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Tuesday 6 October

Quantum Physics

Classical Verification of a Remote Quantum Processor via Degenerate Concordant Computations: An Application to Randomness Generation

Bishal Kumar Das and Lakshya Priyadarshi

A client who rents a cloud quantum processor cannot easily tell whether the bitstrings it receives came from a quantum device. Certification by random circuit sampling runs on present hardware, but scoring each returned sample costs the client time $O(2^n)$. We give a protocol in which the client scores each round in time $O(n)$, built on degenerate concordant ensembles. When a concordant state has a degenerate spectrum, every concordance-preserving gate factorizes as $G_t=U_tP_tB_tU_{t-1}^\dagger$, and the block factor $B_t$, which commutes with the state, cancels from the ensemble while entangling each pure component. The client holds a secret parity whose cosets are the degenerate sectors of every layer, so that it builds the circuit and predicts the ensemble with one inner product per layer. The secret never leaves the client. Inputs are sent uniformly at random, and the client attaches to each round, in its own bookkeeping, the eigenvalue of that input in the hidden state; both verification statistics, an input–output agreement test and a weighted collision test, then compare the server's samples with the non-flat output distribution of the concordant computation. We prove completeness, client efficiency, and soundness in an ideal model in which the compiled circuit reveals nothing about the secret, and we show that the secret must stay hidden: a classical server that holds it passes. For the explicit construction we prove that the single-qubit symmetry test used in efficient simulations of concordant computation fails on non-product inputs with unbiased marginals, and we measure that the local-basis finder of Cable and Browne halts on every layer we tested, that the transmitted component saturates the maximal Schmidt rank, that the circuit is not Clifford, and that matrix-product spoofers are rejected. Soundness of the explicit construction is stated as a conjecture.
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Quantum Physics

The polarization behavior of a system of two interacting identical spins 1/2 in adiabatically varying magnetic field

Yury Belousov and Dim Safin

We derive the density matrix of a nonstationary system in the adiabatic representation and obtain the corresponding Liouville equation. This representation is particularly suitable when the standard interaction representation is inapplicable. We illustrate the approach with two identical, spatially separated spin-1/2 particles coupled by a magnetic dipole-dipole interaction in a slowly varying external magnetic field. Specifically, we analyze the spin polarization of pseudo-qutrit states in regimes where the states become entangled. The application of the adiabatic representation to calculate the dependence of observed physical quantities on time for quantum systems is illustrated. Our results show that entangled states can significantly modify the behavior of these observables and provide additional opportunities for their control.
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Quantum Physics

Operational quasiprobability formulation of non-signaling in time for macrorealism violations

Stefano Gherardini and Paolo Solinas

In this paper, we introduce the concept of quasi non-signaling in time (QNSIT), which extends non-signaling in time (NSIT) to quasiprobability distributions. QNSIT requires that the marginals of the quasiprobability distribution under consideration at intermediate times remain unperturbed by the measurement back-action induced by a sequence of quantum measurements. Associated with QNSIT, we propose a quasiprobabilistic characterization of macrorealism, ${\rm MR}_{\rm qp}$, which requires that both the QNSIT conditions are satisfied and the quasiprobability distribution constructed from the measurement outcomes is positive. We prove that ${\rm MR}_{\rm qp}$ implies NSIT, whereas the converse does not hold because of hidden superposition paths generated or preserved by the system dynamics. We also prove that, for a two-level system, the ${\rm MR}_{\rm qp}$ and NSIT conditions coincide. ${\rm MR}_{\rm qp}$ therefore provides a more sensitive tool for testing macrorealism and can be implemented experimentally through a quantum non-demolition measurement scheme.
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Quantum Physics

Total Ordering of Resources: A Quantum-Classical Separation

Jingsong Ao, Julio I. de Vicente and Alexander Streltsov

A central goal in quantum information science is understanding which physical states can be converted into others. The ideal scenario would be a totally ordered system, where every state can be ranked by a single value, and a higher-ranked state can always be converted into a lower-ranked one. Such an ordering would provide a complete description of state convertibility, making the resource structure maximally simple to characterize and compare. We prove that such a simple ordering is fundamentally impossible for any quantum system with dimension larger than 2. The underlying properties of quantum mechanics guarantee that some states will always be incomparable. Furthermore, we show this is a uniquely quantum phenomenon by constructing classical systems of any size that can be perfectly ranked. This establishes a rigorous and striking divide between quantum and classical state transformations.
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Quantum Physics

Memory Separations from a Collective-Spin Quantum Register

Jamal Slim

A spin one-half driven only by total-spin operators generates a renewal process in which one interevent time $n^\ast$ is forbidden while all others occur. No classical hidden Markov model reproduces it with fewer than $1+\sqrt{n^\ast}$ states, an unbounded separation whose only resource is one relative phase of $\pi$. With $K$ amplitudes the same interference yields $K(K{+}1)/2$ decay channels, and for positive weights and generic rates the classical cost is exactly $K(K{+}1)/2$ states. Both are realized by a depth-three circuit whose parameter count does not grow with the register.
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Quantum Physics

Broadband Telecom Entanglement from an All-Fiber Type-0 Sagnac Source

Alessandro Cestelli, Spyridon Grountas, Scott Mc Haffie et al. (+3 more)

We demonstrate a compact, practical all-fiber source of non-degenerate telecom-band polarization-entangled photon pairs assembled from off-the-shelf components. The compact fiber-integrated architecture is designed to be affordable, straightforward to assemble and align, and compatible with existing optical-fiber infrastructure. Photon pairs are generated through type-0 spontaneous parametric down-conversion in a pigtailed periodically poled lithium-niobate waveguide embedded in a Sagnac interferometer. At 0.5 mW pump power, quantum-state tomography yields a Bell-state fidelity $F_{\Phi^-}=0.850$, purity $P=0.836$, and concurrence $C=0.782$. A separate CHSH measurement gives $S=2.3301\pm0.0075$. The measured Bell-state fidelity is sufficient for the reconstructed polarization state to support qubit teleportation above the classical fidelity limit under ideal protocol operations. Analysis of the density matrix shows that approximately 96.7% of the population remains in the desired $|HH\rangle$-$|VV\rangle$ subspace, with a normalized coherence of approximately 0.86. A residual relative-phase offset indicates that local phase compensation could increase the Bell-state fidelity to approximately 0.8865. We further characterize the brightness–quality trade-off and obtain a corrected pair-rate slope of $(0.918\pm0.040)$ Mcps mW$^{-1}$. The broadband emission and fiber interfaces offer a route to multiuser entanglement distribution through off-the-shelf multichannel wavelength-division multiplexers with passbands matched to the signal and idler wavelengths.
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All authors Alessandro Cestelli, Spyridon Grountas, Scott Mc Haffie, Jun Gao, Val Zwiller, Ali Elshaari

Quantum Physics

Cyclic triorthogonal codes in prime dimension

Shiroman Prakash

We study cyclic triorthogonal codes in prime dimension $p$, on which a transversal third-level diagonal gate acts as a logical non-Clifford gate. For odd $p$, we use a definition of triorthogonality that does not require self-orthogonality. For cyclic codes of length coprime to $p$, triorthogonality reduces to a sumset condition on the spectral support, which makes exhaustive search feasible: for two natural constructions, we enumerate all maximal codes of this kind for $p=3$ up to length $121$ and $p=5,7$ up to length $96$. For qubits, we prove that a cyclic code of odd length is triply even if and only if it is classically triorthogonal, giving a complete description of triply-even cyclic codes, and we determine all such codes up to length $763$. For qudits, we find codes such as $[[19,1,3]]_3$ and $[[7,1,3]]_7$, which improve on the shortest Reed-Muller and Reed-Solomon codes in both overhead exponent and distillation threshold. The latter is the first member of an infinite family of $[[p,1,2\lfloor p/6\rfloor+1]]_p$ codes.
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Quantum Physics

Squeezed light from a semiconductor amplifier

Andrey S. Aleksandrov and Anatoly V. Masalov

The possibility of generating light in a squeezed quantum state during saturated light amplification in a semiconductor amplifier is theoretically analyzed. Due to four-wave mixing process during amplification, weak components of the spontaneous emission of the medium with symmetric frequency shifts relative to the strong amplified wave acquire the quantum properties of two-mode squeezed light. The squeezing of quadrature components occurs in the range of frequency shifts of several GHz ($\sim$ the inverse relaxation time of the charge carrier density) and can reach 10 dB or more under optimal conditions. The formation of a two-mode squeezing phase facilitates the observation of the squeezing effect, in which a strong wave, playing the role of a local wave, exhibits quadrature noise suppression without additional homodyne detection.
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Quantum Physics

A Single Conformal Coordinate Decodes Outcome-Resolved Measurement-Induced Entanglement

Reyhaneh Khasseh and M.A. Rajabpour

A local measurement of a critical many-body state produces exponentially many microscopic outcomes, each defining a different conditioned quantum state. We ask how much information from one such outcome is actually needed to predict the entanglement that remains between two unmeasured regions. We show that, in critical quantum chains, the universal outcome-resolved Rényi entropy is asymptotically determined by a single compact conformal coordinate extracted directly from the measurement record. Compact-boson theory explains this compression: the universal entropy is insensitive to all spatially varying components of the boundary profile, retaining only a single relative compact zero mode. At the free-fermion XX point, exact lattice identities expose the microscopic structure underlying the decoder, while large-scale outcome-by-outcome calculations confirm its scaling prediction across different subsystem geometries and Rényi indices. Numerical calculations in interacting XXZ chains further show that the same microscopic coordinate continues to decode the conditional entropy without interaction-dependent retuning. Thus an exponentially large space of measurement outcomes is reduced, for this universal observable, to a single conformal degree of freedom, revealing an emergent Luttinger-liquid mechanism for outcome-resolved postmeasurement entanglement.
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Quantum Physics

Entanglement Swapping Scheduling for Quantum Repeater Chains under Decoherence during Classical Communications

Jorge Benito Díaz, Andrés Agustí, Jabier Faba et al. (+3 more)

First-generation quantum repeaters are essential in distributing entanglement over long distances since they could mitigate losses in optical fiber. Repeaters split the total distance into shorter, easier to generate links. Then, entanglement swapping establishes long range entanglement by performing local measurements at the repeaters and heralding their results. In this work, we develop an analytical model that estimates the impact of decoherence during heralding for a given protocol. Furthermore, we identify a competition between two phenomena: as repeater count increases, so does entanglement generation, but the additive impact of memory decoherence on final entanglement rate increases too. Therefore, we find that an optimal number of repeaters exists. These findings could lead the design of future quantum networks, both in the inter-repeater distance and swapping timings.
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All authors Jorge Benito Díaz, Andrés Agustí, Jabier Faba, Laura Ortiz, Luis Robledo, Vicente Martín

Quantum Physics

Divisibility of Non-Invertible Quantum Dynamics in the Heisenberg Picture

Federico Settimo and Dariusz Chruściński

We study divisibility of finite-dimensional quantum dynamics in the Heisenberg picture when the dynamical maps need not be invertible. Heisenberg divisible dynamics determines unique reduced propagators on the accessible operator systems, and Heisenberg CP-divisibility is shown to be equivalent to an operator-norm monotonicity. Although each reduced propagator admits a unital completely positive extension, independently chosen extensions need not satisfy a composition law. Generalized inverses restore this law algebraically, but do not, in general, preserve complete positivity. We characterize the resulting obstruction and give a sufficient construction based on unital completely positive projections onto the accessible operator systems. It is shown that for qubit dynamics Heisenberg CP-divisibility always guarantees a completely positive generalized-inverse propagator. Finally, we analyze time-local generators at singular times and identify the associated gauge freedom on dynamically inaccessible directions.
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Quantum Physics · Cross-listed from cond-mat.quant-gas

Polaronic Response of a Supersonic Impurity Strongly Coupled to a Bose Condensate

Sooshin Kim, Yoonsoo Kim, Seokmin Jang et al. (+1 more)

How a mobile impurity exchanges momentum and energy with a many-body environment is a central question in nonequilibrium quantum physics. This exchange becomes particularly complex when the impurity moves rapidly and is strongly coupled to its bath. Here, we realize an interaction-tunable cold-atom collider with fermionic $^{40}$K impurities immersed in a $^{23}$Na Bose–Einstein condensate. A species-selective Raman pulse simultaneously launches the condensate and quenches the interspecies scattering length, producing an initial relative speed 36 times the condensate speed of sound. We track the ensuing relative motion as the interspecies interaction is tuned from weak coupling to resonance. At weak and intermediate coupling, the impurity dynamics are well described by a finite-energy two-body collision model. Near resonance, however, the early-time impurity acceleration is markedly suppressed relative to the two-body prediction, corresponding to a strong enhancement of the apparent dynamical inertia. The enhancement is observed near two distinct Feshbach resonances. These observations provide evidence for a polaronic response in the strongly coupled, supersonic regime of a degenerate Bose–Fermi mixture.
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All authors Sooshin Kim, Yoonsoo Kim, Seokmin Jang, Jee Woo Park

Quantum Physics · Cross-listed from gr-qc

Horizon-Scale Corrections to Black Hole Geometry from a Conserved Quantum Atmosphere

Kashif Ammar Yasir

Black holes are usually described either as classical geometries on which quantum fields propagate or as objects whose complete quantum description lies beyond semiclassical gravity. Hawking radiation already shows, however, that a black hole is not an exactly isolated system: the horizon carries a thermal scale and, after coarse graining, may be associated with fluctuations, dissipation and environmental correlations. Whether this horizon-scale openness can be encoded directly in the gravitational background, before any stochastic treatment of the metric, remains an open question. Here we answer it by introducing, for the first time, a conserved Hawking-scale anisotropic source representing the coarse-grained near-horizon atmosphere and solving the Einstein equations for a static black hole at fixed ADM mass. The dressed geometry has a renormalized mass function, a self-consistently shifted horizon and a modified surface gravity. Because the same lapse governs the horizon and timelike geodesics, the thermal response is accompanied by a correlated orbital restructuring: the effective potential is renormalized, the ISCO moves inward and the near-horizon residence time increases. The construction adds no Langevin force to a fixed Schwarzschild background; rather, it defines a deterministic dressed mean geometry, providing a controlled background on which stochastic metric fluctuations may subsequently be treated in the Einstein-Langevin framework. This establishes the deterministic mean-field sector as the necessary first step for future stochastic gravity programs addressing horizon-scale openness.
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Quantum Physics

Charging Dicke and Tavis–Cummings quantum batteries with a $\mathcal{PT}$-symmetric lossy–gain cavity pair

Shiqing Tang, Qing Yu, Ying Li et al. (+3 more)

We study how a parity–time-symmetric photonic environment changes the charging of Dicke and Tavis–Cummings quantum batteries. For atoms prepared in the collective ground state the excitation number grows as the square of the charging time, with a coefficient set by the initial mean photon number; the counter-rotating terms of the Dicke model add one to four times that number. Conservation of the total excitation number turns this into an exact ceiling on the stored Tavis–Cummings energy, the transition energy times the smaller of the photon and atom numbers, met numerically to within two per cent, which the Dicke terms overshoot by a few per cent at ultrastrong coupling. We then couple a battery to a lossy–gain pair, keeping the parity–time symmetry intact by putting an identical battery in each cavity; the closed moment system puts the exceptional point at the loss rate. Below it the gain cavity fills and the gain-side battery charges while the lossy-side one stays empty; above it photons are shuttled away and dissipated. Solving the full Lindblad equation with batched Monte–Carlo wave functions, we find that the gain-side battery gains thirty-nine, twenty-six and twenty-eight per cent in stored energy, power and ergotropy in the broken phase and loses sixty-five, thirty-five and sixty-five per cent deep in the unbroken phase, where the lossy-side battery overtakes it. Restoring the counter-rotating terms leaves all of this unchanged within the statistical error; what they add is a reactive oscillation at twice the transition frequency that does no net work but nearly doubles the peak of the instantaneous power. The extra energy is of lower quality, the recoverable fraction dropping from about ninety-eight to ninety per cent. The optimum sits on the broken side, close to but not at the exceptional point.
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All authors Shiqing Tang, Qing Yu, Ying Li, Cuilu Zhai, Zhao-Hui Peng, Wangjun Lu

Quantum Physics

Quantum Data Ruler

Wangjun Lu

A ruler reads an unknown length against a calibrated scale. The quantum data ruler reads a matrix element of an unknown state against the signal of a known reference, from their ratio. It is exact for any Hamiltonian and any time, since transition operators change by a state-dependent constant times one function of time, removing the vanishing-signal limit. The readout is unbiased under dephasing and amplitude damping, and recovers the coherent ergotropy of a quantum battery that two-point measurement erases. The structure survives the classical limit as action–angle Fourier modes.
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Quantum Physics

Quantum telescopy with realistic anti-correlated optical fields as references

Xuan Tang and Z. Y. Ou

Quantum telescopy, proposed by Grottesman et al., uses spatially entangled single-photon states to modify HBT stellar interferometry, potentially surpassing Michelson interferometry in resolution without baseline limits. However, finite source bandwidth, non-unit efficiency, detector response times, and celestial light bandwidth all affect its effectiveness. In this work, we consider a spatially entangled singe-photon state generated by beam-splitting a practical anti-bunched photon source from resonance fluorescence, which is a common method of generating single-photon states, and evaluate the signal level in quantum telescopy with inclusion of imperfection of detection system and finite bandwidth of celestial light. Here, given the incoming light, we optimize the parameters of the anti-bunched photon source and the detector system for the best signal-to-noise ratio (SNR), which can match that of the Michelson method in the ideal case but is always smaller in the realistic case. Alternatively, weak coherent states from heavily attenuated lasers can replace the entangled single-photon state but suffer low signal level, similar to HBT method. We find that the SNR with a realistic anti-bunched source for producing the reference fields can still beat the SNR of the coherent state scheme in the weak incoming field limit. The optimized parameters for the anti-bunched source and detection system will be a guide for future experimental endeavor.
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Quantum Physics · Cross-listed from hep-th

Spontaneous Hawking Radiation from Dynamical Black-Hole Formation in Quantum Spin Systems

Shunichiro Kinoshita, Keiju Murata, Daisuke Yamamoto et al. (+1 more)

We show that Hawking radiation spontaneously generated by dynamical black-hole formation can be realized in a local quantum spin model. Starting from the ground state corresponding to the early-time vacuum, with no initially prepared excitations, we follow the dynamics induced by the evolving geometry. In the corresponding continuum quantum field theory, Hawking particle creation is associated with nontrivial Bogoliubov mixing between the in- and out-mode decompositions. We derive the resulting renormalized local energy density in the continuum quantum field theory, which corresponds to an energy-weighted integral of the Hawking spectrum, and identify its counterpart as a local spin observable whose expectation value quantitatively reproduces the continuum prediction, including its dependence on the surface gravity. The agreement persists over a finite time window whose duration grows logarithmically with system size, revealing the crossover from universal continuum Hawking physics to lattice-cutoff dynamics. These results establish a microscopic route to quantum simulation of quantum field theory in curved spacetime, connecting field-theoretic particle creation directly to experimentally accessible many-body observables.
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All authors Shunichiro Kinoshita, Keiju Murata, Daisuke Yamamoto, Ryosuke Yoshii

Quantum Physics · Cross-listed from math-ph

Convergence Without Finite Exactness in the Eigenstate Quantum Bootstrap

Ye Zhou and Daniel Keren

The eigenstate quantum-mechanical bootstrap can converge to the physical moment set while remaining nonexact at every finite order. We prove this for a one-dimensional quasi-exactly solvable sextic oscillator at explicitly known eigenvalues, including excited levels. At every finite order, the lower and upper bounds on $u^2$ remain strictly below and above its physical value, even after adjoining arbitrary finite polynomial Gram tests and all eigenstate-annihilator relations visible in the retained moment space. Hence neither side of the centered observable admits a finite sum-of-squares certificate modulo the eigenstate annihilator. The gap persists under any finite set of additional localizing constraints whose weights are strictly positive on polynomial vectors. We then prove convergence for polynomial Schrödinger operators satisfying explicit sum-of-squares confinement and derivative bounds. The eigenstate relations give moment estimates uniform in the truncation order; these yield a regular Schrödinger representation supported on the prescribed eigenspace. Finally, a four-mode extension combines this fixed-observable obstruction with feasible position moments having no positive representing measure at every finite order. At level three, this construction yields nonphysical feasible points with strictly positive definite moment matrices.
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Quantum Physics

Demonstration of Parallel Multi-QPU Execution for Fragment-Based Quantum Chemistry Using On-Premises Hardware

Nils Herrmann, Mariam Akhtar, Luke W. Bertels et al. (+21 more)

Fragment molecular orbital (FMO) quantum chemistry offers a natural route to parallel quantum computing: large molecular calculations can be decomposed into smaller quantum subproblems that can, in principle, run concurrently. Their practical benefit on real quantum hardware, however, has remained unclear. We demonstrate parallel FMO calculations on three room-temperature diamond-based 'Quoll' quantum computers installed on-site at the Oak Ridge Leadership Computing Facility. Helium clusters (He$_{n}$, $n=2,4,8,10,14$) served as a testbed for total energy calculations using the Quantum Wave Function Sampler (QWFS), a selected configuration interaction (SCI) approach. We compare two execution modes: synchronous shot-parallel execution, in which quantum processors jointly sample QWFS circuits and combine their SPAM-corrected counts into a single probability distribution; and asynchronous fragment-parallel execution, in which processors independently evaluate FMO fragments using device-specific optimized circuits. Both modes increase quantum sampling throughput without systematic loss of accuracy. The three-QPU shot-parallel workflow reaches a parallel efficiency of $93.60\%$, while the fragment-parallel workflow achieves a measured efficiency of $75.01\%$, increasing to $95.17\%$ when excluding the non-useful idle time associated with non-interruptible submitted jobs. In both cases, the assembled FMO-QWFS energies remain chemically accurate compared to the classical ideal. These results provide a proof-of-concept demonstration that parallel quantum execution can improve the practical throughput of quantum chemistry workflows on prototype hardware, motivating the development of future HPC environments with many fault-tolerant quantum processors operating as a distributed resource for molecular and materials simulation.
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All authors Nils Herrmann, Mariam Akhtar, Luke W. Bertels, Leigh Cameron, Raymond Chan, Lei Cheng, Daniel Claudino, Mark de Burgh, Simon Gemmell, Rugang Geng, Geoff Gillett, Travis Humble, Stephan Irle, John P. McMahon, Christian Ortiz, Florian Preis, Andreas Sawadsky, Bianca Sawyer, Reuben Singer, Sai Meghana Tunikipati, Cameron Walters, Lachlan Whichello, Adam Zegelin, Marcus W. Doherty

Quantum Physics

When Expressivity Is Not Enough: Discrete Routing Geometry in Variational Quantum Circuits

Yu Wang

Finding useful variational quantum circuits requires more than representational capacity: training must also have access to directions that improve the objective. We study how discrete CNOT routing shapes both capacity and local accessibility. For circuits built from CNOT and arbitrary one-qubit gates, we separate binary routing matrices from continuous rotations to relate global capacity to local tangent accessibility. An exact finite-field formula for the operator Schmidt rank of a CNOT block and cumulative multiblock bounds yield capacity constraints and a Bell-pair fidelity obstruction. Locally, routing propagates generators into a routing-dependent tangent space, so all parameter gradients can vanish even when the ambient objective gradient is nonzero; frame bounds distinguish loss of gradient coverage from poor conditioning. A routing-conditioned identity insertion preserves the current unitary, state, loss, and existing gradients while adding new tangent directions. Candidate-only and incremental projection scores provide local criteria for ranking insertions. Exact-statevector tests show strong short-horizon score-descent correlations on controlled Bell tasks and effective equal-cost selection on connected Heisenberg chains up to twelve qubits, with complementary TFIM selector controls. A broader TFIM routing ensemble exhibits routing-dependent near-stagnation. Together, these results connect representational capacity, local accessibility, and same-point repair through a common discrete routing description. An architecture need not remain fixed during training: task-dependent geometric information can guide the opening of new descent directions while preserving the learned circuit at insertion. This provides a concrete local approach to a broader problem: how useful quantum circuits can be discovered from the information a task supplies.
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Quantum Physics · Cross-listed from cond-mat.mes-hall

Sparse qubit operation in a 6$\times$6 quantum dot array

Alexader S. Ivlev, Harold L. Co, Damien R. Crielaard et al. (+4 more)

Fault-tolerant quantum computation requires increasing the qubit count while maintaining high performance and connectivity. Semiconductor qubits have demonstrated high-fidelity operation, but growing the system size at sufficient connectivity remains challenging, such that scaling mainly occurred in one spatial direction to preserve access for control lines in the other. Here, we realise an approach that relaxes the requirements on fabrication and demonstrate a 6$\times$6 quantum dot array, defined by a shared barrier gate and individual plunger gates. The quantum dots are tuned through coherent spin shuttling over distances up to 5.2um into a sparse configuration of 10 simultaneously operated qubits, leaving space for shuttling connections. We reduce crosstalk by 42$\times$ in this separated regime, enabling high-fidelity simultaneous single-qubit control, while control-Z operations can be implemented through shuttling and plunger-only control. These results promise high-connectivity quantum circuits, enabled by scaling to two-dimensional geometries and exploiting shuttling.
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All authors Alexader S. Ivlev, Harold L. Co, Damien R. Crielaard, Alice Petrillo, Setareh Kazemzadeh, Giordano Scappucci, Menno Veldhorst

Quantum Physics

Spectroscopy using dipole-phonon quantum logic

Lu Qi, Evan C. Reed, Duanyang Wang et al. (+2 more)

Dipole-phonon quantum logic (DPQL) offers novel approaches for state preparation, measurement, and control of quantum information in molecular ion qubits. Here we report the first experimental demonstration of DPQL as a spectroscopic tool using the shared motional mode of a CaO$^+$-Ca$^+$ ion chain to measure the frequency splitting of the ground $\Omega$-doublet of CaO$^+$. By comparing the dipole-phonon signal across three trap-frequency ramp ranges, we determine the splitting to be $\omega_{mol}=2\pi\times454\pm1$ kHz and place a lower bound on the effective dipole-phonon coupling. The rate of observation is limited by the low thermal population in the molecular ground rotational state.
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All authors Lu Qi, Evan C. Reed, Duanyang Wang, Boyan Yu, Kenneth R. Brown

Quantum Physics

Excitation of subradiant states in atom chains by surface plasmon waves

Giorgio Vimercati and Nicola Piovella

Subradiant states of a finite subwavelength atomic chain in free space offer a powerful resource for photon storage. Yet their preparation and control remain challenging experimentally. A standard technique is to drive the atoms by a laser and observe the florescence light of the long-lived subradiant modes of excitation after the laser is switched off. However, these modes are highly degenerate and form an almost continuous band in the subradiant manifold. Here we propose a new method of exciting a given subradiant state by coupling atoms to a surface plasmon wave propagating along the interface with a metallic film. By controlling the chain/surface distance we may load, store and release photons in the atoms, making them a promising tool for future quantum memories.
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Quantum Physics

Quantum Darwinism through apparatus-mediated environmental recording

Tran P.H Don, Lan Nguyen Tran and Le Bin Ho

Quantum Darwinism explains classical objectivity through redundant environmental records of a system's pointer states. We show that these records can be established through an intermediate measurement apparatus without direct system–environment interactions. Using exact analytical solutions of quantum circuit models, we compare system monitoring with apparatus monitoring, where the environment interacts only with the apparatus. During encoding, system–apparatus correlations decrease as system–environment correlations grow. Without noise, both models yield identical accessible pointer information and redundancy, while the discord accounts for their different mutual information. The reliability of these records depends on where noise acts. Apparatus bit flips introduce errors shared by successive records, limiting the information recoverable even from the complete environment. In contrast, larger fragments can mitigate independent noise on environment qubits after encoding. Dephasing in the pointer basis leaves the record information unchanged in the model considered. These results extend the description of quantum Darwinism to indirect measurements and clarify the conditions for reliable environmental records.
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Monday 5 October

Quantum Physics · Cross-listed from cond-mat.mes-hall

Real-Space Interference from Quantum Metric

Bo-Qing Wu, Xing-Lei Ma, Jin-Xin Hu et al. (+2 more)

Real-space interference of wave packets is governed by their central momentum separation and momentum width, which set the oscillation wavelength and spatial envelope, respectively. Here, we show that in crystalline systems, quantum metric distribution in an isolated band can similarly give rise to real-space interference through the evanescent tails of a localized bound state. The location and width of a quantum metric hot spot in the momentum space set a complex wavevector as one evanescent plane wave component of a bound state. Consequently, with multiple quantum metric hot spots, the density distribution of a bound state or the total density of two spatially overlapped bound states exhibits an interference pattern, where the oscillation period and long-range decay length are determined by the momentum-space separation of the hot spots and the width of the narrowest hot spots, respectively. This quantum-geometric interference is particularly pronounced in (nearly) flat bands, where competing scales from band dispersion are suppressed. Furthermore, we demonstrate that using a Josephson STM the real-space interference can be experimentally observed through the tip-position dependence of the Josephson critical current. Our results reveal a direct correspondence between momentum-space quantum geometry and real-space interference, showing how bound-state profiles encode the momentum-resolved quantum metric.
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All authors Bo-Qing Wu, Xing-Lei Ma, Jin-Xin Hu, Jinchao Zhao, K. T. Law

Quantum Physics

X2C-inspired approximation of the Dirac coupling operator in a multiwavelet basis

Quentin Pitteloud, Jacopo Masotti and Luca Frediani

We present a method to obtain the proven exact 2-component (X2C) method, normally applied to Gaussian-type fixed Hilbert bases, in an adaptative non-uniform grid basis of the Hilbert space, namely Multiwavelets. The X2C method manages to block-diagonalise the Dirac Hamiltonian restricted to the subspace of the basis, in an energy-independent manner. The method takes inspiration from the success of the atomic mean-field X2C method on the Gaussian bases to represent the coupling operator as a projector onto the small components of the eigenstates of the constituent atoms of the system.
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Quantum Physics · Cross-listed from physics.chem-ph

Coupled cluster theory for multireference and open-shell molecules

Sarai Dery Folkestad and Henrik Koch

We introduce a simple coupled cluster approach for multireference and open-shell systems. In the restricted active space coupled cluster framework, the cluster operator only includes excitations from doubly occupied to virtual orbitals, and this ensures the individual operators commute among each other. The ground and excited states are obtained by diagonalisation of the coupled cluster similarity transformed Hamiltonian in a restricted active space. We demonstrate the framework for the RAS-CCSD model, where single and double excitations are included in the cluster operator, and likewise for a complete active space. We consider systems with multireference character and open-shell systems in a spin-adapted manner. The approach captures significantly more dynamical correlation than without the coupled cluster similarity transformation.
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Quantum Physics · Cross-listed from hep-th

Quantum Algorithms for Finding Vacua in the ADK Landscape

Shirabe Endo and Yuta Hamada

We study quantum algorithms for finding a vacuum with a small cosmological constant in the toy model for string Landscape proposed by Arkani-Hamed, Dimopoulos, and Kachru. First, we construct a function-value oracle and use it for Grover search with query complexity $\mathcal{O}(2^{N/2})$, where $N$ is the number of scalar fields. Then, we point out that, by exploiting the linearity of the vacuum energy, the problem can be reduced to a collision problem between two blocks of the bit string. With this structure, we construct two quantum algorithms with query complexity $\mathcal{O}(2^{N/3})$. Compared with the pseudo-polynomial classical algorithm of Denef and Douglas, the quantum algorithms yield a large reduction in computational cost for the physically motivated parameter values.
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Quantum Physics

White paper: 1-10 Hz matter-wave interferometer to test the spin entanglement witness for quantum gravity

Sougato Bose, Anupam Mazumdar, Marko Toroš et al. (+118 more)

In this white paper, we highlight the importance of the ($1-10~{\rm Hz}$) frequency range for laboratory tests of the quantum nature of gravity using the quantum gravity-induced entanglement of masses (QGEM) protocol. QGEM requires matter-wave interferometers with masses ($m\sim10^{-15}-10^{-14}~{\rm kg}$), brought within separations ($d\sim30-50~\mu{\rm m}$), while maintaining spatial superpositions of ($1-20~\mu{\rm m}$) and coherence for ($\tau\sim 0.1 - 1~{\rm s}$). These requirements make low-frequency environmental noise a central experimental challenge and place QGEM in a regime closely related to the low-frequency goals of the Einstein Telescope (ET) and the Cosmic Explorer (CE). In particular, QGEM is sensitive to relative acceleration noise (RAN) and to gravity-gradient noise (GGN) generated by seismic and other environmental mass-density fluctuations. For representative parameters $m=10^{-14}~{\rm kg}$, $\Delta x=10~\mu{\rm m}$, and $\tau=1~{\rm s}$, the differential acceleration-noise amplitude spectral density must be suppressed well below the $10^{-15}~{\rm m\,s^{-2}/\sqrt{Hz}}$ level to keep acceleration-induced dephasing below the relevant experimental scale. Achieving this level of low-frequency noise suppression is therefore a key requirement for QGEM and closely parallels the seismic and gravity-gradient noise challenges that ET and CE address.
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All authors Sougato Bose, Anupam Mazumdar, Marko Toroš, Tian Zhou, Tadeusz Adach, Niayesh Afshordi, Agya Sewara Alam, Alexandre Arbey, Navdeep Arya, Simon Baier, Peter F. Barker, Angelo Bassi, Ettore Bernardi, Lorenzo Braccini, Robert Brandenberger, Daniel Braun, Guri K. Buza, Luigi Cacciapuoti, Carlo Cepollaro, Lin-Qing Chen, Yanbei Chen, Ralph Jason Costales, Marion Cromb, Álvaro de la Cruz-Dombriz, Catalina Curceanu, Shubhang Dadhich, Debarshi Das, Saurya Das, Pratika Dayal, Subhadeep De, Ema Dimastrogiovanni, Lajos Diósi, Or Dobkowski, Kemal Döner, Brian D'Urso, Gurudev Dutt, Shafaq Gulzar Elahi, Samira Elghaayda, Matteo Fadel, Samuel Fedida, Omer Feldman, Fabiano Feleppa, Ron Folman, Joshua Foo, Paolo Fragolino, Laurent Freidel, Giulio Gasbarri, Marco Genovese, Andrew Geraci, Menachem Givon, Cisco Gooding, Jonathan M. H. Gosling, Piotr T. Grochowski, David Groswasser, Mustafa Gündoğan, Ekim Taylan Han{\i}meli, Bas Hensen, Dipankar Home, Richard Howl, Yonathan Japha, Maciej T. Jarema, Rainer Kaltenbaek, Adrian Kent, Eva Kilian-Rademacher, M. S. Kim, Jarosław K. Korbicz, Timothy Kovachy, Samuel Kováčik, Ohkyung Kwon, Gaetano Lambiase, Naor Levi, Iarley P. Lobo, Leon Loveridge, Adrian Lupascu, Paolo Luppi, Marta Maria Marchese, Antonino Marcianò, Aaron G. Markowitz, Chiara Marletto, Ryan J Marshman, J. D. D. Martin, Florian Millo, Gavin W. Morley, Maria Muretova, Sebastian Murk, Robin Oberfrank, Daniel K. L. Oi, Jerzy Paczos, Papadopoulos Stylianos, Matteo G. A. Paris, Mauro Paternostro, Alessandro Pesci, Luciano Petruzziello, Fabrizio Piacentini, Tanmay Kumar Poddar, Sofia Qvarfort, Markus Rademacher, Dennis Rätzel, Anna Chiara Rescigno, Ryan Rizaldy, Albert Roura, Carlos Sabín, Barry C. Sanders, Martine Schut, Helen M Sheehy, Suprit Singh, Aninda Sinha, Urbasi Sinha, Peter Skakunenko, Michael E Tobar, Géza Tóth, Hendrik Ulbricht, Gislaine Varão, Vlatko Vedral, Vincent Vennin, Francesca Vidotto, Giuseppe Filiberto Vitale, Marko Vojinović, Chenan Wei, Qian Xiang, Magdalena Zych

Quantum Physics

Benchmarking exchange-only control of a 48-spin singlet manifold

HRL Quantum Team & Microsoft Collaborators: Stephen Carr, Matt Abbitt, Michael Abraham et al. (+122 more)

Exchange-only quantum computing benefits from high fidelity and straightforward control afforded by the exchange interaction. However, independently controllable qubits must be encoded into subsystems of at least three electron spins, restricting computation to only a fraction of the available spin Hilbert space. The remaining states are treated as leakage and used only transiently in gate sequences. By contrast, a quantitative, system-wide measure of exchange-only performance can exploit the full available Hilbert space, including states conventionally treated as leakage. Here, we apply this approach to arrays of up to 48 electron spins, accessing a total-spin-zero Hilbert space with dimension exceeding $2^{40}$. Measurements of out-of-time-order correlators (OTOCs) reveal rich scrambling dynamics and demonstrate access to regimes relevant to quantum computational advantage. Using generalized forms of cross-entropy and mirror randomized benchmarking, we also assess the aggregate performance of the full exchange-only system per fundamental two-body interaction: the two-spin exchange. We obtain an effective system-level error of $3 \times 10^{-4}$ per exchange, incorporating the complete experimental control sequence and all associated noise sources. This value is up to an order of magnitude lower than those reported from comparable benchmarks on other platforms at the time of writing.
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All authors HRL Quantum Team & Microsoft Collaborators: Stephen Carr, Matt Abbitt, Michael Abraham, Edwin Acuna, I. Alverado, Carter Andrews, Hussein Anton, Katherine M. Beech, Aaron J. Bluestone, Jacob Z. Blumoff, Matthew G. Borselli, Brydon Boyd, Jacob T. Boyer, Peter Brewer, Steven L. Brown, Joseph D. Broz, Tyler A. Cain, John B. Carpenter, Faustin W. Carter, Brittany Carter, Matthew D. Chambers, James M. Chappell, Kevin C. Chen, Edward H. Chen, Maxwell D. Choi, Matthew N. H. Chow, Justin E. Christensen, Aaron M. Chronister, Andrew M. Clapper, Michael D. Cornelius, Gregory M. Crosswhite, Stanislav Culaclii, Erik S. Daniel, Dominic Daprano, John K. David, Charles R. Elliott, Kevin Eng, Colin P. Feeney, David J. Fialkow, Dylan H. Finestone, Bryan H. Fong, Richie Fu, Zachary A. Geiger, Bradley W. Greene, Rafael Guerra-Fuentes, Hrayr K. Gurgenian, Alex Hamill, Brooke M. Hardesty, Jim W. Harrington, Alex Hirman, Donald A. Hitko, Silas Hoffman, Dominic Holloman, Daniel R. Hulbert, Jake Hundley, Clayton A. C. Jackson, Paul C. Jerger, B. Johnson, Aaron M. Jones, Michael P. Jura, Adour V. Kabakian, Tyler Keating, Joseph Kerckhoff, Andrey A. Kiselev, Patrick W. Krantz, Thaddeus D. Ladd, Sanaaya Lakdawala, Elias Lawson-Fox, Alwina R. Liu, Dwight Luhman, Manny Macias, Theodore K. Macioce, Ryan M. Martin, Daniel S. Matic, Gavin C. Mazur, Ryan McGeehan, Olivia Means, Austin Meyer, Samuel Mumford, Tina Niknejad, Riley P. O'Neil, Andrew Pan, Winston Pouse, Eric M. Prophet, Matthew D. Reed, Marcus Rentie, Alec Roberson, Zechariah Rogers, Golam Sabbir, Spencer Sager, Christopher D. Sanborn, Jonathan Sanchez, Rachel H. Sarmiento, Christian J. Schnaible, Cole Scott, Aaron Smith, Daniel E. Smith, James Soash, Kevin C. Staley, Andrea Su, Bo Sun, Christopher M. Swank, Noah Swimmer, Charles Tahan, Bryan J. Thomas, Yessica Torres, Alan Tran, Ivan Tran, James R. van Meter, Franklin Vartanian, John Samuel Venker, Daniel Volya, Annette L. Wagner, Daniel R. Ward, Aaron J. Weinstein, Abigail L. Wessels, Thomas V. Westrick, Evan T. White, Randall M. White, Parker Williams, Catrina E. Wilson, Courtney P. Wilt, Matthew Wingert, Clifford S. YoungSciortino, Andrew Ziegler