Quantum control papers on arXiv, selected and classified automatically by QOC. https://llorebaga.github.io/QOC/topics/feedback-control/ 2026-10-08T00:00:00Z QOC https://arxiv.org/abs/2610.11607 2026-10-08T00:00:00Z 2026-10-08T00:00:00Z Jinhao JiaYingyu LiRan LiangQinghua CaiXin WenMei Zhang

We propose a scheme for generating and controlling remote nonreciprocal magnon--magnon entanglement in a cavity magnonic system with coherent feedback.The two microwave cavities are driven by independent single-mode squeezed vacuum fields, with a rotating YIG sphere in the left cavity and a stationary one in the right cavity. For a fixed rotation direction, reversing the bias magnetic field applied to the rotating sphere changes the sign of the Barnett shift, leading to nonreciprocal magnon--magnon entanglement. By tuning the cavity detuning and feedback phase, entanglement can be generated for one direction of the bias magnetic field while it vanishes for the opposite direction, yielding complete nonreciprocity. Coherent feedback enhances the entanglement at an appropriate reflectivity, whereas stronger feedback can reduce it. The relative squeezing phase provides additional control, and an appropriate choice of unequal squeezing factors improves the entanglement at fixed total input photon occupation. With the squeezed inputs kept fixed, moderate feedback also increases the critical temperature for magnon entanglement.Our results provide a controllable approach to preparing remote nonreciprocal quantum correlations in cavity magnonic systems.

https://arxiv.org/abs/2609.39415 2026-09-30T00:00:00Z 2026-09-30T00:00:00Z Leon C. CamenzindIk Kyeong JinAkito NoiriKenta TakedaTakashi NakajimaTakashi KobayashiSeigo Tarucha

Scalable quantum processors require repeated qubit initialization throughout large arrays. In semiconductor spin qubits, fast initialization commonly relies on local reservoir access or measurement-based feedback, requiring dedicated infrastructure that becomes increasingly difficult to distribute as processors scale. Here, we demonstrate reservoir- and measurement-free initialization of a silicon spin-qubit pair in an industrially fabricated Si/SiGe quantum-dot device using a fixed sequence of microwave and baseband pulses. Odd spin-parity states relax to the singlet charge state, whereas blocked even spin-parity states are microwave-driven through the triplet manifold and subsequently converted to the singlet by singlet-triplet mixing and charge hybridization. Repeated cycles produce the singlet-associated charge outcome with a median probability of 99.4% across the sampled preparation states, while exchange spectroscopy independently verifies mapping to the target $|\uparrow\downarrow\rangle$ operational state. Microwave spectroscopy and time-domain measurements identify the dark-state-limited single-cycle transfer and the subsequent blockade-lifting dynamics that set the initialization time scale. The demonstrated pumping sequence uses approximately $12\,μ\mathrm{s}$ of microwave bursts and mixing dwells, while we project sub-microsecond initialization under improved device conditions. These results establish fixed-sequence microwave initialization as a scalable control primitive for semiconductor spin-qubit processors, based on singlet-triplet physics that can be adapted to platforms with suitable Pauli-blockade transitions.

https://arxiv.org/abs/2609.26949 2026-09-22T00:00:00Z 2026-09-22T00:00:00Z Ming LiJunYan LuoGeorg Engelhardt

Non-Poissonian photon statistics are usually traced to nonlinear interactions, nonclassical states, or engineered reservoirs. Here we show that measurement-based feedback can be alternatively deployed to control the Poissonian character of the measurement statistics in an externally-driven linear cavity. Each photon detection triggers a weak displacement operation whose phase determines how the feedback interferes with the coherent cavity field, resulting in a significant cavity response. Full counting statistics and two-time photon correlations show that the feedback phase controls the sign and strength of the induced correlations, continuously tuning the emission statistics between super- and sub-Poissonian regimes. At generic phases, the Mandel parameter responds linearly to the feedback amplitude, whereas at specific phases, the leading response becomes quadratic. The same phase dependence is also found in higher-order cumulants of the emission statistics. These results establish measurement-based feedback as a route to controlling photon statistics in otherwise linear driven-dissipative systems.

https://arxiv.org/abs/2609.22821 2026-09-19T00:00:00Z 2026-09-19T00:00:00Z Yang XiaoJin Wang

Generally, quantum heat engines driven by Gibbs reservoirs achieve their maximum work and efficiency only in quasi-static processes, a constraint that hampers practical applications due to the resulting vanishing power output. To address this fundamental limitation, we propose a finite-time quantum Otto information engine (OIE) driven by a Maxwell's demon paired with a single Gibbs reservoir. We demonstrate that the demon's measurement and feedback control can preserve quantum coherence to extract coherent work, while harnessing quantum internal friction as a work source. Consequently, the OIE can produce work by only modulating the eigenstates of the Hamiltonian, and surpass the quasi-static limits of its conventional counterpart in the work output and the efficiency, which accounts for the energetic cost of the demon. Notably, the OIE can achieve near-perfect efficiency alongside positive work extraction in the rapid-driving regime.

https://arxiv.org/abs/2609.13988 2026-09-12T00:00:00Z 2026-09-12T00:00:00Z Vedran Vujnović

We study measurement-based feedback control of a damped harmonic oscillator, motivated by optomechanical and interferometric position measurements, in which feedback is synthesized from a displacement measurement. For a band-limited derivative controller, the loop shapes the mechanical susceptibility and reinjects measurement noise, creating a tradeoff between stabilization and noise amplification. We present a compact classical formulation that isolates this tradeoff in the closed-loop displacement spectrum and identifies the parameters governing resonance suppression versus noise-driven actuation. We then extend the same topology to a quantum-limited position detector, enforcing the imprecision-backaction constraint and introducing finite measurement bandwidth that regularizes high-frequency noise reinjection and adds phase lag in the feedback path. In the high-$Q$ regime we obtain an analytic design rule for the optimal feedback gain at fixed controller and measurement bandwidths and compare it with full-spectrum calculations. The comparison shows that a high mechanical quality factor alone does not guarantee quantitative agreement: the accuracy of the near-resonant approximation depends strongly on the controller and measurement bandwidths. Because finite measurement bandwidth changes both the magnitude and phase of the feedback loop, the analysis separates the near-resonant damping contribution from imprecision-noise reinjection and shows that positive near-resonant feedback damping requires the product of the controller cutoff and measurement bandwidth to exceed the square of the mechanical resonance frequency. Finally, we present a dimensionless map of the gain-optimized high-$Q$ occupation and compare its predictions with full-spectrum optimization. All analytical gains evaluated in this map satisfy the full closed-loop stability criterion for these parameters.

https://arxiv.org/abs/2609.06111 2026-09-05T00:00:00Z 2026-09-05T00:00:00Z Weichao LiangJing ZhangGaoyue GuoDaoyi Dong

This review develops a control-theoretic perspective on measurement-based feedback for continuously monitored open quantum systems, with the main analysis focused on finite-dimensional systems governed by diffusive stochastic master equations. We introduce the relevant state-space, invariant-subspace, and quantum non-demolition structures, interpret quantum filtering as nonlinear observer dynamics, and review open-loop asymptotics, filter stability, state-feedback stabilization, robustness, and reduced-order observer-based control. Particular emphasis is placed on a recurrence--contraction framework, in which Hamiltonian feedback removes non-target invariant obstructions while measurement-induced dynamics provide local exponential contraction. Although the detailed analysis is developed for finite-dimensional diffusive models, the underlying measurement--estimation--feedback architecture is relevant across a broad range of quantum platforms. We further discuss implementation challenges and open problems involving scalable estimation, sampling and delay, adaptation, hybrid and non-Markovian dynamics, practical stability, optimal control, and learning-based design. By organizing these developments around invariance, estimation, recurrence, and contraction, the review provides a tutorial bridge between measurement-based quantum feedback and nonlinear stochastic control.

https://arxiv.org/abs/2609.03097 2026-09-02T00:00:00Z 2026-09-02T00:00:00Z Masayuki OhzekiAndrew N. Jordan

Schrödinger bridges and entropic optimal transport are usually formulated as stochastic interpolation problems between initial and final probability distributions. In their computational form, the bridge potentials are obtained by Sinkhorn or iterative proportional fitting, and are often regarded as auxiliary scaling functions. We show that for continuously monitored quantum systems, these potentials acquire a direct measurement-theoretic meaning. The mathematical structure of conditional quantum trajectory theory induces a Fokker--Planck equation on quantum state space. Conditioning this diffusion on a terminal distribution or a terminal measurement effect produces a Doob/Sinkhorn potential whose directional derivative along a unitary control vector field is the imaginary part of a generalized weak value. The same construction connects the Schrödinger-bridge viewpoint to the optimal-path framework for continuously monitored trajectories: the backward bridge potential plays the role of an effect-like costate, and its weak-value directional derivative gives the local control signal. By specifying the desired endpoint distribution, the induced drift produced by the Schrödinger bridge solution is the control solution that minimizes the quadratic cost feedback law, guiding the distribution to its desired endpoint. We quantify the control score of the available control Hamiltonian as a logarithmic directional derivative of the bridge potential, or an imaginary weak value. Three explicit examples identify weak measurement as a natural entropic regularization mechanism for quantum state transport and gives a route from Sinkhorn scaling to quantum feedback and Hamiltonian control synthesis for practical optimal control.

https://arxiv.org/abs/2609.02841 2026-09-02T00:00:00Z 2026-09-02T00:00:00Z Jinyi DuEn Teng LimXingjian ZhangHongwei GaoGeorge F. R. ChenDawn T. H. TanAlexander Ling

Fibre-based quantum networks distributing polarization entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into quantum channels. Entangled-photon-based feedback avoids this noise, but typically interrupts the target entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed quantum networks.

https://arxiv.org/abs/2609.02518 2026-09-02T00:00:00Z 2026-09-18T00:00:00Z Kseniia VodenkovaAndrew PocklingtonFan YangAndrew LingenfelterAashish A. ClerkHannes Pichler

We present a scheme to efficiently simulate nonreciprocal many-body spin models using a non-Markovian open system. Our scheme utilizes a single quantum emitter coupled to a waveguide mode that is fed back to the same emitter after a time delay. With this coherent delayed feedback, an effective nonreciprocal interaction can be engineered, wherein the emitter at earlier times affects itself at later times, creating a scalable spin chain. We show that all the steady-state quantities of such spin models can be accessed through measurement of the emitter and the output field. Furthermore, using classical feedback that resets the emitter, quench dynamics from arbitrary product states can be simulated. We demonstrate striking features of nonreciprocal models, such as the Liouvillian skin effect, anomalous relaxation dynamics, and quasi-long-range order of output photons. Finally, we show that the proposal is amenable to experimental realization and robust to imperfections. Our work establishes a feasible way to scale up a nonreciprocal many-body system, and provides a new route towards generation of exotic states of light.

https://arxiv.org/abs/2608.28506 2026-08-28T00:00:00Z 2026-08-28T00:00:00Z Marc Cuenca-LaràsMing LiCarlos Navarrete-BenllochGermán J. de Valcárcel

Phase-space methods are powerful tools for the treatment of Markovian open quantum systems: they map the reduced dynamics of a system S, in interaction with an environment E, exactly onto Langevin equations for c-number stochastic variables, as opposed to Heisenberg-Langevin equations for operators. Langevin equations provide analytical insight in key regimes and excel at handling strong nonlinearities and couplings, where other methods often falter. Extending phase-space methods to non-Markovian dynamics, however, has remained a long-standing challenge. Here we address this gap by applying phase-space representations to the full S+E system; integrating out the environmental degrees of freedom then yields a general Langevin framework for S that incorporates both deterministic and stochastic contributions from E. Normally ordered representations, such as the Glauber-Sudarshan P representation and its positive variant due to Drummond and Gardiner, lead to Langevin equations in which (i) non-Markovian effects emerge exclusively in the deterministic terms, via a memory kernel, and (ii) noise contributions vanish when E is initially in the vacuum state. To demonstrate the power of this framework, we address the paradigmatic problem of delayed coherent feedback, in which the system is driven by its own past state, and study its impact on the laser linewidth: we recover the narrowing observed well above threshold and predict an enhanced narrowing just above it. Crucially, the number of stochastic variables scales linearly with the system size, making the framework suitable for problems ranging from a few degrees of freedom to genuinely many-body systems. This opens the way to the systematic study of non-Markovian driven-dissipative quantum systems using the same analytical and numerical tools that have long made phase-space methods so successful in the Markovian regime.

https://arxiv.org/abs/2608.25101 2026-08-25T00:00:00Z 2026-08-25T00:00:00Z Mehri EbrahimiYunhu HuangAli RashediJohn P. Davis

Lithium ferrite (LiFe) is a promising material for cavity magnonics because its large spin density enables strong coupling between magnons and microwave photons. Its potential for cavity magnomechanics, however, has remained unexplored. Here, we observe magnomechanical interactions in a single-crystal LiFe sphere using coherent microwave feedback to suppress dissipation of the cavity--magnon polariton. In the absence of sufficient feedback, the narrow mechanical response is difficult to resolve against the much broader polariton background. Increasing the feedback gain reduces the polariton linewidth and correspondingly increases the magnomechanical cooperativity, revealing a clear magnomechanically induced transparency feature. In the measurements presented here, the effective upper-polariton linewidth is reduced from $3.53~\mathrm{MHz}$ without feedback to $5.8~\mathrm{kHz}$ in the presence of feedback, while the measured cooperativity increases from $C=1.9\times10^{-3}$ to $C=0.15$. These measurements provide, to our knowledge, the first observation of cavity magnomechanics in LiFe and demonstrate coherent feedback as a practical route for accessing weak interactions that would otherwise be obscured by dissipation.

https://arxiv.org/abs/2609.27908 2026-08-21T00:00:00Z 2026-08-21T00:00:00Z Eran Kopel

A quantum feedback loop that returns information from a forward simulation to an earlier internal time induces a completely positive trace-preserving map on a message register; iterating that map eventually destroys its ability to carry entanglement. The entanglement-breaking index N = n_EB(Phi), the round at which this happens, is studied here along two axes: the interaction and the bath. On the interaction axis we prove that strict contraction onto a full-rank fixed point forces N finite in every dimension, via an explicit separable ball around the limiting Choi state; the qubit closed form overshoots the exact answer by the factor ln(2 sqrt 3) / ln 3 = 1.1309... on isotropic unital channels, and across 3997 sampled channels index and stability gap are one clock, N [1 - rho(A)] in [0.60, 1.50]. On the bath axis, replacing the zero-temperature ancilla by a thermal one at polarisation p = tanh(hbar omega / 2 k_B T) makes every channel quantity an exact quadratic polynomial in p, and the same one-sided certificates (no eigensolver) establish N = 3 at the reference circuit at every temperature, uniformly on a parameter box. The infinite-temperature index is an architecture floor with closed form and exact measure (46.427% of circuits break entanglement in one round however hot the bath). The floor is not a bound: certified circuits dip below it in interior temperature windows, unit-quantised in depth; over 4.1 million circuits their rate follows a Gaussian cutoff in epsilon = 1 - ||A||_2 with exponent 2.02 [1.90, 2.16], and 12 million more certified circuits show valleys surviving to epsilon = 0.125. Certified endpoints make comparisons exact: temperature moves the index by at most 21/14 = 3/2; the interaction moves it by 1213/3.

https://arxiv.org/abs/2608.16608 2026-08-17T00:00:00Z 2026-08-30T00:00:00Z Luc BoutenJohn Gough

Quantum Feedback Network Theory (also known colloquially as the SLH-framework) is a powerful tool for modeling systems in quantum optics. This paper describes a method for discretising SLH-networks such that they can be implemented on a quantum computer. Building on a discretisation theorem for quantum stochastic differential equations, we establish strong convergence, uniformly on compact time intervals, of the discretised system to the continuous SLH-network as the discretisation mesh goes to zero.

https://arxiv.org/abs/2608.15715 2026-08-16T00:00:00Z 2026-08-16T00:00:00Z Priyanshi SinghKrishna Bhatia

Quantum feedback control requires acting on noisy continuous measurement records without direct access to the underlying quantum state. We propose Kraus-Parameterized Belief Reinforcement Learning, a pipeline in which a recurrent encoder, constrained to the Stiefel manifold, produces density-matrix estimates that are guaranteed positive-semidefinite and trace-normalized by construction, embedding quantum state geometry directly into the learning loop. A Proximal Policy Optimization (PPO) actor then maps these physically valid belief states to continuous control actions. On a simulated continuously monitored qubit, the resulting policy achieves stable feedback control, maintaining a measurement-conditioned belief fidelity of approximately 0.77-0.80 and exhibiting substantially lower return variance than a parameter-matched LSTM-history baseline across both nominal and out-of-distribution conditions. Although gains in raw target fidelity are modest, the geometric constraint guarantees a physically valid, interpretable belief representation and yields markedly more stable control under measurement inefficiency and abrupt dynamics switches. These results indicate that physics-informed neural memory is a practical inductive bias for reliable quantum feedback control.

https://arxiv.org/abs/2608.13764 2026-08-13T00:00:00Z 2026-10-04T00:00:00Z Eran Kopel

A feedback loop that returns a message into the process it describes defines a quantum channel on the message. We ask when such a loop preserves entanglement with a reference, that is, when its channel is not entanglement breaking. For any qubit loop, the negativity of the Choi state is at most half the fidelity between the states that the two values of the message leave in the discarded registers, in any basis. This channel analogue of the duality between fringe visibility and which-way information is attained by pure dephasing, and a perfect record of the message in the discarded registers makes the loop entanglement breaking. We then solve a three-qubit loop in which the message steers a future event, a probe reads the event, and a partial swap feeds the probe back. Every partial feedback is a strict contraction, and the loop is entanglement breaking exactly when an explicit trigonometric polynomial in its coupling angles is not positive. A partial record of the message in the future event can restore entanglement that the feedback alone destroys. Stability, a coherent fixed point, information about the future event and entanglement preservation coexist on about three quarters of the parameter space.

https://arxiv.org/abs/2608.06875 2026-08-07T00:00:00Z 2026-08-07T00:00:00Z Heyue LiYanshu GuoQichun LiuTie-Fu LiZhihua WangHanjun Jiang

Scalable quantum processors require control, readout, feedback, calibration, and error correction to coexist under bounded latency and shared-resource constraints, whereas existing platforms typically optimize only a subset of these capabilities. This article presents QCORE (Quantum-Control-Oriented Real-Time Execution), a QPU-side digital control reference architecture positioned between the Host and a platform-specific analog/mixed-signal front end. QCORE separates task management, shared resources, hard-real-time execution, and long-timescale services into four hardware partitions. A fast-result sideband closes same-round feedback, a Measurement Packet provides a traceable measurement and service interface, and a common service-control skeleton, Tile-local QEC, and versioned safe-point commit organize calibration, error correction, and long-term state updates. Transaction-level, event-driven, and quantum-behavioral models are used for evaluation. At a background load of 0.8, the $P_{99}$ latency of the shared Measurement Packet/Event feedback path is $(1.984\pm0.004)L_{\max}$. Closed-loop operation reduces the mean frequency error by $83.2\%\pm0.8\%$ and lowers the state-assignment error at maximum readout drift from $10.39\%\pm0.54\%$ to $5.37\%\pm0.29\%$. No unsafe acceptance or mixed-version observation is observed in 100,000 configuration transactions, and Tile-local QEC reduces modeled global-boundary demand and yields a $2.08\times$ capacity-normalized scaling estimate.

https://arxiv.org/abs/2608.04779 2026-08-05T00:00:00Z 2026-08-05T00:00:00Z Salman Sajad WaniÁlvaro Perales-EceizaSaif Al-KuwariMir Faizal

Programmable quantum control systems increasingly rely on predictive modules for certification, real-time feedback, and autonomous decision-making. This development raises a fundamental question: can self-analyzing quantum platforms universally predict their own experimental outcomes? Wolpert formalized a general impossibility of universal self-prediction. Here we translate that limitation into an explicit laboratory obstruction that can be realized with finite resources. We consider settings with programmable quantum control in which predictors can be embedded as subroutines within the experiments they analyze. Our diagonal construction uses Kleene's recursion theorem to transform any deterministic bounded-time predictor into a reversible protocol encoding its own specification. The resulting protocol invokes the predictor on that specification and deterministically produces a classical pointer record that contradicts the forecast. For efficient predictors, the compilation has polynomial overhead and admits concrete physical realizations as a fault-tolerant quantum circuit and as a minimal Mach-Zehnder interferometer. These realizations connect computability-theoretic self-reference to programmable quantum hardware. We also introduce and formally define Gödel-safe architectures. These architectures block the forbidden causal path from the protocol description to an actuator that can affect the pointer during the same run. We analyze their implications for real-time quantum error correction, including the resulting expressiveness trade-offs. As quantum control loops grow in computational expressiveness, the limits of self-reference cease to be mere mathematical abstractions and become explicit engineering constraints for the reliable operation of autonomous quantum technologies.

https://arxiv.org/abs/2607.28702 2026-07-30T00:00:00Z 2026-08-19T00:00:00Z Jinfeng WeiYingying HongDehua LiuXi WangZhe WangYiling ZhanKaifeng CuiJintao BuJianhui WangLei-Lei YanGang Chen

Harnessing finite-time nonadiabatic transitions that are conventionally associated with quantum inner friction for useful work extraction remains an open experimental challenge. Here we address this issue by introducing and experimentally realizing an innovative measurement-feedback quantum information engine model in the trapped 40Ca+ ion system, in which the projective measurement replaces the hot reservoir as a nonthermal energy source and the feedback control conditionally steers the system through either unitary compression-expansion strokes or thermalization. We experimentally show that the repeated feedback cycle converges to a stable operating regime with a resolved energetic balance, and, by controlling the measurement angle and stroke duration, measurement-induced coherence and the finite-time nonadiabatic contribution can enhance work extraction and raise the efficiency above the corresponding Otto benchmark. The experimental results further show that, over finite ranges of stroke durations, the efficiency and intrinsic cycle power can increase simultaneously. Our experiment establishes a route toward information-to-work quantum engines that convert finite-time irreversibility into performance-enhancing resources.

https://arxiv.org/abs/2607.26480 2026-07-29T00:00:00Z 2026-07-29T00:00:00Z Hyeongyu JangJaemin ParkYounguk SongJinwoong KimHanseo SohnLucas E. A. StehouwerDavide Degli EspostiGiordano ScappucciDohun Kim

Fast measurement, signal processing, and accurate estimation of Hamiltonian parameters are essential for feedback control in quantum-classical interface circuitry. However, existing frequentist and Bayesian inference methods typically require a large number of measurements to achieve the accuracy needed to mitigate qubit decoherence. Consequently, feedback control of semiconductor qubits has largely been limited to single-qubit frequency stabilization, whereas two-qubit parameter stabilization remains experimentally unexplored. Here, we demonstrate a real-time feedback framework based on sequential Monte Carlo estimation using one bit of data from a single-shot measurement. Using a four-qubit semiconductor quantum dot device, we rapidly estimate individual qubit frequencies, yielding an approximately twofold increase in coherence time compared with a conventional Bayesian strategy. Moreover, sequential two-qubit parameter estimation using two bits of data enables stabilization of qubit-qubit coupling, allowing both quasi-static frequency drift and exchange-interaction noise to be estimated and suppressed. By shortening the time required for precise parameter estimation, these results demonstrate the importance of the synergistic development of classical and quantum electronics for building robust and scalable quantum technologies in fluctuating environments.

https://arxiv.org/abs/2607.23681 2026-07-26T00:00:00Z 2026-07-26T00:00:00Z Wei-En LinLi-Chieh HsiaoChen-Hsun MaErh-Hsiang YehWei-Lun PengHsi-Sheng GoanCen-Shawn WuYueh-Nan ChenYung-Fu ChenChung-Ting KeChii-Dong Chen

High-fidelity and rapid qubit readout is essential for superconducting quantum processors, typically realized through the quantum non-demolition (QND) dispersive interaction within a qubit-resonator architecture. However, the achievable readout speed and fidelity are fundamentally limited by measurement-induced state transitions (MIST). For a transmon qubit, MIST is highly sensitive to the offset charge $n_g$ due to the charge dispersion of its higher-lying energy levels. In this work, we systematically investigate $n_g$-dependent MIST dynamics governed by the diabaticity and symmetry of pulse shaping within a charge-sensitive transmon architecture. We engineer fast-load and fast-clear pulses that effectively suppress resonator photon overshoots, thereby demonstrating a highly practical strategy to mitigate MIST without requiring complex waveforms or real-time feedback. Utilizing active gate-voltage control and rapid feedback, the measurement-induced transition probability is precisely mapped against $n_g$ and the steady-state resonator photon number, exhibiting strong agreement with numerical Floquet branch analysis. Ultimately, we evaluate the $n_g$-averaged total error probabilities for both readout and post-readout stages, verifying that a straightforward three-step pulse scheme consistently minimizes overall readout errors. Within the framework of large-scale superconducting quantum processors, this practical, hardware-free approach inherently offers a better trade-off between the readout signal-to-noise ratio and QND preservation.

https://arxiv.org/abs/2607.16885 2026-07-18T00:00:00Z 2026-07-18T00:00:00Z Evan D. Hall

Optical parametric amplification is a crucial technology for the production of continuous-wave squeezed vacuum, which is now applied to gravitational-wave interferometry and other highly sensitive measurements of optical phase. The amplifiers employed in gravitational-wave detection are resonantly enhanced with optical cavities, which introduces nontrivial dynamics to their operation and requires a frequency-domain model of the relations between optical fields. In this work, input-output relations between optical fields entering, circulating in, and exiting a model amplifier are solved directly to produce frequency-domain relations that fully incorporate the cavity dynamics. We also provide low-order zero-pole-gain expansions for these relations to facilitate their analysis from a feedback control perspective. In the limit of small cavity decay rates, our relations are shown to reduce to relations previously derived under a Hamiltonian formalism. Expressions for amplifier figures of merit and for squeezed quadrature variances are also analyzed. Along the way, we examine how information about the amplifier performance can be extracted from the frequency dependence of the input-output relations, providing characterization methods that are complementary to methods that examine the static amplifier behavior.

https://arxiv.org/abs/2607.14223 2026-07-15T00:00:00Z 2026-07-15T00:00:00Z Hang DongJie RenAndrew HallamHan WangZhengyi CuiYiren ZouJunlin WangHekang LiQiujiang GuoZhen WangLei YingZlatko Papić

Understanding how complex systems transition between order and chaos is a central challenge of nonequilibrium physics. While weak perturbations of classical integrable systems give rise to a mixed phase space of coexisting regular and chaotic trajectories, analogous behavior in interacting quantum many-body systems has remained elusive. Here we develop and experimentally implement a hybrid quantum-classical feedback protocol that autonomously discovers and stabilizes long-lived regular trajectories in a superconducting quantum processor. Each iteration combines short-time quantum evolution with classical optimization that projects the dynamics back onto a low-entanglement variational manifold, effectively distilling coherence from chaotic evolution. The stabilized trajectories reveal a quantum many-body mixed phase space emerging from nonlinear variational dynamics, without a direct analogue in classical or few-body quantum systems. Our results establish a versatile framework for algorithmic discovery and control of coherent dynamics previously inaccessible to experiment.

https://arxiv.org/abs/2607.12581 2026-07-14T00:00:00Z 2026-07-14T00:00:00Z Radhika JoshiYuli V. NazarovMohammad H. Ansari

We show that driven qubit junctions enable programmable control of physical entropy transport, with entropy conductance governed by quantum dynamics rather than by reservoir parameters alone. By comparing two simple quantum architectures -- a driven single-qubit junction and a driven two-qubit junction -- we find that the two-qubit junction enhances entropy transfer while requiring substantially lower driving power than its single-qubit counterpart. We further reveal two non-intuitive effects in both junctions: a sizable coherent contribution to the entropy current that emerges only under resonant driving, and negative differential entropy conductance, where increasing the thermal bias suppresses entropy flow into the probe reservoir. These results identify quantum logic architectures as programmable devices for entropy transport and suggest routes toward quantum feedback control, reservoir protection and refrigeration in driven quantum circuits.

https://arxiv.org/abs/2607.12264 2026-07-14T00:00:00Z 2026-09-28T00:00:00Z Hayato YunokiYoshihiko Hasegawa

Kinetic uncertainty relations impose a fundamental constraint on counting precision by requiring large dynamical activity to suppress fluctuations. For $N$ independent constituents, the activity is additive and scales as $O(N)$, so the corresponding precision bound improves only as $O(1/N)$. A central question is whether many-body quantum systems can instead achieve Heisenberg scaling, with relative variance suppressed as $O(1/N^{2})$. Such scaling could enable substantially enhanced precision in quantum devices, including quantum clocks. Superradiance appears to provide a natural route because its collective emission activity can scale as $O(N^2)$. We show, however, that superradiance alone fails to achieve Heisenberg scaling because this enhanced activity is transient. We then demonstrate that jump-conditioned quantum feedback stabilizes the superradiant high-activity regime and converts this collective enhancement into Heisenberg scaling. We establish this mechanism analytically using a many-body kinetic uncertainty relation and mean-field analysis, and demonstrate through quantum-trajectory simulations that the actual relative counting fluctuation scales as $O(1/N^{2})$.

https://arxiv.org/abs/2607.09414 2026-07-10T00:00:00Z 2026-07-10T00:00:00Z D. A. PershinD. A. KumpilovA. E. RudnevG. E. SubbotinA. M. IbrahimovI. S. CojocaruV. A. KhlebnikovI. A. PyrkhP. A. AksentsevS. A. KuzminA. D. RaskatovA. K. ZykovaV. V. TsyganokA. V. Akimov

Cold neutral atoms is a powerful tool for many experiments ranging from frequency standards and sensing to quantum simulations. Sensitive experiments often demand transferring cold atoms from one vacuum chamber to the other with better optical access and vacuum. In case the transfer is done with a beam waist controlled by a focus-tunable lens, repeatability of the transfer as well as stability of the final position can be below the experimental demands. Here we implement the scheme for stabilization of the axial waist position of the optical dipole trap in the whole range of transfer (38 cm) using speckle patterns, which provides enough stability to achieve Bose-Einstein condensation of around 4*10^4 thulium atoms in the trap formed by the transport beam and the other beam in the target vacuum volume

https://arxiv.org/abs/2607.09118 2026-07-10T00:00:00Z 2026-07-29T00:00:00Z Jiaxin LiuZuoxian WangFeng LiDanyue Ma

Continuous quantum feedback uses time-resolved measurement records to steer monitored systems toward pure states. Yet how the information available to a controller determines the ultimate purification speed remains unresolved. We establish this relation for a qubit under fixed-spectrum Hermitian monitoring with detector loss, obtaining the exact long-time impurity-moment spectrum optimized over causal basis controls at each horizon. Rare records with nearly canceled evidence then make all moments from half order upward decay at the Bhattacharyya information rate between two quantum nondemolition record laws. Aligned quantum nondemolition monitoring preserves that binary distinguishability and attains the limit, while complete detection restores an order-dependent branch. The mechanism extends to higher dimensions, where an attainable rank-two ceiling lies above the full-rank qutrit upper bound over a finite moment interval, establishing retained record distinguishability as a purification resource.

https://arxiv.org/abs/2607.07372 2026-07-08T00:00:00Z 2026-08-18T00:00:00Z Xiaorang GuoKun QinYanbin ChenCarsten TrinitisMartin Schulz

The Quantum Control Processor (QCP) bridges the gap between compiler toolchains and control electronics, and is responsible for translating compiled quantum circuits into executable instructions that directly manipulate qubits and handle measurement feedback. However, existing designs rely primarily on customized instruction sets, limiting design reuse and requiring significant effort to build supporting toolchains. Furthermore, efficiently addressing qubits and scheduling operations in highly scalable scenarios remains a critical challenge. In this work, we present a vectorized quantum control approach built upon the RISC-V Vector (RVV) engine with a quantum-oriented extension. Leveraging the high parallelism of RVV, our approach can address up to 128 qubits in a single instruction. We also embed parameterized rotation information into the instruction set, enabling dynamic tuning of gate rotations in hybrid quantum-classical programs. To support mid-circuit measurements, we design a hardware-based halt-resume protocol that resumes pipeline execution within 80 $ns$ of receiving the measurement result. Comprehensive evaluation using both RISC-V toolchains and FPGA prototypes demonstrates that our design achieves up to 2.52$\times$ speedup over the baseline in program execution time, with excellent scalability.

https://arxiv.org/abs/2606.31321 2026-06-30T00:00:00Z 2026-06-30T00:00:00Z Hendra I. Nurdin

Quantum Markov models have been successfully used to accurately model various physical quantum systems in fields such as quantum optics, optomechanics and superconducting circuits and they provide the basis for (measurement-based) quantum feedback control. However, the quantum Markov assumption is a strong one and it is not expected to hold for general quantum systems of interest. The projection operator approach is one approach that has been developed to model non-Markovian quantum systems by considering its embedding in a larger Markovian quantum system, but mainly in the context of quantum master equations for the dynamics of the unmonitored reduced quantum state of a quantum system. This approach was recently adapted for continuously measured non-Markovian quantum systems, which enables open-loop control but did not yet consider the presence of feedback of the stochastic measurement record, deriving non-Markovian SDEs for the evolution of the projected state of the Markovian embedding. This paper generalizes these stochastic equations to the setting of stochastic feedback based on the continuous-measurement record and shows that the equations take the same form but that previously deterministic terms become stochastic ones which depend on the measurement record, as would be intuitively expected. The stochastic equations are obtained for a generalized class of measurements that includes continuous (possibly adaptive) homodyne and photon counting measurements.

https://arxiv.org/abs/2606.30765 2026-06-29T00:00:00Z 2026-06-29T00:00:00Z Matthew L. PetersGuoqing WangDavid C. SpieringsNiv DruckerMeng-Wei ChenAudrey BartlettIsaac L. ChuangVladan Vuletić

Real-time feedback control of quantum systems is often limited by partial observations, nonlinear dynamics and measurement noise, which make accurate model-based controllers difficult to design. Here we show that deep reinforcement learning can cool the motion of a single neutral atom coupled to a high-finesse optical cavity using only the continuously monitored cavity transmission. We first train the controller in simulation and then transfer it to the experiment, where online fine-tuning adapts it to unmodeled experimental dynamics. The learned policy damps the atom's motion in real time and achieves a cooling time constant of 388 +/- 14 microseconds, corresponding to only two motional periods in the trap. It also outperforms a standard linear differentiator controller in cooling speed while maintaining comparable atom retention over a broad range of operating conditions. These results establish reinforcement learning as a practical strategy for feedback control in quantum-limited experiments where compact analytical models are incomplete.

https://arxiv.org/abs/2606.25196 2026-06-23T00:00:00Z 2026-06-23T00:00:00Z Gökhan ElmasIgor LitvinPaul KohlJanis Nötzel

Achieving both reconfigurability and stable output signals is a critical challenge in the development of integrated photonic circuits for large-scale optical quantum information processing. This has led to the creation of multimode photonic processors, also known as reconfigurable multimode interferometers, which have wide-ranging applications in quantum and classical information processing. However, maintaining phase stability in multi-port input signals remains a significant hurdle, particularly due to the phase instabilities introduced by active cooling systems and temperature drifts in the photonic processor. In this study, we propose theoretical models to simulate phase instability in photonic processors and validate them against experimental results. Two distinct modeling approaches were employed: a Brownian random walk and phase reconstruction based on experimentally observed oscillating harmonics. Additionally, we verified and applied our model to a specific application for input phase correction using self-feedback control within the photonic processor.