the
quantum limit. Here, we present an overview of the proposed method and the current status of the project, with special emphasis on the new cryogenic multi-Penning-trap stack and the
proton detection system.
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All authors J.M. Cornejo, J.-A. Coenders, A. Lissel, N. Poljakov, M. Prasse, Y. Priewich, J. Schaper, M. Schubert, B. Hampel, M. Schilling, S. Ulmer, C. Ospelkaus
Quantum Physics
No Absolute Hierarchy of Quantum Complementarity
Kunika Agarwal, Sahil Gopalkrishna Naik, Ananya Chakraborty et al. (+4 more)
Bohr's principle of complementarity, prohibiting simultaneous access to certain physical properties within a single experimental arrangement, is considered to be a defining feature of
quantum mechanics. It is commonly viewed as inducing an intrinsic hierarchy among incompatible observables: some sets of
quantum properties are fundamentally more incompatible than others, as quantified by the maximal sharpness permitting their joint measurement. We show that this hierarchy ceases to be absolute in the multi-copy regime. Analyzing
qubit spin observables, we prove a No-Comparison
Theorem establishing that no global ordering of incompatible observable sets is preserved across all finite-copy configurations. In particular, two sets of observables can exhibit reversed complementarity ordering depending solely on whether the available resources are arranged as identical copies or as parallel-antiparallel pairs. Thus, the degree of
quantum incompatibility is not an intrinsic property of observables alone but depends on the global configuration of the prepared
quantum probes. Our results uncover a configuration-dependent structure of complementarity, reveal a subtle role of
entanglement in shaping the structure of measurement limitations, and call for a reassessment of
quantum information protocols under finite resources.
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All authors Kunika Agarwal, Sahil Gopalkrishna Naik, Ananya Chakraborty, Guruprasad Kar, Ram Krishna Patra, Snehasish Roy Chowdhury, Manik Banik
Quantum Physics
Generating entangled polaritonic condensates by pumping with entangled pairs of photons
N.A. Asriyan, A.A. Elistratov and A.V. Kavokin
We investigate the steady state of two single-mode uniform spatially separated polaritonic conden- sates exposed to resonant pumping with entangled pairs of
photons. We demonstrate the principal possibility of driving the system to an entangled state despite its exposure to noises arising from the excitonic reservoir and
photon leakage through the microcavity mirrors. Estimates are provided for the flux of entangled particles required to drive the system into a steady state that violates the partial-transpose criterion for
entanglement. Furthermore, we trace the
evolution of the system after a sudden disappearance of the entangled pumping. Our analysis provides estimates for the
entanglement lifetime in a system of two
exciton-polariton condensates
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Quantum Physics
Optimization-based Unfolding in High-Energy Physics
Simone Gasperini, Gianluca Bianco, Marco Lorusso et al. (+2 more)
In experimental High-Energy Physics, unfolding refers to the problem of estimating the underlying distribution of a physical observable from detector-level data, in the presence of statistical fluctuations and systematic uncertainties. Starting from its reformulation as a regularized quadratic optimization problem, we develop a framework to address unfolding using both classical and
quantum-compatible methods. In particular, we derive a Quadratic Unconstrained Binary Optimization (QUBO) representation of the unfolding objective, allowing direct implementation on
quantum annealing and hybrid
quantum-classical solvers. The proposed approach is implemented in QUnfold, an open-source Python package integrating classical mixed-integer solvers and D-Wave's hybrid
quantum solver. We
benchmark the method against widely used unfolding techniques in RooUnfold, including response
Matrix Inversion, Iterative
Bayesian Unfolding, and Singular Value Decomposition unfolding, using synthetic
datasets with controlled distortion effects. Our results demonstrate that the optimization-based approach achieves competitive reconstruction accuracy across multiple distributions while naturally accommodating regularization within the objective function. This work establishes a unified optimization perspective on unfolding and provides a practical pathway for exploring
quantum-enhanced methods in experimental HEP data analysis.
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All authors Simone Gasperini, Gianluca Bianco, Marco Lorusso, Carla Rieger, Michele Grossi
Quantum Physics
A matching decoder for bivariate bicycle codes
Kaavya Sahay, Dominic J. Williamson and Benjamin J. Brown
The discovery of new
quantum error-correcting codes that encode several logical
qubits into relatively few physical
qubits motivates the development of efficient and accurate methods of decoding these systems. Here, we adopt the minimum-weight perfect matching
algorithm, a subroutine invaluable to decoding topological codes, to decode bivariate bicycle codes. Using the equivalence of bivariate bicycle codes to copies of the toric code, we propose a method we call the `cylinder trick' to rapidly find a correction using matching on code
symmetries. We
benchmark our decoder on the gross code family, cyclic hypergraph-product codes, generalized toric codes, and recently proposed directional codes under code capacity and phenomenological noise models, demonstrating the general applicability of our protocol. For a subset of these codes, we find that our decoder can be significantly improved by augmenting matching with strategies including belief propagation and `over-matching', thus achieving performance competitive with state-of-the-art approaches.
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Quantum Physics
Effective Repulsive Action of Gravitational Quantum Superpositions Under Postselection
Sougato Bose and Lev Vaidman
A classic feature of
gravity is that it is an attractive force. If a source mass is prepared in a localized (classical- like) state, it will cause another probe mass to move towards it. Here we consider the situation in which a source mass is prepared in a
quantum superposition of distinct spatial states while a probe mass interacts with it. Conditional on the detection of the source mass in a specific state, the probe mass will be found to move away from the source mass (repulsion). This signifies the
quantum superposition of gravitational forces acting on the probe mass and thereby the fact that spacetime can exist in
quantum superpositions. The technique used is the repulsive effect arising from an anomalous negative weak value. A potential experimental implementation with
spin bearing nanocrystals is outlined.
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Quantum Physics · Cross-listed from physics.optics
Quantum squeezing in an all-resonant periodically poled lithium niobate microresonator
Xinyi Ren, Reshma Kopparapu, Tushar Sanjay Karnik et al. (+12 more)
Quantum noise limits the sensitivity of optical measurements, but squeezed states of light enable
quantum-enhanced metrology, sensing, and information processing. Most on-chip squeezed-light sources rely on Kerr ($\chi^{(3)}$) nonlinearities, remain limited by pump power and excess loss constraints. Quadratic ($\chi^{(2)}$) platforms instead provide stronger parametric interactions, lower pump power requirements, and greater spectral engineering flexibility. Here, we demonstrate strong, broadband squeezed-light generation on a thin-film lithium niobate (TFLN) photonic chip using a dual-resonant optical parametric amplifier implemented in a single periodically poled LN (PPLN) microresonator. Near-full-depth domain inversion is achieved simultaneously with highly over-coupled resonances, exhibiting escape efficiencies exceeding 90% and intrinsic quality factors above 2.5 million in a 0.6 mm$^2$ X-cut TF-PPLN resonator, enabling efficient squeezing at 1587 nm when pumped at 793.5 nm. Operating in the continuous-wave regime, we directly measure -0.81 dB of squeezing below the shot-noise limit with a pump power of 27 mW, together with +4.29 dB of anti-squeezing. From these measurements, we infer an on-chip squeezing level of -7.52 dB $\pm$ 0.22 dB (95% confidence interval: [-7.96,-7.10] dB), and an on-chip anti-squeezing level of +9.62 dB $\pm$ 0.25 dB. We demonstrate single-mode squeezing at degeneracy with a squeezed-light spectrum exceeding 10.3 THz. This work reports the highest squeezing ratio among integrated $\chi^{(2)}$ cavity platforms and the first quasi-phase matched, fully resonant $\chi^{(2)}$ cavity squeezer on chip, establishing a scalable route to fully integrated power-efficient squeezed-light sources for
quantum-enhanced sensing and metrology.
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All authors Xinyi Ren, Reshma Kopparapu, Tushar Sanjay Karnik, Chun-Ho Lee, Kiwon Kwon, Clayton Cheung, Yue Yu, Shi-Yuan Ma, Bo-Han Wu, Ran Yin, Lian Zhou, Quntao Zhuang, Dirk Englund, Zaijun Chen, Mengjie Yu
Quantum Physics
Ideal random quantum circuits pass the LXEB test
Nicholas Hunter-Jones and Jonas Haferkamp
We show that noiseless random
quantum circuits pass the linear cross-
entropy benchmark (LXEB) test with high probability. If the
circuits are linear depth, and thus form
unitary 4-designs, the LXEB test is passed with probability $1-O(1/\sqrt{k})$, where $k$ is the number of independently drawn samples from the output distribution of the random
circuit. If the
circuits are of depth $\tilde O(n^2)$, and thus form
unitary $n$-designs, the LXEB test is passed with probability $1-O(e^{-k \log(n)/n})$. In proving our results, we show strong concentration of the random
circuit collision probability at linear depth and establish that the tails of the distribution of random
circuit output probabilities start to resemble Porter-Thomas at near-quadratic depths. Our analysis employs higher moments and high-degree approximate designs.
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Quantum Physics
Quantum-Optically Resolving the Number of Colloidal Quantum Dots in a Subwavelength Volume
Zhi-Bo Ni, Jia-Wang Yu, Jiong-Zhao Li et al. (+9 more)
The number resolution of solid-state artificial atoms is of fundamental interest for the study of
quantum few-body systems, yet remains experimentally challenging.
Quantum optical experiments offer a non-invasive approach which links up macroscopic measurements with the quantity of
quantum emitters. In this work, we propose a time-domain
quantum optical methodology for the strict numbering of colloidal CdSe/CdS/ZnS
quantum dots (QDs) confined in subwavelength-size polystyrene capsules. The non-polarized, homogeneously broadened emission of colloidal QDs in the subwavelength volume satisfies the description of Dicke's superradiance of identical
quantum emitters. An analytic relation describes the numerical dependence of the second-order
photon correlation on the number and the collective lifetime of emitters, yielding an experimental counting range of colloidal QDs from one to ten. This work provides a robust pathway for the non-invasive numbering of artificial atoms and the investigation of collective light-matter interactions at the nanoscale.
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All authors Zhi-Bo Ni, Jia-Wang Yu, Jiong-Zhao Li, Xiao-Tian Cheng, Mei-Na Jiang, Zi-Xuan Song, Xiao-Qing Zhou, Wei Fang, Chen-Hui Li, Feng Liu, Xing Lin, Chao-Yuan Jin
Quantum Physics
Thermodynamic Uncertainty Relation with Quantum Feedback
Ryotaro Honma and Tan Van Vu
Fluctuations are intrinsic to microscopic systems and impose fundamental limits on nonequilibrium precision, as captured by the thermodynamic uncertainty relation (TUR), which links current fluctuations to
entropy production. While feedback control is expected to further suppress fluctuations, its role within the TUR framework has remained unclear, particularly in
quantum systems where control is inherently information-driven. In this Letter, we consider open
quantum systems weakly coupled to a thermal environment, in which
quantum jumps are continuously monitored, and Markovian feedback is applied. Using
quantum mutual information to quantify the information contribution induced by feedback, we derive a finite-time TUR for arbitrary time-integrated currents in terms of
entropy production and mutual information. Our results uncover how feedback control suppresses fluctuations together with thermodynamic cost and establish a fundamental precision bound imposed by information-based control. As an application, we analyze a
quantum clock model and demonstrate that the clock precision can be enhanced by feedback control in the presence of a single thermal reservoir.
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Quantum Physics
Finite key analysis of experimentally realized practical COW-QKD protocol
Neha Pathania, Sandeep Mishra and Anirban Pathak
An experimental implementation of the Coherent One-Way
Quantum Key Distribution (COW-QKD) protocol is reported under realistic conditions, and a clean and easy-to-use framework for performing finite key analysis of the COW-QKD protocol is provided by extending a set of existing results. The framework provided here is used to perform finite key rate analysis of the COW-QKD protocol with respect to the actual parameters used in the experimental realization reported here. The system is kept running for several hours with different experimental parameters and stable secure key rates between 1.2 to 1.6 kbps are observed. In addition, QBER, phase error rate and secure key rate are obtained under finite key analysis, and it is shown that COW-QKD is secure for medium-range transmissions (up-to ~ 156 (171) km of optical fiber with 0.2 dB loss per km if detector efficiency is 0.1 (0.2) and other parameters are same as those used in this experiment).
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Quantum Physics
Vibration induced transparency and absorption with two ion ensembles in a linear trap
Wenjun Shao and Jian Li
We study the spectra of collective low excitations of two atomic ion ensembles which are confined in a liner trap by addressing lases. When the left ensemble is driven by an external optical field, its corresponding response spectrum to the incident optical light shows a vibration-induced transparency phenomenon when the detuning of the
laser addressing the ion is tuned to the first red sideband. In the case of the detuning tuned to the first blue sideband, the response spectrum shows a conversion from the absorption peak to the transparency window. Furthermore, we investigate the fluctuation spectra of the collective excitation modes of ion ensemble and show the similar phenomena.
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Quantum Physics
SYK thermal expectations are classically easy at any temperature
Alexander Zlokapa and Bobak T. Kiani
Estimating thermal expectations of local observables is a natural target for
quantum advantage. We give a simple classical
algorithm that approximates thermal expectations for Gibbs states of local
Hamiltonians, and we show it has quasi-
polynomial cost $n^{O(\log (n/\epsilon))}$ for all temperatures above a phase transition in the free energy. For many natural models, this coincides with the entire fast-mixing, quantumly easy phase. Our results apply to the Sachdev-Ye-Kitaev (SYK) model at any constant temperature due to its absence of a phase transition – despite its
entanglement, sign problem, and
polynomial quantum circuit lower bounds. Beyond SYK, we rigorously establish a universal classically easy high-temperature phase for all local, bounded-degree
Hamiltonians and show that it extends to temperatures strictly colder than the death of
entanglement transition.
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Quantum Physics
Loss-insensitive quantum noise reduction in a Raman amplifier with coherent feedback
Jianmin Wang, Rong Zhu and Z. Y. Ou
A
quantum amplifier usually adds extra noise inevitably through coupling to internal degrees of freedom while amplifying the signal. The introduction of
quantum correlations can effectively suppress this extra noise. In this work, we utilize the established
quantum correlation between the Stokes field and atomic
spin waves in the Raman amplification process to feedback a portion of the Stokes field into the amplifier. This leads to a reduction in
quantum noise that is independent of the feedback loss at high gain. A maximum of 6 dB noise reduction is observed. The single-path feedback amplifier is found to be sensitive to the feedback phase, a property that expands its potential for applications in
quantum precision measurement, and the general concept can be extended to integrated optics and fiber optic systems.
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Quantum Physics
Optimizing Doppler laser cooling protocols for quantum sensing with 3D ion crystals in a Penning trap
John Zaris, Wes Johnson, Athreya Shankar et al. (+4 more)
Large, 3D trapped ion
crystals offer improved sensitivity in
quantum sensing protocols, and are expected to be implemented as platforms in near-future experiments. However, numerical techniques used to study the
laser cooling of such
crystals are inefficient as the number of ions, $N$, in the
crystal increases. Here we develop a powerful numerical framework to simulate
laser cooling of up to $10^5$ ions stored in a Penning trap. We apply this framework to characterize and optimize the cooling of ellipsoidal 3D
crystals. We document new pathways to enhanced cooling based on the addition of an axial component to the potential energy-dominated $\boldsymbol{E}\times\boldsymbol{B}$ modes. Furthermore, we observe greatly enhanced cooling of the perpendicular kinetic energy to below 1 mK in prolate ion
crystals, enabling a simplified cooling beam setup for such
crystals. We propose specific values of trap and
laser beam parameters which lead to optimal cooling in a variety of examples. This work illustrates the feasibility of preparing large 3D
crystals for high-sensitivity
quantum science protocols, motivating their use in future experiments.
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All authors John Zaris, Wes Johnson, Athreya Shankar, John J. Bollinger, Allison L. Carter, Daniel H.E. Dubin, Scott E. Parker
Quantum Physics
The Road to Useful Quantum Computers
Timothy Proctor, Robin Blume-Kohout and Andrew Baczewski
Building a useful
quantum computer is a grand science and engineering challenge, currently pursued intensely by teams around the world. In the 1980s, Richard Feynman and Yuri Manin observed independently that computers based on
quantum mechanics might enable better simulations of
quantum phenomena. Their
vision remained an intellectual curiosity until Peter Shor published his famous
quantum algorithm for integer factoring, and shortly thereafter a proof that errors in
quantum computations can be corrected. Since then,
quantum computing R&D has progressed rapidly, from small-scale experiments in university physics laboratories to well-funded industrial efforts and prototypes. Hype notwithstanding,
quantum computers have yet to solve scientifically or practically important problems – a target often called
quantum utility. In this article, we describe the capabilities of contemporary
quantum computers, compare them to the requirements of
quantum utility, and illustrate how to track progress from today to utility. We highlight key science and engineering challenges on the road to
quantum utility, touching on relevant aspects of our own research.
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Quantum Physics
Gravitational decoherence of a composite particle: the interplay between gravitons and a classical Newtonian potential
Thiago H. Moreira and Lucas Chibebe Céleri
The fact that gravitational environments cannot be shielded (since
gravity is universal) makes them of great theoretical interest to decoherence mechanisms and to the
quantum-to-classical transition. While past results seemed to indicate that graviton-induced decoherence of spatial superpositions happens only for macroscopic systems, recently it was shown that this mechanism can be enhanced through the system's own dynamical internal structure. In this work, we extend this analysis by including the interaction with a classical Newtonian potential. We show that, although the graviton bath alone dominates the mechanism for short times compared to a timescale established by the size of the
quantum spatial superposition, the interaction between the gravitons and the internal degrees of freedom of the system renders decoherence inevitable in the long-time limit, even for microscopic masses. We also show that this mechanism is slightly slowed down by the interplay with the classical Newtonian potential, which, for systems without dynamical internal degrees of freedom, can even lead to recoherence-like behaviour, at least in principle.
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Quantum Physics
Quantified convergence of general homodyne measurements with applications to continuous variable quantum computing
Emanuel Knill, Ezad Shojaee, James R. van Meter et al. (+2 more)
In arXiv:2503.00188 we introduced broadband pulsed (BBP) homodyne measurements as a generalization of standard pulsed homodyne quadrature measurements. BBP can take advantage of detectors such as calorimeters that have the potential for high efficiency over a broad spectral range. BBP homodyne retains the advantages of standard pulsed homodyne, enabling measurement of arbitrary quadratures in the limit of large amplitude local oscillators (LO). Here we quantify the convergence of standard and BBP homodyne quadrature measurements to those of the quadrature of interest. We obtain lower bounds on the fidelity of the post-measurement classical-
quantum state of outcomes and unmeasured modes, and the fidelity of the states obtained after applying operations conditional on measurement outcomes. The bounds depend on the LO amplitude and the moments of number operators. We demonstrate the practical relevance of these bounds by evaluating them for standard pulsed homodyne used for estimating values of the characteristic function of the Wigner distribution, expectations of moments, for
quantum teleportation and for continuous variable error correction with GKP codes.
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All authors Emanuel Knill, Ezad Shojaee, James R. van Meter, Akira Kyle, Scott Glancy
Quantum Physics · Cross-listed from physics.atom-ph
Stabilization of Rydberg Dissipative Time Crystals Using a Scanning Fabry Perot Interferometer Transfer Lock
Darmindra Arumugam and Brook Feyissa
Stabilization of
laser frequencies is critical for sensitive Rydberg measurements, including in applications such as dissipative time
crystal (DTC) dynamics, yet conventional approaches often require complex or costly hardware. We demonstrate a compact, low cost stabilization method using a scanning Fabry Perot interferometer (SFPI) to transfer lock a 960nm coupler
laser to an 852nm probe. The lock suppresses coupler multi MHz free running drift and improves the Allan deviation by up to an order of magnitude, reaching <75kHz at 66s. Applied to DTC oscillations using a Rb 2
photon D2 transition, the second harmonic generated 480nm (from 960nm lock) reduces DTC frequency drift from >20kHz to a few kHz and lowers instability by more than an order of magnitude with a minimum Allan deviation of 0.2kHz at <10s. These results establish SFPI-based transfer locking as a practical and accurate approach for scalable multi
laser Rydberg experiments requiring long-term stability in a compact and low cost system.
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