Quantum Physics
Thermodynamic Limits of Quantum Search
Ralf Riedinger
Modern cryptography relies on keyed symmetric ciphers to ensure the secrecy and authenticity of high bandwidth data transfer. While the advent of
quantum computers poses a challenge for public key cryptography, unbroken ciphers are considered safe against
quantum attacks if their key is sufficiently long. However, concrete bounds on the required key length thus far remain elusive: Despite the well known asymptotic complexity of Grover's
quantum search, the optimal
algorithm to recover a secret key, no implementation-agnostic tight bounds were established. Here, we discuss the
quantum thermodynamic limits of generic search
algorithms, and find a work-runtime trade-off for autonomous computers with a fundamental lower bound. By devising an application-specific
quantum protocol, which outperforms
circuit and adiabatic implementations, and saturates this bound, we demonstrate that it is tight. Applying this limit, we find that a secret key of 831 bit length cannot be reconstructed deterministically in an expanding, dark-energy-dominated universe until
star formation is expected to cease. Implications for post
quantum cryptography, and
quantum key distribution are discussed.
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Quantum Physics
Aashish Sah, Suman Kundu, Priyank Singh et al. (+3 more)
Accurate control of
quantum degrees of freedom is promising for sensing, communication, and computing, but building a useful
quantum computer faces a central isolation-and-control challenge:
qubits must remain well isolated from their environment to preserve coherence, yet also be coupled strongly enough for control, readout, and reset. Existing approaches address this challenge only partially, using separate reset elements, drive schemes, and Purcell filters, often with added complexity and tradeoffs such as heating and crosstalk. Here we introduce and demonstrate a first-generation
quantum dial: a device that on demand mediates the coupling of a
qubit to selected auxiliary degrees of freedom. Our implementation uses a band-stop filter between a high-coherence transmon
qubit and a broadband transmission line, enabling the coupling strength to be tuned by several orders of magnitude on nanosecond timescales without significant Stark shift. In the reset configuration, we reduce the
qubit energy relaxation time T1 from >150 $\mu$s to about 200 ns and demonstrate reset independent of the initial state. In the control configuration, we obtain 99.99% idle fidelity and 99.9% gate fidelities for 40 ns pulses at about -110 dBm. The same device also enables thermometry of the
qubit environment, reaching a noise-equivalent temperature of 0.6 mK/$\sqrt{Hz}$ at 60 mK and approaching the Cramér-Rao bound at higher temperatures. The
quantum dial thus offers fast, on-demand switching between isolation and strong coupling, with potential to reduce noise and errors in future
quantum processors.
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All authors Aashish Sah, Suman Kundu, Priyank Singh, Eemeli Forsbom, Vasilii Vadimov, Mikko Möttönen
Quantum Physics
Quantum correlations in mutually unbiased bases and non-Markovian dephasing
Hermann L. Albrecht and David M. Bellorin
Wu et al. introduced a symmetric measure $Q^{s}$ in terms of mutually unbiased bases (MUBs) that quantifies residual
quantum correlations (RQC). The definition of this measure involves determining two complementary local bases via an optimization of the
quantum mutual information. One defines the first local bases, the optimal computational ones, by maximizing the classical correlations measure. Next, one defines a new set of local bases that are complementary to the first ones. In the latter, one also calculates the corresponding measure of
quantum correlations by maximization. Considering alternative optimizations of the mentioned measures, one can define other quantifiers. In particular, Mundarain and Ladrón de Guevara introduced local available
quantum correlations (LAQC, $\mathcal{L}$) in terms of the complementary local bases to the ones defining the least classically correlated state. That is, they characterize LAQC by defining the optimal bases as those that minimize the classical correlations measure. This type of
quantum correlation is closely related to an
entanglement activation protocol proposed by Piani et al. In previous articles, we derived an exact solution for the LAQC measure for 2-
qubit X states. Based on these previous results and methodology, we determine the corresponding exact analytical expression for $Q^s$ of 2-
qubit X states. We then analyze the behavior of these two measures for two non-Markovian
quantum dephasing channels: Random Telegraph (RT) and Modified Ornstein-Uhlenbeck (MOU) noises. We derive general conditions for sudden death and revival of these measures in X states and illustrate these results with four families of bipartite
qubit states: Werner states, Maximally Nonlocal Mixed States (MNMS), Maximally Entangled Mixed States (MEMS), and a mixed symmetric separable state.
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Quantum Physics
Quantum contextuality with mixed states of 1D symmetry-protected topological order
Leroy Fagan and Akimasa Miyake
Bell
theorems of many-body nonlocality and contextuality serve as a
benchmark for proving
quantum advantage in that a
quantum computer outperforms a classical computer for a certain problem. In practice, however, near-term
quantum devices do not prepare perfectly pure states but rather mixed states produced from noisy channels. We investigate noisy
quantum advantage by considering thermal mixed states of one-dimensional many-body systems with a
symmetry-protected topological (SPT) order. In the pure-state (or zero-temperature) case, these states are known to be useful for measurement-based
quantum computation, and to outperform classical computers in a many-body contextuality game, provided string order parameters (SOPs) of SPT are sufficiently large. Here, we show that
quantum advantage in mixed states is measured by a combination of twisted SOP and
symmetry representation expectation values. Using the minimally entangled typical thermal states
algorithm, it is demonstrated that
quantum advantage persists to a nonzero critical temperature for finite-sized instances of the many-body contextuality game. While this critical temperature goes to zero in the thermodynamic limit, it is relatively robust to system size, suggesting that these states remain useful for demonstrating genuine "quantumness" of noisy hardware in a scalable fashion. Finally, we show that the
quantum winning probability is lower bounded by the global fidelity with the 1D cluster state, so that our contextuality game can provide an operational meaning to
benchmark the capacity to create long-range order like SPT states in near-term experimental devices.
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Quantum Physics
The Quest for Quantum Advantage in Combinatorial Optimization: End-to-end Benchmarking of Quantum Solvers vs. Multi-core Classical Solvers
Pranav Chandarana, Alejandro Gomez Cadavid, Enrique Solano et al. (+3 more)
We perform an end-to-end
benchmark of a hybrid sequential
quantum computing (HSQC) solver for higher-order unconstrained binary optimization (HUBO), executed on IBM Heron r3
quantum processors to evaluate the potential of current
quantum hardware for combinatorial optimization with sub-second end-to-end runtimes. All reported runtimes include the complete pipeline–from preprocessing to QPU execution and postprocessing–under strict wall-clock accounting. Across 20
benchmark instances, a single hybrid attempt produces high-quality solutions in less than one second, matching the ground-state energy in 14 cases. At the same runtime, CPU-based solvers, including simulated annealing, memetic tabu search, and EasySolve, do not reach the value obtained by HSQC, whereas an enhanced parallel tempering method and the GPU-accelerated solver ABS3 reach or surpass it. These results show that HSQC, executed on a single QPU, can achieve performance competitive with strong classical solvers running on 128 vCPUs or 8 NVIDIA A100 GPUs, while also providing a reproducible system-level
benchmark for tracking progress as
quantum hardware and hybrid sequential workflows improve.
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All authors Pranav Chandarana, Alejandro Gomez Cadavid, Enrique Solano, Thorsten Koch, Stefan Woerner, Narendra N. Hegade
Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Co2SeO3Cl2: Studies of Emerging Magnetoelectric Coupling in a Polar, Buckled Honeycomb Material
Faith O. Adeyemi, Xudong Huai, Mohamed Kandil et al. (+3 more)
The development of magnetoelectric materials requires chemical design strategies that integrate structural polarity with
magnetic lattices capable of supporting competing
spin interactions. Here, we demonstrate such an approach in the polar, buckled honeycomb magnet Co2SeO3Cl2. Magnetization and heat-capacity measurements reveal strong
magnetic anisotropy and four successive
magnetic transitions at 25.4, 16.8, 11, and 3 K. The recovered
magnetic entropy through the ordering regime is only around half of the expected 2Rln(2), indicating persistent
spin fluctuations. Second-harmonic generation measurements show three pronounced intensity anomalies at 11, 17, and 26 K that coincide with
magnetic transitions while revealing that the crystallographic
symmetry is preserved. Together, these results demonstrate that polar, buckled honeycomb magnets offer an unconventional phase space for coupling
magnetic and electric dipoles in magnetoelectric materials.
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All authors Faith O. Adeyemi, Xudong Huai, Mohamed Kandil, Pradip Karki, Wencan Jin, Thao T. Tran
Quantum Physics
Bit-Vector Abstractions to Formally Verify Quantum Error Detection & Entanglement
Arun Govindankutty
As the number of
qubits increases,
quantum circuits become more complex and their state space grows rapidly. This makes functional verification challenging for conventional techniques. Ensuring correctness is especially critical for
quantum error correction and
entanglement generation. This paper presents a novel application of bit-vector based abstraction methodology for formal verification of
quantum circuits where superposition and functional behaviour can be decoupled. The approach is applied to error detection
circuits for 2-
qubit, 3-
qubit, and Shor 9-
qubit quantum codes, as well as Bell-state and GHZ-state generation
circuits. The error detection
circuits and the Bell-state generation
circuit are verified in less than a second and 25MB
memory. GHZ
circuits with up to 8,192
qubits are verified in under three minutes using a maximum of 23.2 GB of
memory. The results demonstrate the versatility, scalability, and effectiveness of the proposed approach.
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Quantum Physics
Quantum Process Realization of LDPC Code Dualities and Product Constructions
Shuhan Zhang, Deepak Aryal and Yi-Zhuang You
We realize a broad class of code transformations, including Kramers-Wannier duality, tensor product, and check product, as
quantum processes consisting of ancilla initialization, local
unitaries, and projective measurements. Using ZX-calculus, we formulate these code transformations diagrammatically and develop a systematic
algorithm for extracting their
quantum-process realizations. Central to our framework is the observation that the physical content of a classical LDPC code is captured by the operator
algebra associated with its Tanner graph, and that code transformations correspond to maps between such
algebras. Kramers-Wannier duality then admits a natural interpretation as gauging, while tensor and check products correspond to coupled-layer constructions in which interlayer coupling and projection implement a quotient on stacked operator
algebras. Together, these results establish a unified framework connecting code transformations,
quantum circuits, and mappings between distinct
quantum phases of matter.
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Quantum Physics
Active Sampling Sample-based Quantum Diagonalization from Finite-Shot Measurements
Rinka Miura
Near-term
quantum devices provide only finite-shot measurements and prepare imperfect, contaminated states. This motivates
algorithms that convert samples into reliable low-energy estimates without full
tomography or exhaustive measurements. We propose Active Sampling Sample-based
Quantum Diagonalization (AS-SQD), framing SQD as an active learning problem: given measured bitstrings, which additional basis states should be included to efficiently recover the ground-state energy? SQD restricts the
Hamiltonian to a selected set of basis states and classically diagonalizes the restricted
matrix. However, naive SQD using only sampled states suffers from bias under finite-shot sampling and excited-state contamination, while blind random expansion is inefficient as system size grows. We introduce a perturbation-theoretic acquisition function based on Epstein–Nesbet second-order energy corrections to rank candidate basis states connected to the current subspace. At each iteration, AS-SQD diagonalizes the restricted
Hamiltonian, generates connected candidates, and adds the most valuable ones according to this score. We evaluate AS-SQD on disordered Heisenberg and Transverse-Field Ising (TFIM)
spin chains up to 16
qubits under a preparation model mixing 80% ground state and 20% first excited state. Furthermore, we validate its robustness against real-world state preparation and measurement (SPAM) errors using physical samples from an IBM
Quantum processor. Across simulated and hardware evaluations, AS-SQD consistently achieves substantially lower absolute energy errors than standard SQD and random expansion. Detailed ablation studies demonstrate that physics-guided basis acquisition effectively concentrates computation on energetically relevant directions, bypassing exponential combinatorial bottlenecks.
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Quantum Physics
Perspective: Interactions and Nonlinearity in Non-Hermitian Physics
Federico Roccati and Federico Balducci
For decades, Hermiticity was considered an immutable axiom of
quantum mechanics, essential for ensuring real energies and
unitary evolution. This perspective has shifted radically, driven by the realization that non-Hermitian
Hamiltonians provide a powerful effective description of open
quantum systems, granting access to unique phenomena such as Exceptional Points and the Non-Hermitian Skin Effect. In this Perspective, we chart the trajectory of this field, moving from its established foundations in single-particle, linear models to the emerging frontier of interacting many-body systems. We first clarify the physical origins of non-Hermitian dynamics, distinguishing between mean-field approximations, conditional "no-click"
evolution, and exact Liouvillian dynamics. We then focus on the rich phenomenology arising from the interplay of non-Hermiticity and interactions. We discuss interaction-induced topological phases, the generalization of skin effects to the many-body
Hilbert space, and the distinct signatures of dissipative
quantum chaos and complexity. Finally, we highlight collective phenomena in nonlinear regimes, including skin
solitons and dissipative phase transitions. We also comment on measurement-induced
entanglement transitions and their relation to non-Hermitian spectra and
topology. By synthesizing these diverse developments, we provide a roadmap for the future of non-Hermitian physics.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Spin qubit gates via phonon buses in electron nanowires
Dylan Lewis, Roopayan Ghosh, Sanjeev Kumar et al. (+4 more)
Scalable architectures for
quantum computing using
semiconductor quantum dots require interactions between
qubits beyond adjacent
quantum dots. Here, we propose using nanowires of electrons to mediate the interaction between two
quantum dots. Virtual
phonons in the linear chain of electrons can mediate an interaction that gives rise to effective
spin-
spin coupling of the electrons in distant
quantum dots. We find coupling strengths of more than 30 MHz for experimentally realisable parameters in GaAs
quantum dots.
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All authors Dylan Lewis, Roopayan Ghosh, Sanjeev Kumar, Michael Pepper, Charles Smith, Karyn Le Hur, Sougato Bose
Quantum Physics · Cross-listed from cond-mat.mes-hall
Overcoming intrinsic material limitations through cavity feedback
M. Ebrahimi, Y. Huang, V.A.S.V. Bittencourt et al. (+3 more)
Magnons, the quanta of
spin waves, have significant potential for use in modern technologies, especially when strongly coupled to another mode for read-out and control. However, while
magnons strongly interact with microwave
photons via the
magnetic-dipole interaction to form hybrid cavity-
magnon polariton modes, the weak magnetostrictive
magnon-
phonon interaction, together with large polariton linewidths dominated by
magnon dissipation, has so far restricted magnonic-spheres to the weak-coupling regime. The material-limited
magnon dissipation rate in particular has been regarded as an unavoidable limitation in these systems. Here, we surpass this long-standing limitation by implementing an active microwave feedback loop to suppress the linewidth of cavity-
magnon polaritons and strongly suppress their effective decay rate below the
magnon-limited linewidth, thereby enhancing the polariton-
phonon cooperativity from C=1 to C=150. As a key milestone, we achieve normal-mode splitting between a cavity-
magnon polariton and a mechanical mode, providing direct evidence of three-mode hybridization among
photons,
magnons, and
phonons. Our results establish feedback as a general route to accessing strong-coupling regimes in systems previously thought to be limited by material properties and hence open new opportunities for coherent control in hybrid
quantum systems.
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All authors M. Ebrahimi, Y. Huang, V.A.S.V. Bittencourt, A. Rashedi, A. Metelmann, J.P. Davis
Quantum Physics
Resource-Optimal Importance Sampling for Randomized Quantum Algorithms
Davide Cugini, Touheed Anwar Atif and Yigit Subasi
Randomized protocols are procedures that incorporate probabilistic choices during their execution and they play a central role in
quantum algorithms, spanning
Hamiltonian simulation, noise mitigation, and measurement tasks. In practical implementations, the dominant cost of such protocols typically arises from
circuit execution and measurement, and depends on hardware-specific resources such as gate counts,
circuit depth, runtime, or dissipated energy. We introduce a general framework for applying classical importance sampling to randomized
quantum protocols. Given a cost function for running
quantum circuits, the proposed approach minimizes a net-cost figure of merit that jointly captures the computational expense per
circuit and the
estimator variance. We further extend the framework to scenarios where the
quantum computation is subject to errors arising either from algorithmic approximations or from physical noise, proving that importance sampling preserves
estimator bias despite altering the sampling distribution, and to settings with error-detection schemes, where we characterize the resulting changes in the optimal sampling strategy and achievable net-cost reduction. Representative applications include the Qdrift protocol, dephasing channels, mixed-states simulation, composite observables estimation, classical shadows, and probabilistic error cancellation. Overall, our results establish a principled approach for reducing the computational resources required by randomized
quantum protocols through classical sampling optimization.
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Quantum Physics
Qubit syndrome measurements with a high fidelity Rb-Cs Rydberg gate
J. Miles, M. T. Lichtman, A. M. Scott et al. (+20 more)
We demonstrate an inter-species entangling Rydberg gate between rubidium (Rb) and cesium (Cs) atoms with fidelity $\mathcal F = 0.975\pm 0.002$. The two-species atom array enables in-place
quantum non-demolition (QND)
qubit measurements which are a key capability for
quantum error correction. We demonstrate this functionality with multi-atom error syndrome measurements achieving QND measurement fidelities of ${\mathcal F}_{\rm QND} = 0.933(12)$ and $0.865(17)$ for two- and three-
qubit plaquettes, respectively.
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All authors J. Miles, M. T. Lichtman, A. M. Scott, J. Scott, S. A. Norrell, M. J. Bedalov, D. A. Belknap, D. C. Cole, S. Y. Eubanks, M. Gillette, P. Gokhale, J. Goldwin, M. Iliev, R. A. Jones, K. W. Kuper, D. Mason, P. T. Mitchell, J. D. Murphree, N. A. Neff-Mallon, T. W. Noel, A. G. Radnaev, I. V. Vinogradov, M. Saffman
Quantum Physics
Distance learning from projective measurements as an information-geometric probe of many-body physics
Oleksii Malyshev, Simon M. Linsel, Fabian Grusdt et al. (+3 more)
The ability of modern
quantum simulators–both digital and analogue–to generate large ensembles of single-shot projective "snapshots" has opened a data-rich avenue for the study of
quantum many-body systems. Unsupervised machine learning analysis of such snapshots has gained traction, with numerous works reconstructing phase diagrams by learning and clustering low-dimensional representations of
quantum states. Here, we forgo such representation learning in favour of distance learning: we infer the pairwise distances between
quantum states–already sufficient for clustering–directly from snapshots. Specifically, we use a single neural discriminator to estimate Csiszar f-divergences–statistical distances between distributions–in an unsupervised manner. The resulting clusters reveal regimes with different dominant correlations, often coinciding with, but not limited to, conventionally defined phases of matter. Beyond phase-diagram exploration, we connect the infinitesimal limit of the inferred divergences to the Fisher information metric and analyse its finite-size scaling. This yields critical exponents of the discovered transitions and enables snapshot-based analysis of universality classes. We apply distance learning to a diverse set of systems characterised by conventional local order parameters (1D transverse-field and 2D classical Ising models), non-local topological order (extended toric code), and higher-order correlations (fermionic t-J model on a triangular
lattice). In all cases, we correctly recover boundaries between distinct correlation regimes and, where applicable, quantitatively match established critical behaviour. Finally, we show that distances to suitably chosen reference snapshot distributions help identify the dominant correlations within the discovered clusters, positioning distance learning as a versatile information-
geometric probe of
quantum many-body physics.
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All authors Oleksii Malyshev, Simon M. Linsel, Fabian Grusdt, Annabelle Bohrdt, Eugene Demler, Ivan Morera
Quantum Physics
Chip-Scale Transmitter Module for Real-Time Continuous-Variable QKD
Igor Servello, Martin Hauer, Moritz Baier et al. (+6 more)
Continuous-variable
quantum key distribution (CV-QKD) enables secure communication over standard telecom infrastructure, but scaling is stalled by bulky, discrete optical hardware. We address this bottleneck by demonstrating a real-time CV-QKD system driven by a chip-scale hybrid transmitter using commercial telecom components. Combining a micro-optic external-cavity
laser with a monolithic photonic integrated IQ modulator we enable secure secret-key generation over 102 km of optical fiber while reducing optical volume by 95% relative to the commercial discrete-component counterpart. Moreover, real-time operation overcomes offline post-processing bottlenecks of experimental setups. This work bridges laboratory demonstrations and field-deployable technology for cost-effective
quantum networks.
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All authors Igor Servello, Martin Hauer, Moritz Baier, Emmeran Sollner, Peter Gleißner, Sebastian Randel, Ulrich Eismann, Emanuel Eichhammer, Imran Khan
Quantum Physics
Agnostic Dynamical Decoupling for Single-Qubit Gates
Gumaro Rendon
We introduce a method for designing smooth single-
qubit control pulses that implement a desired gate while suppressing the effect of unknown static error sources to first order. Unlike dynamically corrected gate constructions that require prior knowledge of the noise model, the present approach is agnostic to the detailed form of the target-bath interaction. The method parametrizes the control propagator through an auxiliary
matrix expansion over orthogonal basis functions and enforces decoupling through algebraic orthogonality and equal-norm constraints on the expansion coefficients. These conditions guarantee that the leading Magnus contribution of an arbitrary static interaction reduces to a term proportional to the identity on the target system, thereby cancelling first-order error effects independently of the microscopic origin of the noise. We further show that the same construction suppresses, to first order, mediated couplings between simultaneously controlled
qubits when their interaction occurs through intermediate environmental degrees of freedom, yielding effective second-order decoupling of the induced inter-
qubit interaction. By using a discrete cosine transform parametrization, the pulse-synthesis problem is cast into a numerically stable constrained optimization with a minimal number of free parameters. Numerical examples for $R_z$ rotations and random single-
qubit unitaries demonstrate smooth control fields that realize the target gates while remaining robust against arbitrary static single-
qubit noise and mediated multi-
qubit couplings. These results provide a hardware-friendly route toward noise-agnostic dynamically corrected single-
qubit gates.
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Quantum Physics · Cross-listed from hep-th
State-dependent geometries from magic-enriched quantum codes
ChunJun Cao, Gong Cheng, Krishnanand Karthikeyan et al. (+2 more)
Quantum error-correcting codes provide a powerful framework for emergent spacetime, yet existing subsystem erasure-correcting
holographic code models describe only
quantum fields on a fixed background: in such codes, the entropic area term is state independent and cannot capture gravitational backreaction. We argue that this limitation is intrinsic to exact subsystem complementary recovery and that incorporating backreaction naturally leads to approximate
quantum error correction. We introduce a Ryu-Takayanagi-like
entropy decomposition for approximate subsystem erasure-correcting codes, defining bulk matter
entropy via optimal recovery and a complementary proto-area
entropy as the difference between boundary
entropy and recoverable bulk
entropy. For a broad class of skewed
quantum codes obtained by small nonlocal perturbations of exact codes, the proto-area increases monotonically with bulk
entropy, closely aligning with the changes in extremal surface areas in gravitational systems. We identify the origin of this response as a form of tripartite non-local magic in the Choi state of the encoding map, which vanishes in stabilizer codes and controls the leading matter-geometry coupling in approximate subsystem erasure-correcting codes.
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All authors ChunJun Cao, Gong Cheng, Krishnanand Karthikeyan, Cathy Li, John Preskill
Quantum Physics · Cross-listed from cs.CR
An Ideal Random Number Generator Based on Quantum Fluctuations and Rotating Wheel for Secure Image Encryption
Subhadip Rana, Sanku Paul and Mrinal Kanti Mandal
In the era of digitization secure transmission of digital images has become essential in real world applications. Image encryption is an effective technique for protecting image data from unauthorized access. The security of encrypted data strongly depends on the quality of the random numbers used as the encryption key. In this paper, we proposed a hybrid random number generator based on
quantum fluctuations and an algorithmically inspired rotating wheel. The wheel contains integer values from 0 to 255 that are shuffled using
quantum fluctuations generated by time-evolving the
quantum kicked rotor model. There are four pre-defined tapping positions in the rotating wheel to collect the number sequences. The wheel rotation speed is dynamically varied after each set of tapping to enhance unpredictability. The
entropy of the number sequence obtained from the rotating wheel attains the ideal value of 8 (in an 8 bit representation). Further, the generated number sequences exhibit a flat histogram and nearly zero correlation, indicating strong randomness. The generated sequences are applied to the image encryption and analyzed cryptographically. Experimental results demonstrate a near ideal
entropy of 7.997, an NPCR of 99.60%, low correlation in all directions, and low PSNR for encrypted images. These results confirm that the proposed random number generator achieves efficient and high-security performance, making it suitable for the security of consumer applications such as mobile healthcare imaging, biometric authentication, QR-based and multimedia communication on smart devices.
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Quantum Physics
Hadamard regularization of open quantum systems coupled to unstructured environments in the Schwinger-Keldysh formalism
Jakob Dolgner
The theory of open
quantum systems addresses how coupling to external degrees of freedom modifies observables and
quantum coherence, a situation central to fundamental condensed-matter research and emerging
quantum technologies. Schwinger-Keldysh field theory is a natural framework for both open- and nonequilibrium
quantum systems in terms of functional integrals. However, its numerical solution is limited by a cubic scaling with the number of time steps. This is particularly prohibitive for scenarios with widely separated time scales, as is often the case for system and environmental scales. We consider a damped
quantum harmonic oscillator as a toy model to study a separation-of-scales ansatz based on Hadamard regularization. A time-stepping
algorithm for the Kadanoff-Baym equations on the slow system time-scale is presented that captures both low-temperature non-Markovianity and renormalization effects arising from the much faster environment scale.
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Quantum Physics
Characterization of Radiation-Induced Errors in Superconducting Qubits Protected with Various Gap-Engineering Strategies
H. Douglas Pinckney, Thomas McJunkin, Alan W. Hunt et al. (+26 more)
Impacts from high-energy particles cause correlated errors in superconducting
qubits by increasing the quasiparticle density in the vicinity of the Josephson junctions (JJs). Such errors are particularly harmful as they cannot be easily remedied via conventional error correcting codes. Recent experiments reduced correlated errors by making the difference in superconducting gap energy across the JJ larger than the
qubit transition energy. In this work, we assess gap engineering near the JJ ($\delta\Delta_{\mathrm{JJ}}$) and the capacitor/ground-plane ($\delta\Delta_{\mathrm{M1}}$) by exposing arrays of transmon
qubits to two sources of radiation. For $\alpha$-particles from an $^{241}$Am source, we observe $T_1$ errors correlated in space and time, supporting a hypothesis that hadronic
cosmic rays are a major contributor to the $10^{-10}$ error floor observed in Ref. 1. For electrons from a pulsed linear accelerator, we observe temporally correlated $T_1$ and $T_2$ errors, this measurement is insensitive to spatial correlations. We observe that the severity of correlated $T_1$ errors is reduced for
qubit arrays with a greater degree of gap engineering at the JJ. For both $T_1$ and $T_2$ errors, the recovery time is hastened by an increased $\delta\Delta_{\mathrm{M1}}$, which we attribute to the trapping of quasiparticles into the capacitor/ground-plane. We construct a model of quasiparticle dynamics that qualitatively agrees with our observations. This work reinforces the multifaceted influence of radiation on superconducting
qubits and provides strategies for improving radiation resilience.
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All authors H. Douglas Pinckney, Thomas McJunkin, Alan W. Hunt, Patrick M. Harrington, Hannah P. Binney, Max Hays, Yenuel Jones-Alberty, Kate Azar, Felipe Contipelli, Renée DePencier Piñero, Jeffrey M. Gertler, Michael Gingras, Aranya Goswami, Cyrus F. Hirjibehedin, Mingyu Li, Mathis Moes, Bethany M. Niedzielski, Mallika T. Randeria, Ryan Sitler, Matthew K. Spear, Hannah Stickler, Jiatong Yang, Wouter Van De Pontseele, Mollie E. Schwartz, Jeffrey A. Grover, Kevin Schultz, Kyle Serniak, Joseph A. Formaggio, William D. Oliver
Quantum Physics
Algebraic Structure of Quantum Controlled States and Operators
Edwin Agnew, Lia Yeh and Richie Yeung
Quantum control is an important logical primitive of
quantum computing programs, and an important concept for equational
reasoning in
quantum graphical calculi. We show that controlled diagrams in the ZXW-calculus admit rich algebraic structure. The perspective of the higher-order map Ctrl recovers the standard notion of
quantum controlled gates, while respecting sequential and parallel composition and multiple-control. In this work, we prove that controlled square
matrices form a ring and therefore satisfy powerful rewrite rules. We also show that controlled states form a ring isomorphic to multilinear
polynomials. Putting these together, we have completeness for
polynomials over same-size square
matrices. These properties supply new rewrite rules that make factorisation of arbitrary
qubit Hamiltonians achievable inside a single graphical calculus.
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Quantum Physics · Cross-listed from physics.class-ph
Zeeman effect in hydrogen treated in classical physics with classical zero-point radiation
Timothy H. Boyer
The Zeeman effect for the low resonant energy states of hydrogen is treated with classical electrodynamics including classical zero-point radiation. The electron is regarded as a classical charged particle in a Coulomb potential. The "space quantization" of old
quantum theory, the Sommerfeld relativistic result, and the Stern-Gerlach experiment are all considered.
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Quantum Physics · Cross-listed from physics.class-ph
Relativistic hydrogen in classical electrodynamics with classical zero-point radiation
Timothy H. Boyer
Classical electrodynamics including classical electromagnetic zero-point radiation leads to a ground state and resonant excited states for a charged particle in a Coulomb potential. These resonant states correspond to integer values of the action variables analogous to those appearing in the Bohr-Sommerfeld theory of the hydrogen atom. The work on classical zero-point radiation reported here is a continuation of the analysis reported in 1975, but with the addition of the ideas of relativity and resonance between the charged-particle orbit and classical zero-point radiation.
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Quantum Physics · Cross-listed from physics.class-ph
Classical linear oscillator in classical electrodynamics with classical zero-point radiation
Timothy H. Boyer
A classical linear oscillator is treated in the small amplitude limit so that it will be approximately relativistic. The oscillator involves a charge particle in a linear potential in classical zero-point radiation. It is found that the ground state is energy balanced with the power lost in radiation emission equal to the average power gained from resonance with the classical zero-point radiation. Also the oscillator is found to have resonant excited states where the energy emitted as dipole radiation is balanced on average by the energy gained from the zero-point radiation when the action variable of the mechanical system is given by J=(n+1/2)(h/2pi).
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Quantum Physics
Non-existence of stabilizer absolutely maximally entangled states across infinitely many configurations
Hyunho Cha
We prove a general reduction
theorem for stabilizer absolutely maximally entangled states in composite local dimension. If a stabilizer $\mathrm{AME}(n,D)$ state exists and $D=\prod_{i=1}^m q_i$ is the
prime-power factorization of $D$, then for every nonempty subset of factors there exists a stabilizer $\mathrm{AME}\bigl(n,\prod_{i\in M} q_i\bigr)$ state. Thus any obstruction at a
prime-power factor immediately obstructs stabilizer AME states in the composite dimension.
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Quantum Physics · Cross-listed from cond-mat.str-el
Two-channel physics in a lightly doped antiferromagnetic Mott insulator revealed by two-hole spectroscopy
Pit Bermes, Sebastian Paeckel, Annabelle Bohrdt et al. (+2 more)
Understanding pairing in the strong-coupling regime of doped Mott
insulators remains an open problem in the context of cuprate
superconductors. We perform ultra-high resolution numerical simulations of spectral functions in the highly underdoped $t-J$ model and discover two coupled branches of hole pairs emerging at low energies in the largely unexplored two-particle spectrum. As
spin anisotropy is tuned from the Ising limit to the $SU(2)$-symmetric Heisenberg regime, the lowest $d$-wave pair evolves from a single bipolaronic branch into two hybridized branches separated by an avoided crossing. We explain this behaviour using an effective two-channel model involving a tightly bound bipolaronic state and a second channel associated with two
magnetic polarons. The model reproduces the qualitative low-energy spectra and implies near-resonant $d$-wave interactions in the $SU(2)$-symmetric $t-J$ model, consistent with proximity to an emergent Feshbach-type resonance. To probe these predictions experimentally, we propose a Raman spectroscopy scheme for the attractive Hubbard model that can be directly implemented using ultracold atoms in optical
lattices. Our work establishes two-particle spectroscopy, beyond single-particle Green's functions, as a powerful tool for revealing the microscopic origins of unconventional superconductivity.
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All authors Pit Bermes, Sebastian Paeckel, Annabelle Bohrdt, Lukas Homeier, Fabian Grusdt
Quantum Physics · Cross-listed from cond-mat.mes-hall
First-principles predictions of band alignment in strained Si/Si1-xGex and Ge/Si1-xGex heterostructures
Nathaniel M. Vegh, Pericles Philippopoulos, Raphaël J. Prentki et al. (+4 more)
Accurate band offsets are essential for predictive continuum modeling of nanostructures such as
quantum wells and
quantum dots formed in strained Si/Si1-xGex and Ge/Si1-xGex
heterostructures. Experimental offset data for these systems remain sparse away from endpoint compositions, making composition-dependent design difficult. We use atomistic first-principles density functional theory to compute valence- and conduction-band offsets across the full range 0 <= x <= 1. Random alloying is treated with special quasirandom structures, interface lineup terms are extracted from macroscopically averaged local Kohn-Sham potentials in thick periodic superlattices, valence-band
spin-orbit coupling is included through species-resolved Mulliken weights, and conduction-band edges are refined using the screened hybrid Heyd-Scuseria-Ernzerhof functional. The resulting offsets show pronounced composition nonlinearity beyond the linear models explored in previous works, agree with experimental
benchmarks, and reproduce the high-Ge slope change in the relaxed-
alloy band gap. Analytic fitting expressions are provided for direct use in simulations, facilitating practical design of modern
quantum technology devices.
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All authors Nathaniel M. Vegh, Pericles Philippopoulos, Raphaël J. Prentki, Wanting Zhang, Yu Zhu, Félix Beaudoin, Hong Guo
Quantum Physics · Cross-listed from math-ph
Robust symmetry breaking in gapless quantum magnets
Chao Yin and Andrew Lucas
We prove the existence of spontaneous
symmetry breaking in suitably low-energy eigenstates of certain gapless and frustrated many-body
quantum systems, namely symmetric
quantum perturbations to classical models which exhibit spontaneous
symmetry breaking of a finite group at some positive temperature. Additionally, the classical model need not be local in space, as long as it satisfies a
quantum analogue of the Peierls condition. As an example of our technique, we establish robust ferromagnetism in random-bond Ising models in $d= 2$ dimensions with sufficiently biased random couplings, with weak transverse field. Our mathematical technique is based on establishing
quantum bottlenecks, similar to a "many-body WKB" method for evaluating tunneling rates. Using these same methods, we provide new proofs of metastability and the slow decay of the false
vacuum, applicable to gapless metastable states. Our work represents a first step towards a rigorous classification of stable gapless
quantum phases.
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Quantum Physics
Superposed quantum evolutions across chaotic and regular regimes
Amit Anand, Anne-Catherine de la Hamette, Robert Mann et al. (+1 more)
While the superposition of
quantum evolutions is known to produce interference effects, the interference between evolutions with regular and chaotic classical limits remains largely unexplored. Here, we use a Mach-Zehnder interferometer to investigate the superposition of two
quantum evolutions, implemented via post-selection, and to compare it with the corresponding classical mixture. The
quantum kicked top provides a natural platform for this study, as its classical dynamics ranges from regular to mixed to fully chaotic depending on the
Hamiltonian parameters. We show that when a regular
evolution is superposed with a chaotic one, the resulting subsystem
entropy can exceed that of the classical mixture, provided the contribution of the chaotic branch dominates in the superposed
quantum evolution. We further demonstrate that
entropy production in such superpositions is strongly influenced by the structure of the underlying classical phase space. We further show that increased
entropy generation can occur for purely regular dynamics at small values of the chaos parameter, given an appropriate choice of post-selection. These results reveal a nontrivial interplay between classical chaos and
quantum interference in superposed
quantum dynamics
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All authors Amit Anand, Anne-Catherine de la Hamette, Robert Mann, Shohini Ghose