Quantum Physics · Cross-listed from physics.optics
Correlated inhomogeneous absorption profiles across distinct optical transitions in a rare-earth doped crystal
Flora Segur, Sacha Welinski, Alban Ferrier et al. (+2 more)
In rare-earth ion-doped
crystals, inhomogeneous absorption profiles reflect the distribution of local environments experienced by individual ions. While each optical transition probes this distribution differently, their fine spectral structures may retain correlations arising from shared local perturbations. In this paper, we present a low-temperature, high-resolution spectroscopic study in Er$^{3+}$:YSO of the transition $^{4}I_{15/2}$ - $^{4}I_{11/2}$ at 980 nm and compare it to the well-known transition $^{4}I_{15/2}$ - $^{4}I_{13/2}$ at 1.5 $\mu$m. Using spectral hole burning on one transition while monitoring the other, we uncover for the first time spectral correlations between two optical transitions, providing new insight into the microscopic origin of inhomogeneous distributions in rare-earth-doped
crystals.
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All authors Flora Segur, Sacha Welinski, Alban Ferrier, Perrine Berger, Anne Louchet-Chauvet
Quantum Physics · Cross-listed from cond-mat.quant-gas
Controlled symmetry breaking of the Fermi surface in ultracold polar molecules
Shrestha Biswas, Sebastian Eppelt, Weikun Tian et al. (+6 more)
Long-range anisotropic dipole-dipole interactions between ultracold polar molecules are predicted to drive exotic
quantum phases, yet direct many-body signatures of these interactions in degenerate
Fermi gases have remained elusive. Here, we report the observation of an interaction-induced controlled deformation of the
Fermi surface, providing a clear many-body signature in a deeply degenerate
Fermi gas of $^{23}\text{Na}^{40}\text{K}$ molecules. Using double microwave (MW) shielding, we prepare $8 \times 10^3$ molecules at $0.23(1)$ times the
Fermi temperature, achieving a three-fold suppression of inelastic losses compared to single MW shielding while preserving strong elastic dipolar scattering. We observe
Fermi surface deformations of up to $7\,\%$, more than two times larger than those observed in
magnetic atoms, despite operating at two orders of magnitude lower densities. Crucially, we demonstrate continuous tuning of the interaction potential from axial U(1) to biaxial C$_{2}$
symmetry, directly imprinting this geometry onto the
Fermi surface. We find excellent agreement between our experimental results and parameter-free Hartree-Fock theory. These results establish MW-shielded polar molecules as a highly tunable platform for exploring strongly correlated dipolar
Fermi matter and offer a promising path towards topological superfluidity.
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All authors Shrestha Biswas, Sebastian Eppelt, Weikun Tian, Wei Zhang, Fulin Deng, Christine Frank, Tao Shi, Immanuel Bloch, Xin-Yu Luo
Quantum Physics · Cross-listed from cond-mat.str-el
Hidden $Z_{2}\times Z_{2}$ subspace symmetry protection for quantum scars
Ayush Sharma and Vikram Tripathi
We study the paradigmatic
spin-1 XY chain under open boundary conditions, which hosts exact
quantum many-body scars generated by an emergent Spectrum Generating
Algebra (SGA). We show that the scar subspace possesses a
symmetry-protected trivial (SPt) character that we attribute to a hidden $Z_{2}\times Z_{2}$
symmetry of another model, namely the commutant
Hamiltonian, for which the scars are the ground states. We construct a Lieb-Schultz-Mattis (LSM) type twist operator, which, for scar states, takes the value $-1,$ and, for ergodic states, approaches zero in the thermodynamic limit. A complementary understanding of the stability of the scars under different perturbations is obtained by analyzing the Loschmidt echo and
Quantum Fisher Information (QFI) of the scars. Finite-size scaling analysis of the QFI reveals that the scars are much more sensitive to perturbations as compared to the nearby thermal states. Based on the analysis of QFI and different LSM twist operators, we obtain a classification of different SGA-preserving and SGA-breaking perturbations.
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Quantum Physics
A field-biased quantum master equation and its Markovian limit
M. Gabriela Boada G., Andrea Delgado and Jose Morales E
We present a non-
equilibrium quantum master equation for a driven open
quantum system in the presence of a continuously applied electromagnetic field. Starting from a driven Caldeira-Leggett (CL) model in which the external electromagnetic field couples simultaneously to the subsystem and reservoir degrees of freedom, the canonical fluctuation-dissipation
theorem (FDT) relations that encode the coefficients of the master equation can no longer be expected to hold. The bath statistics acquire an explicit dependence on the two-time autocorrelation function of the applied field, leading to drive-biased noise correlations and the potential for non-Markovian dynamics. By eliminating the reservoir degrees of freedom at the operator level, we obtain a modified Hu-Paz-Zhang (HPZ) master equation, in which the
diffusion coefficients and coherent forces inherit an explicit
memory dependence on the external field. We demonstrate that the physically observable resonant frequency remains encoded in the homogeneous Green's function of the Generalized Langevin equation (GLE), while the drive-induced corrections manifest exclusively through modified
diffusion and drift terms, should the drive be treated classically. The resultant field-modified HPZ master provides a unified microscopic framework for understanding field-biased open
quantum systems with direct pertinence to a wide variety of experiments in
quantum optics and microscopic
quantum circuits.
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Quantum Physics
Numerical Experiments with Parameter Setting of Trotterized Quantum Phase Estimation for Quantum Hamiltonian Ground State Computation
Elijah Pelofske and Stephan Eidenbenz
We numerically investigate
quantum circuit elementary-gate level instantiations of the standard
Quantum Phase Estimation (QPE)
algorithm for the task of computing the ground-state energy of a
quantum magnet; the disordered fully-connected
quantum Heisenberg
spin glass model. We consider (classical simulations of) QPE
circuit computations on relatively small
quantum Hamiltonians ($3$
qubits) with up to $10$ phase bits of precision, using up to Trotter order $10$. We systematically study the inputs of QPE, specifically time
evolution, Trotter order, Trotter steps, and initial state, and illustrate how these inputs practically determine how QPE operates. From this we outline a coherent set of
quantum algorithm input and tuning guidelines. One of the notable properties we characterize is that QPE sampling of the optimal digitized phase converges to a fixed rate. This results in strong diminishing returns of optimal phase sampling rates which can occur when the Trotter error is surprisingly high.
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Quantum Physics
Basis-independent stabilizerness and maximally noisy magic states
Michael Zurel and Jack Davis
Absolutely stabilizer states are those that remain convex mixtures of stabilizer states after conjugation by any
unitary. Here we give a characterization of such states for multiple qudits of all
prime dimensions by introducing a polytope of their allowed spectra. We illustrate this through the examples of one
qubit, two
qubits, and one qutrit. In particular, the set of absolutely stabilizer states for a single
qubit is a ball inscribed in the stabilizer octahedron, but for higher dimensions the geometry is more complicated. For odd-
prime-dimensional qudits, we also give a complete characterization of absolutely Wigner-positive states, i.e., states whose Wigner function remains nonnegative after conjugation by any
unitary. In so doing, we show there are absolutely Wigner-positive states that are not absolutely stabilizer, which can be seen as a unitarily invariant version of bound magic. We then study the radii of the largest balls contained in the sets of absolutely stabilizer states and absolutely Wigner-positive states. These radii respectively tell us the lowest possible purity of nonstabilizer and Wigner-negative states. Conversely, we also find the radius of the smallest ball containing the set of absolutely Wigner-positive states, giving a tight purity-based necessary condition thereof. Finally, we apply our methods to magic witnessing and find simple sufficient conditions for determining if an $n$-
qubit state is reducible to an $m$-
qubit magic state for any $m<n$.
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Quantum Physics
Schwinger-Keldysh field theory for operator Rényi entropy and entanglement growth in non-interacting systems with sub-ballistic transports
Priesh Roy and Sumilan Banerjee
The notion of operator growth in
quantum systems furnishes a bridge between transport and the generation of
entanglement between different parts of the system under
quantum dynamics. We define a measure of operator growth in terms of subsystem operator Rényi
entropy, which provides a state-independent measure of operator growth, unlike
entanglement entropies, and the usual measures of operator growth like out-of-time-order correlators. We show that the subsystem operator Rényi
entropy encodes both spatial and temporal information, and thus can directly connect to transport for a local operator related to a conserved quantity. We construct a unified Schwinger-Keldysh (SK) field theory formalism for the time
evolution of operator Rényi
entropy and
entanglement entropies of initial pure states. We use the SK field theory to obtain the operator Rényi and state
entanglement entropies in terms of infinite-temperature and
vacuum Keldysh Green's functions, respectively, for non-interacting systems. We apply the method to explore the connection between operator and
entanglement growth, and transport in non-interacting systems with quasiperiodic and random disorder, like the one- and two-dimensional Aubry-André models and the two-dimensional Anderson model. In particular, we show that the growth of subsystem operator Rényi
entropy and state von Neumann and Rényi
entanglement entropies can capture both ballistic and sub-ballistic transport behaviors, like diffusive and anomalous diffusive transport, as well as localization in these systems.
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Quantum Physics
The unbearable hardness of deciding about magic
Lorenzo Leone, Jens Eisert and Salvatore F.E. Oliviero
Identifying the boundary between classical and
quantum computation is a central challenge in
quantum information. In multi-
qubit systems,
entanglement and magic are the key resources underlying genuinely
quantum behaviour. While
entanglement is well understood, magic - essential for universal
quantum computation - remains relatively poorly characterised. Here we show that determining membership in the stabilizer polytope, which defines the free states of magic-state resource theory, requires super-exponential time $\class{exp} ( n^2)$ in the number of
qubits $n$, even approximately. We reduce the problem to solving a $3$-\class{SAT} instance on $n^2$ variables and, by invoking the exponential time hypothesis, the result follows. As a consequence, both quantifying and certifying magic are fundamentally intractable: any magic monotone for general states must be super-exponentially hard to compute, and deciding whether an operator is a valid magic witness is equally difficult. As a corollary, we establish the robustness of magic as computationally optimal among monotones. This barrier extends even to classically simulable regimes: deciding whether a state lies in the convex hull of states generated by a logarithmic number of non-Clifford gates is also super-exponentially hard. Together, these results reveal intrinsic computational limits on assessing classical simulability, distilling pathological magic states, and ultimately probing and exploiting magic as a
quantum resource.
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Quantum Physics
Shu Long, Hong-Sen Yin, Chao Yang et al. (+3 more)
We establish a direct connection between
Hamiltonian band
topology and the Liouvillian spectral winding of open
quantum systems with quadratic dissipations. This allows the band
topology to act as a knob for controlling Liouvillian
topology and corresponding non-
equilibrium dynamics. In particular, we show analytically that, for
chiral Hamiltonians under sublattice-unidirectional
chiral dissipation, the Liouvillian spectral winding depends on the
Hamiltonian solely through its band winding number, thereby enabling direct band-topological control of the Liouvillian skin effect. We further identify two distinguished coherent and incoherent Liouvillian skin effects therein: the former hosts a band-topological dark state of the
Hamiltonian, whereas the latter yields a mixed steady state driven by dissipative nonreciprocity. Our results establish a systematic framework for topological control of spectral and spatial organization in open
quantum systems and provide a unified perspective on
topology in
Hamiltonian and dissipative dynamics.
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All authors Shu Long, Hong-Sen Yin, Chao Yang, Sen Mu, Jia-Wei Zhang, Linhu Li
Quantum Physics
Fundamental Limits on QBER and Distance in Quantum Key Distribution
Stefano Pirandola
Quantum key distribution (QKD) enables information-theoretic secure communication, yet its ultimate tolerance to noise and achievable transmission distance remain fundamentally constrained. We establish the maximum
quantum bit error rate (QBER) compatible with secure QKD and derive corresponding upper bounds on communication distance. Our results follow from a fundamental capacity threshold for
qubit Pauli channels and apply to protocols based on two or more mutually unbiased bases, using either single-
photon or weak coherent sources. By connecting information-theoretic limits to realistic physical noise models, we obtain universal bounds on achievable distances in fiber and free-space links, including diffraction-limited constraints relevant to deep-space
quantum communications. These findings clarify the ultimate noise robustness of QKD and delineate the fundamental boundaries of secure
quantum communication.
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Quantum Physics
Partial Reversibility and Counterdiabatic Driving in Nearly Integrable Systems
Rohan Banerjee, Shahyad Khamnei, Anatoli Polkovnikov et al. (+1 more)
Adiabatic (or reversible) processes are the key concept unifying our understanding of thermodynamics and dynamical systems. Reversibility in the thermodynamic sense is understood as
entropy-preserving processes, such as in the idealized Carnot engine, whereas in integrable dynamical systems it is understood as the conservation of the action variables. Between these two idealized limits, however, things are much less clear. In this work, we study several toy models to assess the extent to which reversible processes are even possible in this regime. We then explore how the dissipative losses resulting from rapidly driving these kinds of systems can be fought by approximate counterdiabatic driving. Finally, we provide numerical evidence that much of the phenomenology should be the same for
quantum many-body systems with large degeneracy in the presence of integrability breaking perturbations.
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All authors Rohan Banerjee, Shahyad Khamnei, Anatoli Polkovnikov, Stewart Morawetz
Quantum Physics
Taxonomy of integrable and ground-state solvable models: Jastrow wave functions on graphs and parent Hamiltonians
Nilanjan Sasmal and Adolfo del Campo
We introduce a family of many-body systems of distinguishable continuous-variable particles in which interparticle interactions are set by the adjacency
matrix of a graph. The ground-state wave function of such systems is of a generalized Jastrow form involving the product of pair-correlation functions over the edge set of the graph. These systems describe
quantum fluids when the graph is complete, and the pair function has a well-defined permutation
symmetry. In general, they provide the continuous-variable generalization of
spin systems on graphs, with broken permutation
symmetry. The corresponding parent
Hamiltonian is shown to include (a) two-body interactions determined by the graph adjacency
matrix and (b) three-body interactions over all possible 2-paths on the graph. Employing elements of graph theory, we chart the landscape of models, recovering known instances in the literature and providing numerous new examples of ground-state solvable models for which the system
Hamiltonian, ground-state wave function, and corresponding energy
eigenvalue are specified.
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Quantum Physics
Quantum simulation of massive Thirring and Gross–Neveu models for arbitrary number of flavors
Bojko N. Bakalov, Joao C. Getelina, Raghav G. Jha et al. (+2 more)
The study of fermionic
quantum field theories is an important problem for realizing the standard model of particle physics on a
quantum computer. As a step towards this goal, we consider the massive Thirring and Gross–Neveu models with arbitrary number of
fermion flavors, $N_f$, discretized on a spatial one-dimensional
lattice of size $L$ in the
Hamiltonian formulation. We compute the gate complexity using the higher-order product formula and using block-encoding/qubitization and
quantum singular value transformations in the limit of large $N_f$ and $L$. We also prepare the ground states of both models with excellent fidelity for system sizes up to 20
qubits with $N_f = 1,2,3,4$ using the adaptive-variational
quantum imaginary time
algorithm. In addition, we also classify the dynamical Lie
algebras of these relativistic fermionic models and show that they belong to the same isomorphism class. Our work is a concrete step towards the
quantum simulation of real-time dynamics of large $N_f$ fermionic
quantum field theories models relevant for
chiral symmetry breaking, understanding dimensional transmutation, and exploring the conformal window of field theories on near-term and early fault-tolerant
quantum computers.
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All authors Bojko N. Bakalov, Joao C. Getelina, Raghav G. Jha, Alexander F. Kemper, Yuan Liu
Quantum Physics
Robustness-Runtime Tradeoff for Quantum State Transfer
Twesh Upadhyaya, Yifan Hong, T. C. Mooney et al. (+1 more)
Quantum state transfer is the primitive of transporting an unknown state on one site of a
lattice to another. Using power-law interactions, recent state transfer protocols achieve speedup by utilizing the intermediate ancilla sites. However, these protocols require the ancillas to be in a perfectly initialized state, which, due to noise or imperfect control, may not be the case. In this work we introduce the $\textit{robustness}$ of a state transfer protocol, which quantifies the protocol's tolerance to error in the initial ancilla state. In the Heisenberg picture, state transfer grows operators supported on the final site such that they no longer commute with all operators on the starting site. We prove that this robustness tightly bounds the Schatten $p$-norms of these commutators between initial and final-site operators. This generalizes the known cases of $p=\infty$ and $p=2$, which govern completely state-dependent and state-independent state transfer respectively, demonstrating that intermediate values of $p$ govern partially state-dependent state transfer. In conjunction with existing power-law light cones, our result gives new minimum runtimes for partially state-dependent protocols which, in certain regimes, are parametrically better than existing bounds. We introduce new robust state transfer protocols, charting the landscape between complete state-dependence and state-independence.
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All authors Twesh Upadhyaya, Yifan Hong, T. C. Mooney, Alexey V. Gorshkov
Quantum Physics · Cross-listed from cond-mat.mes-hall
Atomic-Scale Quantum Control of Single Spin Defects in a Two-Dimensional Semiconductor
Kwan Ho Au-Yeung, Wantong Huang, Johanna Matusche et al. (+15 more)
Individual
spin defects in solids are promising building blocks for
quantum technologies, but their deterministic creation, individual addressability, and operation near surfaces remain major challenges. Two-dimensional materials provide an attractive alternative, as their single-layer thickness enables direct atomic-scale access to defect states. Here, we demonstrate single-
spin control of solid-state defects in a two-dimensional
semiconductor by a combination of scanning tunneling microscopy and electron
spin resonance. We create and manipulate individual sulfur vacancies and carbon substitution defects in monolayer molybdenum disulfide and characterize their
spin dynamics, including coherent control, at the single-defect level. Using atomic manipulation, we further engineer and probe
spin-
spin interactions between defect pairs. Our results demonstrate deterministic creation, addressability, coherent manipulation, and controlled coupling of individual
spin defects within a single experimental platform. This establishes atomically engineered
spin defects in two-dimensional
semiconductors as a versatile class of controllable solid-state
quantum systems and opens a route towards tailored
quantum sensing experiments.
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All authors Kwan Ho Au-Yeung, Wantong Huang, Johanna Matusche, Paul Greule, Jonas Arnold, Lovis Hardeweg, Máté Stark, Luise Renz, Affan Safeer, Daniel Jansen, Thomas Michely, Jeison Fischer, Wolfgang Wernsdorfer, Christoph Sürgers, Hannu-Pekka Komsa, Johannes Schwenk, Wouter Jolie, Philip Willke
Quantum Physics
Computing with many encoded logical qubits beyond break-even
Shival Dasu, Matthew DeCross, Andrew Y. Guo et al. (+42 more)
High-rate
quantum error correcting (QEC) codes encode many logical
qubits in a given number of physical
qubits, making them promising candidates for
quantum computation. Implementing high-rate codes at a scale that both frustrates classical computing and improves performance by encoding requires both high fidelity gates and long-range
qubit connectivity – both of which are offered by trapped-ion
quantum computers. Here, we demonstrate computations that outperform their unencoded counterparts in the high-rate $[[ k+2,\, k,\, 2 ]]$ iceberg
quantum error detecting (QED) and $[[ (k_2 + 2)(k_1 + 2),\, k_2k_1,\, 4 ]]$ two-level concatenated iceberg QEC codes, using the 98-
qubit Quantinuum Helios trapped-ion
quantum processor. Utilizing new gadgets for encoded operations, we realize this "beyond break-even" performance with reasonable postselection rates across a range of fault-tolerant (FT) and partially-fault-tolerant (pFT) component and application
benchmarks with between $48$ and $94$ logical
qubits. These
benchmarks include FT state preparation and measurement, QEC cycle benchmarking, logical gate benchmarking, GHZ state preparation, and a pFT
quantum simulation of the three-dimensional $XY$ model of
quantum magnetism. Additionally, we illustrate that postselection rates can be suppressed by increasing the code distance via concatenation. Our results represent state-of-the-art logical component and state fidelities and provide evidence that high-rate QED/QEC codes are viable on contemporary
quantum computers for near-term beyond-classical-scale computation.
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All authors Shival Dasu, Matthew DeCross, Andrew Y. Guo, Ali Lavasani, Jan Behrends, Asmae Benhemou, Yi-Hsiang Chen, Karl Mayer, Chris N. Self, Selwyn Simsek, Basudha Srivastava, M.S. Allman, Jake Arkinstall, Justin G. Bohnet, Nathaniel Q. Burdick, J.P. Campora III, Alex Chernoguzov, Samuel F. Cooper, Robert D. Delaney, Joan M. Dreiling, Brian Estey, Caroline Figgatt, Cameron Foltz, John P. Gaebler, Alex Hall, Craig A. Holliman, Ali A. Husain, Akhil Isanaka, Colin J. Kennedy, Yuga Kodama, Nikhil Kotibhaskar, Nathan K. Lysne, Ivaylo S. Madjarov, Michael Mills, Alistair R. Milne, Brian Neyenhuis, Annie J. Park, Anthony Ransford, Adam P. Reed, Steven J. Sanders, Charles H. Baldwin, David Hayes, Ben Criger, Andrew C. Potter, David Amaro
Quantum Physics
Quantum jumps in open cavity optomechanics and Liouvillian versus Hamiltonian exceptional points
Aritra Ghosh and M. Bhattacharya
Exceptional points, where two or more eigenstates of a non-Hermitian system coalesce, are now of interest across many fields of physics, from the perspective of open-system dynamics, sensing, nonreciprocal transport, and topological phase transitions. In this work, we investigate exceptional points in cavity optomechanics, a platform of interest to diverse communities working on gravitational-wave detection, macroscopic
quantum mechanics,
quantum transduction, etc. Specifically, we clarify the role of
quantum jumps in making a clear distinction between Liouvillian and
Hamiltonian exceptional points in optomechanical systems. While the Liouvillian exceptional point arises from the unconditional Lindblad dynamics and is independent of the
phonon-bath temperature, the
Hamiltonian exceptional point emerges from the conditional no-jump
evolution and acquires a thermal shift due to an enhanced conditional damping. Employing the thermofield formalism, we derive a unified spectral framework that interpolates between these regimes via an analytical hybrid-Liouvillian description. Remarkably, in the weak-
quantum-jump regime, the exceptional point is perturbed only at the second order, highlighting the robustness of the
Hamiltonian exceptional point under small hybrid perturbations. Our work reveals a continuous family of hybrid exceptional points, clarifies the operational and physical differences between the conditional and unconditional dissipative dynamics in optomechanical systems, and provides a probe for thermal baths.
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Quantum Physics
Controlled jump in the Clifford hierarchy
Yichen Xu and Xiao Wang
We develop a simple and systematic route to higher levels of the
qubit Clifford hierarchy by coherently controlling Clifford operations. Our approach is based on Pauli periodicity, defined for a Clifford
unitary $U$ as the smallest integer $m\ge 1$ such that $U^{2^{m}}$ is a Pauli operator up to phase. We prove a sharp controlled-jump rule showing that the controlled gate $CU$ lies strictly in level $m+2$ of the hierarchy, and equivalently that $CU$ lies in level $k$ if $U^{2^{k-2}}$ is Pauli while no smaller positive power of $U$ is Pauli. We further quantify the resources required to realize large level jumps in the Clifford hierarchy by proving an essentially tight upper bound on Pauli periodicity as a function of the number of
qubits, which implies that accessing high hierarchy levels through controlled Cliffords requires a number of target
qubits that grows exponentially with the desired level. We complement this limitation with explicit infinite families of Pauli-periodic Cliffords whose controlled versions achieve asymptotically optimal jumps. As an application, we propose a protocol for preparing logical catalyst states that enable logical $Z^{1/2^k}$ phase gates via phase kickback from a single jumped Clifford.
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Quantum Physics
Decentralized Consensus from Quantum Proof of Position
Dar Gilboa, Siddhartha Jain and Or Sattath
We construct an energy efficient consensus protocol with geospatial position as an external resource by using
quantum computers. Economic resources of some form are essential in decentralized consensus protocols in order to prevent attacks in which malicious parties costlessly generate fake identities. The resulting protocol,
Quantum Proof of Position (QPoP), improves on existing classical mechanisms: it dispenses with the notorious electricity consumption of Proof-of-Work, and, because its resource is external to the protocol, with the wealth concentration of Proof-of-Stake. QPoP also has two properties that, to our knowledge, no other permissionless Sybil resistance mechanism possesses. First, its Sybil resistance is non-rival: position verification occupies a
quantum computer only briefly, so a single device can secure any number of chains at once, each enjoying its full benefit. Second, the protocol generates geographic decentralization, because its resource cannot be agglomerated: a participant's power is the number of distinct locations at which it operates a
quantum computer, and a second computer at an occupied location adds nothing.
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Quantum Physics
Beyond Single-Shot Fidelity: Chernoff-Based Throughput Optimization in Superconducting Qubit Readout
Sinan Bugu
Single-shot fidelity is the standard
benchmark for superconducting
qubit readout, yet it does not directly minimize the wall-clock time needed to certify a
quantum state. We treat the dispersive measurement record as a
stochastic communication channel and compute the classical Chernoff information governing the multi-shot error exponent, using a trajectory model that incorporates T1 relaxation with full cavity
memory. The integration time that maximizes single-shot fidelity and the time that minimizes total certification time do not coincide. For representative transmon parameters and hardware overheads, the throughput-optimal window is longer, cutting certification time by roughly 9-11%, with the gain saturating near 1.13x in the high-readout-power and high-overhead regime. Benchmarking the extracted classical information against the unit-efficiency Gaussian Chernoff limit defines an information-extraction efficiency: dispersive schemes capture ~45% at short integration times, dropping to eta_info(tau_rate) ~ 12% at tau_rate ~ 1.22 us as T1-induced trajectory smearing accumulates. These results connect readout calibration directly to the operational objective of minimizing certification time in high-throughput superconducting processors.
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Quantum Physics
Energy efficient optical tracking for space quantum communication
Eric Vokes, Vinod N. Rao, Elinore Spencer et al. (+1 more)
Power consumption is a critical constraint for CubeSat based
quantum communication, where tracking systems often dominate the onboard power budget. We demonstrate an energy-efficient approach that enables reliable satellite tracking at substantially reduced beacon power by treating tracking as a weak-signal estimation task. Using a closed-loop system with fine steering mirrors and higher-order Kalman filters on ground, we can maintain stable tracking at a transmitted power equivalent to 34 mW over a -60 dB satellite to ground optical channel. Our results show that the resulting penalties on QKD bit error rates and signal-to-noise ratios are negligible, allowing for more efficient power allocation to
quantum payloads in CubeSat missions.
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All authors Eric Vokes, Vinod N. Rao, Elinore Spencer, Rupesh Kumar
Quantum Physics · Cross-listed from hep-th
Time in gravitational subregions and in closed universes
Andreas Blommaert and Chang-Han Chen
What are gauge-invariant local observables in a subregion in
quantum gravity? How does one even define such a subregion non-perturbatively? We study these questions in JT
gravity. One can define a subregion by specifying the value of the dilaton at the boundary of the region. We study conformal matter correlators in such a subregion. There is a gravitational constraint associated with York time
evolution within the causal diamond of the subregion. This constraint can be leveraged to construct gauge-invariant observables in
quantum gravity, using a crossed product construction. The extrinsic
curvature of Cauchy slices acts as the physical clock. This is a simple example of how gauge-invariant observables can be obtained by dressing to features of a spacetime (or other fields), without the need for introducing an external observer. The
entropy associated with this
algebra of observables is not an area, or any boundary term. We show that gravitational constraints only give boundary formulas for
entropy when gauging isometric diffeomorphisms. York time flow is merely a conformal isometry, not an actual isometry, and thus leads to bulk contributions to
entropy. We repeat our construction for Milne-type closed Big-Bang universes, which may be of independent interest.
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Quantum Physics
Exponential speedup in measurement property learning with post-measurement states
Zhenhuan Liu, Qi Ye, Zhenyu Cai et al. (+1 more)
Learning properties of
quantum states and channels is known to benefit from resources such as entangled operations, auxiliary
qubits, and adaptivity, whereas the resource structure of measurement learning, namely, learning properties of
quantum measurement operators, remains poorly understood. In this work, we identify a measurement learning task for which access limited to classical measurement outcomes leads to an exponential lower bound on the query complexity, established via a distinguishing task between a genuine
quantum projective measurement and a purely classical random number generator. Remarkably, this hardness persists even when arbitrary entangled operations, auxiliary systems, and fully adaptive strategies are allowed, indicating that conventional resources for state and channel learning are ineffective in this task. In contrast, when access to the post-measurement
quantum state is available, the same task can be solved with constant query complexity using a simple measuring-twice protocol, without requiring resources that are useful for state and channel learning. Our results reveal post-measurement states as a qualitatively new and decisive resource for measurement learning, suggesting potential implications for the design of practical
quantum certification protocols.
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All authors Zhenhuan Liu, Qi Ye, Zhenyu Cai, Jens Eisert
Quantum Physics
Trade-offs in Gauss's law error correction for lattice gauge theory quantum simulations
Balint Pato and Natalie Klco
Gauss's law-based
quantum error correction (GLQEC) offers a promising approach to reducing
qubit overhead in
lattice gauge theory simulations by leveraging built-in
symmetries. For applications of GLQEC to 1+1D
lattice quantum electrodynamics (QED), we identify two significant trade-offs. First, we prove via dimension-counting arguments that GLQEC requires periodic electric fields, thereby constraining the design space for
lattice QED simulations. Second, we numerically compare GLQEC with a universal
quantum error correction (UQEC) code, specifically the $d=3$ bitflip repetition code, and find that while GLQEC can achieve lower logical error rates in single-round error correction, it exhibits faster decoherence to the steady-state mixed ensemble under multiple rounds. The mixing speed penalty is manifest in observables of interest for both
memory experiments and
Hamiltonian evolution. We identify a mixing speed threshold, $p_{th}=0.277(2)$, above which using GLQEC exhibits even faster decoherence than without error correction. Our results highlight fundamental limitations of
symmetry-based error correction schemes and inform corresponding constraints on formulations of
lattice gauge theories compatible with error-robust
quantum simulation techniques.
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Quantum Physics
Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits
Raquel Garcia Belles, Alexander Anferov, Lukas F. Deeg et al. (+14 more)
Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in
quantum science, all of which depend crucially on the
quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting
circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with
phonon lifetimes up to 400 $\mu$s and lifetime-limited coherence times approaching one millisecond in the
quantum regime. When coupled to a superconducting
qubit, this leads to a hybrid system with a large
quantum coherence cooperativity of $C_{T_2}=1.1\times10^5$. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
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All authors Raquel Garcia Belles, Alexander Anferov, Lukas F. Deeg, Loris Colicchio, Arianne Brooks, Tom Schatteburg, Maxwell Drimmer, Ines C. Rodrigues, Rodrigo Benevides, Marco Liffredo, Jyotish Patidar, Oleksandr Pshyk, Matteo Fadel, Luis Guillermo Villanueva, Sebastian Siol, Gerhard Kirchmair, Yiwen Chu
Quantum Physics · Cross-listed from cond-mat.str-el
Lowering the temperature of two-dimensional fermionic tensor networks with cluster expansions
Sander De Meyer, Atsushi Ueda, Yuchi He et al. (+2 more)
Representing the time-
evolution operator as a tensor network constitutes a key ingredient in several
algorithms for studying
quantum lattice systems at finite temperature or in a non-
equilibrium setting. For a
Hamiltonian composed of strictly short-ranged interactions, the Suzuki-Trotter decomposition is the main technique for obtaining such a representation. In [B.~Vanhecke, L.~Vanderstraeten and F.~Verstraete, Physical Review A, L020402 (2021)], an alternative strategy, the cluster expansion, was introduced. This approach naturally preserves internal and
lattice symmetries and can more easily be extended to higher-order representations or longer-ranged interactions. We extend the cluster expansion to two-dimensional fermionic systems, and employ it to construct projected entangled-pair operator (PEPO) approximations of Gibbs states. We also discuss and
benchmark different truncation schemes for multiplying layers of PEPOs together. Applying the resulting framework to a two-dimensional spinless
fermion model with attractive interactions, we resolve a clear phase boundary at finite temperature.
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All authors Sander De Meyer, Atsushi Ueda, Yuchi He, Nick Bultinck, Jutho Haegeman
Quantum Physics
Self-stabilized high-dimensional quantum key distribution on a metropolitan free-space link
Karolina Dziwulska, Christopher Spiess, Sarika Mishra et al. (+3 more)
Quantum communication technologies capable of operating reliably across heterogeneous optical channels are essential for scalable metropolitan
quantum networks. Here we demonstrate high-dimensional time-bin-encoded
quantum key distribution over a hybrid metropolitan link comprising 1.7 km free-space transmission and 685 m of optical fiber. Operating at a clock rate of 500 MHz in the C-band, we implement both 2- and 4-dimensional protocols, and obtain estimated secure finite-key rates of (95 +- 28) kbit/s for 4D at (25.0 +- 2.0) dB loss and (59 +- 27) kbit/s for 2D at (23.5 +- 2.3) dB loss. Crucially, we achieve continuous operation over 48 h in a fully self-referenced architecture: initial synchronization, interferometric phase stabilization, and long-term drift compensation are performed exclusively using the detected
quantum signals, without auxiliary optical reference channels. Our results thus establish a practical and versatile platform for hybrid free-space-to-fiber
quantum communication and show that the encoding dimensionality can be adapted to the optimal operating regime of realistic metropolitan channels, providing a pathway toward efficient, autonomous and deployable
quantum network nodes.
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All authors Karolina Dziwulska, Christopher Spiess, Sarika Mishra, Markus Leipe, Yugant Hadiyal, Fabian Steinlechner
Quantum Physics
On the emergence of quantum mechanics from stochastic processes
Jason Doukas
The
stochastic–
quantum correspondence reinterprets
quantum dynamics in terms of an underlying
stochastic process on a configuration space. We generalize its lifting prescription, overcoming restrictions on the reachable density
matrices without enlarging the configuration space or its associated
Hilbert space. A compatibility relation and Kraus dictionary connect
stochastic kernels to completely positive, trace-preserving (CPTP) maps: every finite-dimensional kernel admits a compatible lift, and these lifts collectively encompass all
quantum channels as the kernels vary. We describe sufficiently regular composable CPTP dynamics through time-local Lindblad generators. We establish that every rooted
quantum dynamical family admits a
stochastic realizer on the same configuration space, and that a revival of configuration-space distinguishability rules out every Markovian realizer on that space. We show how the predictive information carried by off-diagonal degrees of freedom in a compatible composable lift is encoded in the continuation probabilities of a family of context-dependent
stochastic processes. We also analyze physical measurement by separating active basis rotations from configuration recording. The rooted
quantum probabilities alone do not enforce faithfulness in idealized recording interactions; nevertheless, we show that a faithful
stochastic realizer always exists.
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