Quantum Physics
Scaling-optimal purification of noisy qubit unitary channels
Ryotaro Niwa, Satoshi Yoshida, Koki Ono et al. (+5 more)
We consider the problem of purifying noisy
qubit unitary channels. Given the ability to apply an unknown
qubit unitary channel followed by depolarizing noise, we aim to construct a superchannel that purifies the noisy
unitary back to the original unknown
unitary. We first provide numerical evidence that sequential strategies can strictly outperform parallel strategies when the number of channel uses is finite, highlighting the fundamental distinction from state purification. We then provide a concrete $\mathrm{U}(2)$-covariant parallel protocol based on a novel
entanglement-assisted
quantum error-correcting code that suppresses the first-order noise strength as $O(1/n)$ with $n$ channel uses and show this scaling is asymptotically optimal in the low-noise regime, even when sequential strategies are allowed.
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All authors Ryotaro Niwa, Satoshi Yoshida, Koki Ono, Takeru Utsumi, Zhaoyi Li, Yuxiang Yang, Ryuji Takagi, Mio Murao
Quantum Physics
Minimality of the Stabilizer ZX Calculus
Harry K. Stoltz
The stabilizer fragment of the ZX calculus is amongst the most important fragments of the theory. Crucially, the stabilizer calculus can be described by a small collection of rewrites, most of which have been shown to be necessary by Backens–Perdrix–Wang (arXiv:1709.08903). However, two rules, describing the red/green compact-structure coincidence and the important bialgebra law, had not been shown to be necessary. We present two alternate interpretations, showing that both of these rules are individually necessary. Thus, this resolves a problem which has remained open for over a decade, and gives the first complete, minimal ruleset for the stabilizer ZX Calculus.
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Quantum Physics
Fermions are fundamentally more nonlocal than Bosons
Fatemeh Moradi Kalarde, Sadra Boreiri, Xiangling Xu et al. (+5 more)
Bell's
theorem shows that entangled
quantum particles can exhibit correlations that classical particles cannot reproduce without an additional nonlocal resource, such as communication. In this sense,
quantum particles are fundamentally more nonlocal than classical ones, and
entanglement becomes unavoidable in physics. Here we prove the analogous result within
quantum theory itself: indistinguishable
fermions transmitted through a
quantum network can generate correlations that distinguishable particles or indistinguishable
bosons cannot reproduce without additional communication. In the same sense,
fermions are fundamentally more nonlocal than
bosons or distinguishable particles, motivating fermionic anticommutation and indistinguishability as unavoidable operational resources. Our result further implies that
fermions can strictly surpass all
qubit-based protocols for certain distributed computing tasks, demonstrating that a complete understanding of information processing requires going beyond
qubits to fermionic information carriers - febits.
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All authors Fatemeh Moradi Kalarde, Sadra Boreiri, Xiangling Xu, Lucas Tendick, Salman Beigi, Paolo Perinotti, Tommaso Guaita, Marc-Olivier Renou
Quantum Physics · Cross-listed from cond-mat.mes-hall
Gate-tunable spin-valley transport via carrier velocity in monolayer WSe$_2$
Otman Bouladiane, Hocine Bahlouli, Clarence Cortes et al. (+2 more)
We theoretically investigate
spin- and valley-resolved
quantum transport in monolayer tungsten diselenide (WSe$_2$) described by an effective massive Dirac
Hamiltonian. Particular
attention is devoted to a finite barrier region characterized by simultaneously modulated
Fermi velocity and scalar potential. The barrier velocity $v_2$ is related to the external velocity $v_1$ through a velocity ratio $\xi=v_2/v_1$, motivated by an optical analogy with the Snell-Descartes law. The exact refraction condition depends on the full
spin- and valley-resolved dispersion, and the simple ratio $\xi=v_2/v_1$ is recovered only in the massless, symmetric limit. The interplay of intrinsic
spin-orbit coupling in the conduction and valence bands, quantified by $\lambda_c$ and $\lambda_v$, with
spin- and valley-dependent Zeeman fields, $M_s$ and $M_v$, gives rise to substantial changes in the quasiparticle dispersion, leading to pronounced modifications of the transport characteristics. By solving the Dirac equation and enforcing current-conserving matching conditions at the interfaces, we compute the
spin- and valley-dependent transmission probability and conductance. Our results demonstrate that the barrier velocity, scalar potential, incidence angle, incident energy, and barrier width serve as effective control parameters for transport, giving rise to strong anisotropy and resonant tunneling features. Furthermore, we show that both the magnitude and orientation of
spin- and valley-polarized currents can be continuously tuned via velocity and potential modulation. These findings establish combined velocity and potential engineering as a powerful theoretical framework for controlling
spin-valley physics in two-dimensional transition-metal dichalcogenides.
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All authors Otman Bouladiane, Hocine Bahlouli, Clarence Cortes, David Laroze, Ahmed Jellal
Quantum Physics · Cross-listed from hep-th
Entanglement generation between field modes mediated by a fluctuating conducting wall
Luca Giovanni Cammarata, Tommaso Fazio, Roberto Passante et al. (+1 more)
We consider a movable conducting plate of finite mass, between two fixed ones, whose mechanical degrees of freedom are treated
quantum-mechanically and bound to its
equilibrium position by a harmonic potential. The movable wall is thus subjected to
quantum fluctuations of its position. This creates a system of two subcavities separated by the movable fluctuating plate, and two massless one-dimensional scalar fields, one in each subcavity. This system is described by an appropriate generalization of the Law
Hamiltonian. The presence of the movable wall yields an effective plate-fields interaction, as well as an effective interaction between the field modes. We obtain, at the second order in perturbation theory, the ground state of the interacting system and the reduced density operator of the fields in each subcavity by tracing out the wall's degrees of freedom. We calculate the
entanglement between two field modes, one in each cavity, by evaluating analytically the negativity; we then evaluate numerically also the total multimode negativity. Our results show that in both cases the fields in the two subcavities are entangled, in contrast to the case in which the wall is fixed in space. We discuss the amount of the field
entanglement present as a function of relevant physical parameters of the system such as the mass and oscillation frequency of the movable wall, its distance from the fixed walls and the frequencies of the field modes considered.
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All authors Luca Giovanni Cammarata, Tommaso Fazio, Roberto Passante, Lucia Rizzuto
Quantum Physics
Quantum repeater segment with free-space coupled co-trapped ions using telecom photon interference
Max Bergerhoff, Pascal Baumgart, Christian Haen et al. (+5 more)
A
quantum repeater segment is a basic building block of a
quantum repeater, generating buffered
entanglement of
quantum memories to connect
quantum repeater
cells. It also enables the connection between
quantum computers. In the implementation we present here,
photons emitted from two co-trapped free-space coupled $^{40}$Ca$^+$ ions are converted to the telecom-C band and interfered after transmission over 440$\,$m of optical fiber (220$\,$m per arm), where a photonic Bell measurement is performed to create
entanglement between the
memories. With this scheme we generate an entangled $\left|\Psi^+\right\rangle$ Bell state with $\ge 68(8)\,$% fidelity, highlighting trapped $^{40}$Ca$^+$ ions as a promising
quantum repeater hardware platform.
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All authors Max Bergerhoff, Pascal Baumgart, Christian Haen, Jonas Meiers, Tobias Bauer, Jonas Haferkamp, Christoph Becher, Jürgen Eschner
Quantum Physics
Partitioned Iterative Quantum Scheduling of Satellites for Urgent Disaster Response: Case study of Wildfire
Lucas T. Braydwood, Taejin Park, Hirofumi Hashimoto et al. (+4 more)
The standard in Earth-observation tasks today is having near real-time access to surface images in response to changing conditions. For instance, as urban environments interface more with wildlands and wildfires become less predictable, their tracking with satellite resources becomes essential. This requires the coordination of increasingly large constellations of satellites, giving rise to challenging computational problems. With wildfire detection and tracking as a backdrop, we investigate the power of special purpose and novel computing paradigms to tackle the ensuing satellite scheduling problems, making a compelling case for
quantum algorithms. We bring
quantum scheduling
algorithms closer to implementation by examining both the emerging iterative
quantum algorithm framework, which comes with analytic guarantees compared to some classical
algorithms, and distributed
quantum computing methods whose relevance is on the rise as utility-scale problems begin to get solved with
quantum computers. Drawing strength from several computing fronts, we develop a distributed/parallelization scheme in conjunction with the
quantum algorithm design and apply these techniques to real-world
datasets for wildfire detection. While our
quantum subprocesses are currently too small to see significant
quantum advantage, our results validate the utility of these techniques, and continue forging the path toward distributed
quantum computing.
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All authors Lucas T. Braydwood, Taejin Park, Hirofumi Hashimoto, Zoe Gonzalez Izquierdo, Andrew Michaelis, Eleanor Rieffel, Shon Grabbe
Quantum Physics
Super-Link Fragility in Asymmetric W-Class States under Quantum Noise
Sougata Bhattacharyya, Fatih Ozaydin and Sovik Roy
The asymmetric three-
qubit W-class state $|\overline{W_3^L}\rangle$ defines an isosceles
entanglement-network geometry, (a) two vertex-base (VB) links form stronger bipartite connections, (b) while the base-base (BB) link is weaker. This suggests that concentrating
entanglement into a super-link may be advantageous for
quantum-network tasks. Here, we show that this intuition is incomplete. We analytically compare the bipartite concurrence dynamics of the symmetric |W> state and the asymmetric $|\overline{W_3^L}\rangle$ state, which differ both in
entanglement-network geometry and excitation sector under standard noise models. In the absence of noise, the concurrence hierarchy is $C_{VB} > C_W > C_{BB}$. Under phase damping, this hierarchy is preserved for all noise strengths and no
entanglement sudden death occurs. Under amplitude damping, however, the hierarchy is reordered. The symmetric |W> state becomes the most robust, while the base-base concurrence of $|\overline{W_3^L}\rangle$ vanishes at the finite threshold of parameter $\gamma$. We term this reordering as the Super-Link Fragility Effect. The same structural asymmetry that produces a stronger vertex-base link also makes it more vulnerable to energy dissipation when coupled with multi-excitation amplitudes. Under depolarization, the asymmetry advantage is erased, with $C_W$ and $C_{VB}$ sharing the same sudden-death threshold for some value of the parameter p, while $C_{BB}$ disappears earlier at some other value of the parameter p. The generalized amplitude damping channel continuously connects the damping-dominated regime to the pure-excitation limit, where the initial hierarchy is restored. These results show that
entanglement robustness in $W$-class resources is controlled not by initial concurrence alone, but by the joint structure of
entanglement-network geometry, excitation sector, and noise
symmetry.
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Quantum Physics
An iterative Ising decoder for quantum error correction codes
Yuanqi Liu, Weilei Zeng, Peixiang Li et al. (+6 more)
The Ising framework maps the decoding problem in
quantum error correction onto ground-state optimization of a classical
Hamiltonian, in which $X$-$Z$ error correlations enter as cross terms. Under phenomenological depolarizing noise, the exact joint formulation contains up to 8-body interactions for the toric code and 10-body for the $6.6.6$ color code. These high-order terms degrade solver convergence, inflate runtime, and raise the auxiliary
spin overhead when
embedding into native 2-body Ising hardware. In this work, we propose the iterative low-order decoding (ILOD)
algorithm, which alternates between $X$- and $Z$-type sub-
Hamiltonians, approximating cross-type correlations through
Bayesian priors that reweight each type's couplings using the other type's inferred error configuration. This halves the maximum body count of interaction terms in the
Hamiltonian, accelerating the solver, restoring convergence at larger code distances, and reducing the total
spin count for 2-body
embedding by a factor of $2.5$. For the toric code, ILOD attains a threshold of $4.73%$ versus $4.83%$ for the joint formulation, with the empirical runtime ratio scaling as $(0.81)^d$. For the $6.6.6$ color code, their thresholds agree within statistical uncertainty for small code distances, and ILOD remains convergent for larger distances where the joint formulation fails to converge despite a larger annealing budget.
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All authors Yuanqi Liu, Weilei Zeng, Peixiang Li, Yantong Liu, Guangyao Huang, Yingwen Liu, Dongyang Wang, Junjie Wu, Lingling Lao
Quantum Physics · Cross-listed from gr-qc
A post-selected quantum model of cosmic acceleration
Dimitris Lionas, Charis Anastopoulos and Konstantinos Gourgouliatos
The origin of
cosmic acceleration remains a central problem in cosmology, commonly attributed to a cosmological constant within the $\Lambda$CDM model or to dynamical dark energy. Here, we develop an alternative approach in which acceleration emerges from
quantum post-selection, a standard feature of
quantum theory that is not usually incorporated into cosmological modelling. While
quantum theory admits both pre-selected and post-selected ensembles,
quantum cosmological models are almost exclusively formulated in terms of initial conditions. Building on previous work on post-selected quasiclassical dynamics, we construct a minimal predictive cosmological model in which post-selection and coarse-graining generate effective late-time acceleration without introducing a cosmological constant, dark energy, or modifications of general relativity. The resulting expansion history is highly constrained theoretically and depends on at most two parameters beyond standard Friedmann
evolution. Confrontation with type Ia
supernova and
cosmic chronometer data yields statistically competitive fits while naturally avoiding the coincidence problem. The model also reproduces the standard radiation- and matter-dominated behaviour at early times and predicts a present-day jerk parameter significantly different from the $\Lambda$CDM value. These results suggest that
cosmic acceleration may arise as a macroscopic
quantum cosmological effect rather than from additional cosmological fluids or modified gravitational dynamics.
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Quantum Physics
Multipartite reference-frame-independent quantum cryptographic communication
Donghwa Lee, Kyujin Shin, Hyang-Tag Lim et al. (+2 more)
Reference frame mismatch among communication parties introduces errors in
quantum cryptographic protocols. As the number of participants increases, aligning reference frames becomes increasingly difficult, complicating multipartite
quantum cryptographic implementations. Here, we theoretically and experimentally investigate multipartite reference-frame-independent (RFI)
quantum cryptographic communication using Greenberger-Horne-Zeilinger (GHZ) states. We generalize the bipartite RFI security parameter $C$ to an $N$-party parameter $C_N$ and derive the asymptotic secret key rate expressed solely in terms of experimentally accessible quantities. We analyze the key rate under global and local depolarizing noise models and find that increasing the number of parties $N$ enhances robustness against global depolarizing noise while increasing vulnerability to local channel noise. We also present a proof-of-principle experimental demonstration of four-party RFI
quantum cryptographic communication using four-
photon GHZ states, confirming the reference-frame invariance of both the $C_4$ parameter and the secret key rate under various reference frame rotations.
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All authors Donghwa Lee, Kyujin Shin, Hyang-Tag Lim, Yosep Kim, Yong-Su Kim
Quantum Physics
Time-Frequency Grid States for Reconstruction and Correction of Channel-Induced Distortion in Entangled Photons
Siang-Yun Liu, Bo-Ren Huang, Zhi-Xuan Zen et al. (+2 more)
Characterization of time-frequency (TF)
quantum states requires reliable reconstruction of their TF distributions. However, imperfect transmission or measurement channels can distort reconstructed joint spectral intensities (JSIs), especially when the underlying perturbation mechanism is unknown. Here, we experimentally demonstrate a reconstruction and correction framework that uses a TF grid state as an intrinsic frequency-domain reference. By analyzing the displacement of the grid points, a Gaussian process regression model is employed to reconstruct a correction mapping for the nonlinear coordinate deformation without assuming a prior physical model of the distortion. The learned mapping reduces the residual coordinate deviation of the TF grid state by approximately a factor of 11 and, when applied to an independent frequency-entangled test state, improves the Gaussian-shape fidelity from 76.2% to 90.0%. These results establish TF grid states as practical metrological resources for diagnosing and correcting distortions in TF
quantum systems, providing a pathway toward distortion-resilient
quantum communication and high-dimensional
quantum information processing.
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All authors Siang-Yun Liu, Bo-Ren Huang, Zhi-Xuan Zen, Yen-Hung Chen, Pin-Ju Tsai
Quantum Physics
Quantum Occam Learning: Sample-Supported Expressibility for Circuit-Based Quantum Learning
Jeongho Bang, Kyoungho Cho and Jeongwoo Jae
A central principle in
quantum machine learning is that an ansatz should be expressive enough to represent the
quantum data of interest. Yet, the expressibility is statistically meaningful only insofar as it can be learned from finitely many copies of an unknown
quantum state. In this work, we develop an information-theoretic Occam theory for
quantum data generated by finite-size
quantum circuits. For the class $S_{n,G}$ of $n$-
qubit pure states preparable with at most $G$ two-
qubit gates, a metric-
entropy argument gives the realizable sample law $\widetilde{\Theta}(G/\epsilon^2)$ in the
circuit-limited regime. For an arbitrary source $\hat{\rho}$, we introduce the best $G$-gate approximation error $d_G(\hat{\rho})$ and the approximate
circuit complexity $C_\eta(\hat{\rho})$. We prove an agnostic
quantum Occam
theorem: with $M$ copies, one can learn up to the best $G$-gate approximation error plus a statistical penalty $\widetilde{O}(\sqrt{G/M})$. We then remove the need to know $G$ in advance through an adaptive model-selection
theorem whose oracle inequality selects the
circuit complexity justified by the data. Matching lower bounds yield a sample-supported expressibility law: at trace-distance accuracy $\epsilon$, $M$ samples can support only $G_{\rm supported} \simeq M\epsilon^2$ gates, up to logarithmic factors and
tomography saturation at $2^n$. Thus, the
circuit complexity becomes an adaptive statistical resource rather than a static promise. Our framework turns bounded
circuit complexity into a model-selection principle for
quantum machine learning.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Experimental straintronics in nanotube quantum dots
L. Huang, I. G. Rebollo and A. R. Champagne
Single-wall carbon nanotubes (SWCNTs) are narrow ribbons of
graphene with atomically precise edges and a single
quantum transport channel, at experimentally-relevant dopings. This makes them ideal systems to harness
quantum transport straintronics (QTS), i.e. using mechanical strain to control accurately
quantum transport. We present QTS data from three single-wall carbon nanotube
quantum dot (SWCNT-QD) transistors over a broad range of in-situ tunable and reversible uniaxial strain ($\Delta\varepsilon_\text{mech}\approx$ 0 to 3 %). We first present the nanofabrication of the suspended SWCNT transistors whose channel lengths are $\approx$ 30 nm. The channels are strained by moving gold clamps holding firmly the nanotubes. We present detailed charge transport data, $dI/dV_{\text{B}} - V_{\text{B}} - V_{\text{G}}$ and $dI/dV_{\text{B}} - V_{\text{B}} - \Delta\varepsilon_\text{mech}$, showing a large mechanical-gating effect of the SWCNT-QDs. The precise reversibility of the data, and their agreement with QTS theory, confirms that the tubes are strained elastically. We demonstrate that the mechanical control of the QD doping is not due to capacitive-gating effects, but to quantitatively predictable bandstructure changes including a strain-tunable bandgap. This precise mechanical control of the doping and bandgap of SWCNT-QDs could find applications in
qubits, condensed matter physics, and homojunction molecular transistors.
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Quantum Physics
On-Chip Quantum Randomness Amplification
Lang Li, Yutian Wu, Giulio Chiribella et al. (+1 more)
Randomness amplification, the task of extracting uniform private bits from biased seeds that may be partly known by a malicious third party, is of central importance in cryptography. The highest security in this task is provided by a class of
quantum protocols known as device-independent, which however are challenging to integrate into scalable devices. Semi-device-independent (SDI) protocols are a promising alternative that guarantees security under few natural assumptions, such as bounds on the amount of energy used by the devices. Here, we provide the first demonstration of SDI randomness amplification on an integrated silicon photonic chip, achieving a throughput rate of 20 Mbps suitable for practical applications. This rate is achieved through a novel technique for SDI
entropy certification, which delivers strictly tighter von Neumann
entropy bounds compared to existing methods and remains valid even if the preparation and measurement devices share
quantum correlations. Overall, the methods developed in this work enable the integration of SDI technology into portable telecom devices, opening up a new generation of
quantum cryptographic hardware.
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All authors Lang Li, Yutian Wu, Giulio Chiribella, Ravishankar Ramanathan
Quantum Physics · Cross-listed from physics.optics
Fabricating fiber cavity mirror substrates compatible with high coupling efficiency
Michael Caouette-Mansour, Thomas J. Clark, Valeria Mosso Tsedilkina et al. (+1 more)
Fiber optical cavities offer small mode volumes and correspondingly strong light-matter interactions in an open Fabry-Perot geometry. However, existing fabrication techniques do not reliably produce substrates with surface profiles amenable to high mode matching between the cavity mode and fiber core, thereby limiting the achievable collection efficiency. Here we present a technique to fabricate fiber mirror substrates while using $\textit{in situ}$ reflectometry to constrain the achievable mode matching prior to coating. By measuring the back-reflection from freshly cleaved fiber tips, we pre-select 138 fibers compatible with 96.5-99.5% mode matching, and after a single CO$_2$
laser ablation pulse, these fibers remained compatible with 95.3-99.2%. This simple technique provides rapid feedback during each stage of substrate fabrication, greatly enhancing the yield of viable fiber mirror substrates prior to (expensive) coating runs.
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All authors Michael Caouette-Mansour, Thomas J. Clark, Valeria Mosso Tsedilkina, Jack C. Sankey
Quantum Physics
Quantum ergodicity and semiclassical measures: mathematical results
Stéphane Nonnenmacher
In this chapter we review some results describing the high-frequency eigenmodes of the
Laplacian on compact
manifolds, or Euclidean domains, for which the geodesic flow is chaotic. We focus on the macroscopic distribution of these eigenmodes, which is described by the concept of semiclassical measure. The main result on the question is the
Quantum Ergodicity
theorem, originally due to Schnirelman. We provide the detailed proof of this
theorem, including the adjustments necessary to treat the case of
manifolds with boundary. We also discuss the
Quantum Unique Ergodicity
conjecture, and some progress towards this
conjecture for strongly chaotic (Anosov) systems. In particular, we describe the constraints on admissible semiclassical measures, in terms of their Kolmogorov-Sinai
entropy, as well as more recent delocalization results.
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Quantum Physics
Necessary and Sufficient Conditions for Universal Gates with Pauli Strings and Beyond
Isaac D. Smith, Hans J. Briegel and Hendrik Poulsen Nautrup
Any
quantum computation consists of a sequence of
unitary evolutions described by a finite set of
Hamiltonians. For the case where this set consists of only products of Pauli operators, known as Pauli strings, we provide a necessary and sufficient condition for it to generate $\mathfrak{su}(2^n)$, i.e., to be universal for
quantum computation on $n$
qubits. When combining Pauli strings with a general
Hamiltonian, we show a sufficient (and in certain circumstances even necessary) condition for universality based on the Pauli-basis expansion of the
Hamiltonian. As an application of these results, we prove two corollaries: (i) a necessary and sufficient condition for the universality of a general
Hamiltonian given arbitrary single-
qubit control on all
qubits, and (ii) the universality of an XYZ Heisenberg
Hamiltonian with local control of just two adjacent
qubits.
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Quantum Physics · Cross-listed from cond-mat.dis-nn
Non-Hermitian Delocalization Realizes Random Dirac Criticality in One Dimension
Bo Li, Shen Zhang and Ren Zhang
Non-Hermitian systems can evade Anderson localization and exhibit delocalized states even in one dimension. Here, we show that such non-Hermitian delocalized states under periodic boundary conditions (PBC) are intrinsically critical, realizing the universality class of one-dimensional random Dirac
fermions. By linking spectral winding to topological Anderson transitions via Hermitization, we demonstrate that the delocalized PBC states exhibit a Dirac-type criticality with universal algebraic correlations. In contrast to Hermitian systems, where this criticality occurs only at fine-tuned transition points, it emerges generically in non-Hermitian systems as a consequence of spectral
topology. These results identify a universal mechanism by which non-Hermiticity promotes criticality, providing a unified description of non-Hermitian delocalization in one dimension.
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Quantum Physics
Bound State Solutions of the Relativistic Finite-difference Equation for the Ring-shaped Quesne Oscillator Potential
Sh.M.Nagiyev, Narmin Nasibova, V. A. Tarverdiyeva et al. (+1 more)
We solve exactly the relativistic finite-difference equation for the
quantum three-dimensional ring-shaped Quesne oscillator potential. Our investigation is based on a finite-difference version of relativistic
quantum mechanics. So-called relativistic configurational r-space is a key concept here. We show that the radial wavefunctions and angular wavefunctions are expressed through the continuous dual Hahn
polynomials and Jacobi
polynomials, respectively. A discrete energy spectrum has been found. The radial wave functions and energy spectrum have the correct nonrelativistic limit. We also build a dynamical
symmetry group SU (1, 1) for the radial part of the equation of motion, which allows us to find the energy spectrum purely algebraically.
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All authors Sh.M.Nagiyev, Narmin Nasibova, V. A. Tarverdiyeva, G. H. Guliyeva
Quantum Physics
A semi-definite programming formulation of the device-dependent guessing probability
Raffaele D'Avino, Aurora Mugnai, Miguel Navascués et al. (+2 more)
In
quantum mechanics, a measurement applied to a state in general produces some amount of intrinsic randomness. This is not only a fundamental feature of the theory, but is also at the basis of any
quantum process to generate random numbers. The simplest of such processes consists of a single, fully charaterized, measurement acting on a single, fully characterized, state. Unfortunately, no general method to estimate the intrinsic randomness produced in such setups is known. In this work, we address this issue by presenting a semidefinite programming formulation of the maximum probability with which an adversary, Eve, can guess the outcomes of characterized but untrusted prepare-and-measure setups. We then present several applications of this construction. First, we apply our method to a variety of specific setups, allowing us both to
benchmark the approach and, more importantly, to determine the exact amount of certifiable randomness in scenarios where only upper bounds were previously available. Then, we show that the presence of
entanglement between the device preparing the state and the measurement strictly increases Eve's predictive power, already in the most elementary setup of a binary measurement acting on a
qubit state.
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All authors Raffaele D'Avino, Aurora Mugnai, Miguel Navascués, Antonio Acín, Gabriel Senno
Quantum Physics
The quantum harmonic oscillator and the real Hilbert space
Sergio Giardino
The harmonic oscillator is considered within generalized frameworks using complex and quaternionic numbers. The classical oscillator is considered in terms of a complex position function, and
quantum oscillators are examined in terms of complex wave functions, and in terms of quaternionic wave functions as well. Both of the
quantum solutions are obtained within the real
Hilbert space formalism. The results reveal the complex and quaternionic descriptions as suitable frameworks for non-stationary processes, including damped oscillations, forced oscillations, and additionally self-interacting processes that cannot be appropriately described otherwise.
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Quantum Physics
Clifford disentanglers for entanglement reduction in molecular electronic structure simulations
Longfei Chang, Zibo Wu, Yunzhi Li et al. (+5 more)
Entanglement is a key bottleneck limiting the efficiency of tensor-network and
quantum simulations of molecular electronic structures. Here, we systematically assess and extend Clifford disentanglers as a structure-preserving approach to
entanglement reduction: they can modify the
entanglement structure of
qubit wavefunctions while retaining the Pauli-string form of
qubit Hamiltonians. To enable a practical search over Clifford transformations, we classify Clifford operators by their action on the Schmidt spectrum across a bipartition, reducing the two- and four-
qubit search spaces to 20 and 91392 representatives, respectively. Embedded in an iterative Clifford-augmented
matrix product state framework, these transformations reduce the energy errors at fixed bond dimension for the molecular test cases studied and mitigate the dependence on orbital orderings and
fermion-to-
qubit mappings. We further show that Clifford disentanglers can also benefit
quantum simulations such as the shallow-
circuit variational
quantum eigensolver calculations. Together, these results establish Clifford disentanglers as a useful structure-preserving
entanglement-engineering tool for tensor-network and
quantum simulations of molecular electronic structure, while also clarifying their correlation dependence and motivating future developments.
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All authors Longfei Chang, Zibo Wu, Yunzhi Li, Haiqi Liu, Jiajun Ren, Mingpu Qin, Zhendong Li, Wei-Hai Fang
Quantum Physics
A Geometric Family of Correlations Containing the Quantum Singlet
E. Aldo Arroyo
We introduce a geometrically constrained hidden-variable framework that generates a family of correlations parametrized by a boundary function, within which the
quantum singlet correlation appears as a particular member. Exact expressions for the correlation function are derived. Several structural results are established, including admissibility conditions,
symmetry properties, a universal stationary point of the associated CHSH function, and an exact relation between the CHSH value at $\nu=\pi/4$ and a
geometric contrast measure defined on the underlying hidden-variable distributions. Rather than treating the
quantum singlet correlation as an isolated target to be reproduced, the present framework places it within a broader
geometric structure of correlations. These results suggest the existence of a nontrivial
geometric structure underlying the family of correlations and motivate the search for a principle capable of selecting the
quantum singlet solution from within that family.
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Quantum Physics
Shadow Engineering of Quantum Processes
Tian-Ci Tian, De-Tao Jiang, Wei-Ming Zhu et al. (+3 more)
Characterizing
quantum processes is essential for hardware benchmarking, error diagnosis, and
algorithm verification. While recent work [PRX
QUANTUM 4, 040337 (2023)] extended classical shadows from
quantum state to
quantum process, enabling efficient single-channel $\mathcal{E}$ property prediction, its applicability to composite processes $f(\mathcal{E}_1, \mathcal{E}_2,\cdots, \mathcal{E}_k)$ remains unexplored. We introduce shadow engineering, a framework encoding the classical shadows of processes into sparse transfer
matrices to predict $f(\mathcal{E}_1, \mathcal{E}_2,\cdots, \mathcal{E}_k)$ properties with proven
polynomial sample complexity, matching single-channel efficiency while exponentially lower than
quantum process
tomography. Crucially, this approach repurposes existing $\mathcal{E}_m$-shadow data without physical execution of $f(\mathcal{E}_1, \mathcal{E}_2,\cdots, \mathcal{E}_k)$, enabling flexible
quantum process characterization with minimal hardware overhead. We demonstrate the framework's effectiveness and practicality on a superconducting
quantum processor for typical applications such as error mitigation and
Hamiltonian dynamical simulation. This framework unlocks new capabilities for predicting complex
quantum behaviors without physical re-execution, with immediate applications in near-term device calibration and
quantum simulation.
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All authors Tian-Ci Tian, De-Tao Jiang, Wei-Ming Zhu, Wei-You Liao, Hong-Wei Li, He-Liang Huang
Quantum Physics
Measurement-Free Toric-Code Memory in Array Globally Controlled Rydberg Array
Han Wang, Yusheng Zhao, Xiuhao Deng et al. (+1 more)
The central prerequisite of any fault-tolerant
quantum architecture is a
quantum memory: a block of encoded physical
qubits whose logical state is actively preserved against noise across many rounds of error correction. In neutral-atom Rydberg arrays, realizing such a
memory is obstructed not by the entangling gates themselves, which are already fast and high-fidelity, but by the auxiliary operations that a conventional error-correction cycle requires: mid-
circuit fluorescence measurement, inter-zone atom transport, and locally focused single-
qubit addressing. Each of these introduces latency, atom loss, or optical crosstalk that exceeds the cost of the underlying gates by orders of magnitude. These costs accumulate cycle after cycle, progressively degrading the very logical information the code is meant to protect. Here we propose a protocol that stabilizes a toric-code
quantum memory without moving, measuring or local addressing atoms. The key is to use a three-species Rydberg atom array for the complete stabilizer cycle, including syndrome extraction, coherent correction, and ancilla reset, under global, species-selective
laser pulses. Numerical simulation of a $4 \times 4$ rotated toric code shows a longer
qubit lifetime when the physical error rate is below a pseudo-threshold $p^\star \approx 0.034$. The scheme offers a concrete, hardware-efficient route to topological
quantum memory in neutral-atom platforms.
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All authors Han Wang, Yusheng Zhao, Xiuhao Deng, Jinguo Liu
Quantum Physics
Experimental Tabletop Petz recovery of a photonic qubit
Hui Li, Jinyan Chen, Yue Pan et al. (+4 more)
The
quantum information lost in open evolutions cannot be fully recovered, but partial recovery is possible. The Petz recovery map guarantees almost optimal recovery, notably if the chosen reference state is close to the real one. This map has been widely used in theoretical studies, but has been the object of only a handful of experimental realisations, typically under a single fixed noise model. In this work, we describe and implement the Petz recovery map for a versatile class of
qubit channels with tunable decoherence and dissipation. The setup we realize is also the first experimental example of “tabletop reversibility”: for a good range of choices of the reference state, the Petz recovery map can be implemented with the same devices as the forward dissipative
evolution, whose effect it is partially undoing. Our results demonstrate that the Petz recovery map can be resource-efficiently realized without requiring complex ancillary resources, providing a feasible pathway for mitigating information loss in
quantum systems.
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All authors Hui Li, Jinyan Chen, Yue Pan, Liang Xu, Minjeong Song, Valerio Scarani, Lijian Zhang
Quantum Physics
Dark state spectroscopy in nonlinear waveguide quantum electrodynamics
Shay Nadel, Amir Sivan and Aviv Karnieli
Quantum systems face a fundamental trade-off: they must remain decoupled from the environment to maintain long coherence times, yet they require interactions with the environment to be accessible for measurement. As a
prime example, emitter arrays coupled to waveguides facilitate collective modes that, owing to interference, can suppress radiation into the waveguide. While complete destructive interference creates perfectly dark states with infinite lifetimes, their inherent decoupling makes them unmeasurable in standard waveguide
quantum electrodynamics. Consequently, current approaches must rely on system non-idealities that permit measurement but limit the coherence times. In this work, we lift this limitation by proposing the use of weakly squeezed light generated in \{chi}(2) nonlinear waveguides for the spectroscopy of completely dark states. We show that the fluorescence spectrum probes transitions between the dressed dark states of the emitter array. This work paves the way towards the measurement and control of dark states, with applications for robust
quantum memories, computation, and communication.
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Quantum Physics
Super-Heisenberg Non-Equilibrium Quantum Sensing with Waveguide-Coupled Emitters
Mohammad B. Arjmandi
We explore an array of
quantum emitters as non-
equilibrium probes, coupled to a one-dimensional photonic waveguide, aiming to estimate its properties such as wave number which encodes the waveguide frequency and dispersive characteristics. By considering transient dynamics following initial excitation, we show that the
quantum Fisher information (QFI) can be significantly enhanced through careful emitter positioning. For two-emitter probes, optimal spacing stabilizes populations and coherences in the single-excitation subspace, suppressing super radiant decay and extending both the magnitude and longevity of QFI. Randomized emitter configurations also reveal that vanishing waveguide-mediated cross decay maximizes both achievable sensitivity and the temporal duration over which information about the parameter remains accessible. Extending to multipartite probes, we demonstrate that the maximum QFI and its temporal integral scale with system size, exceeding the Heisenberg limit for all positioning strategies. Our results highlight the potential of waveguide-coupled emitter arrays as versatile
quantum sensors, where collective radiative dynamics can be harnessed to achieve tunable, long-lived, and enhanced precision.
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Quantum Physics
Controlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu$^{3+}$ complex
Evgenij Vasilenko, Vishnu Unni Chorakkunnath, Barbora Brachnakova et al. (+4 more)
Molecular rare-earth-ion complexes offer unique opportunities for
quantum technologies by combining the intrinsic coherence properties of rare-earth ions with chemically tunable molecular environments. A crucial capability is the realization of multi-
qubit architectures with defined
qubit couplings to enable two-
qubit quantum gates. Here, we investigate the optical coherence properties and excitation-induced interactions of two Eu$^{3+}$-based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu$^{3+}$ ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and
photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times $T_{2,\text{o}}$ of up to 9 $\mu$s. As a key step toward scalable multi-
qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the $\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_0$ transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable
quantum technologies.
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All authors Evgenij Vasilenko, Vishnu Unni Chorakkunnath, Barbora Brachnakova, Nicholas Lester Jobbitt, Senthil Kumar Kuppusamy, David Hunger, Mario Ruben