01721nas a2200169 4500008004100000245003000041210003000071260001400101520125300115100001801368700002001386700002801406700002801434700003501462700001701497856003701514 2024 eng d00aAgnostic Phase Estimation0 aAgnostic Phase Estimation c2/29/20243 a

The goal of quantum metrology is to improve measurements' sensitivities by harnessing quantum resources. Metrologists often aim to maximize the quantum Fisher information, which bounds the measurement setup's sensitivity. In studies of fundamental limits on metrology, a paradigmatic setup features a qubit (spin-half system) subject to an unknown rotation. One obtains the maximal quantum Fisher information about the rotation if the spin begins in a state that maximizes the variance of the rotation-inducing operator. If the rotation axis is unknown, however, no optimal single-qubit sensor can be prepared. Inspired by simulations of closed timelike curves, we circumvent this limitation. We obtain the maximum quantum Fisher information about a rotation angle, regardless of the unknown rotation axis. To achieve this result, we initially entangle the probe qubit with an ancilla qubit. Then, we measure the pair in an entangled basis, obtaining more information about the rotation angle than any single-qubit sensor can achieve. We demonstrate this metrological advantage using a two-qubit superconducting quantum processor. Our measurement approach achieves a quantum advantage, outperforming every entanglement-free strategy.

1 aSong, Xingrui1 aSalvati, Flavio1 aGaikwad, Chandrashekhar1 aHalpern, Nicole, Yunger1 aArvidsson-Shukur, David, R. M.1 aMurch, Kater uhttps://arxiv.org/abs/2403.0005402447nas a2200169 4500008004100000245005000041210004900091260001300140520194800153100002002101700003302121700002102154700002802175700001702203700002002220856003702240 2024 eng d00aComplexity-constrained quantum thermodynamics0 aComplexityconstrained quantum thermodynamics c3/7/20243 a

Quantum complexity measures the difficulty of realizing a quantum process, such as preparing a state or implementing a unitary. We present an approach to quantifying the thermodynamic resources required to implement a process if the process's complexity is restricted. We focus on the prototypical task of information erasure, or Landauer erasure, wherein an n-qubit memory is reset to the all-zero state. We show that the minimum thermodynamic work required to reset an arbitrary state, via a complexity-constrained process, is quantified by the state's complexity entropy. The complexity entropy therefore quantifies a trade-off between the work cost and complexity cost of resetting a state. If the qubits have a nontrivial (but product) Hamiltonian, the optimal work cost is determined by the complexity relative entropy. The complexity entropy quantifies the amount of randomness a system appears to have to a computationally limited observer. Similarly, the complexity relative entropy quantifies such an observer's ability to distinguish two states. We prove elementary properties of the complexity (relative) entropy and determine the complexity entropy's behavior under random circuits. Also, we identify information-theoretic applications of the complexity entropy. The complexity entropy quantifies the resources required for data compression if the compression algorithm must use a restricted number of gates. We further introduce a complexity conditional entropy, which arises naturally in a complexity-constrained variant of information-theoretic decoupling. Assuming that this entropy obeys a conjectured chain rule, we show that the entropy bounds the number of qubits that one can decouple from a reference system, as judged by a computationally bounded referee. Overall, our framework extends the resource-theoretic approach to thermodynamics to integrate a notion of time, as quantified by complexity.

1 aMunson, Anthony1 aKothakonda, Naga, Bhavya Tej1 aHaferkamp, Jonas1 aHalpern, Nicole, Yunger1 aEisert, Jens1 aFaist, Philippe uhttps://arxiv.org/abs/2403.0482802447nas a2200169 4500008004100000245005000041210004900091260001300140520194800153100002002101700003302121700002102154700002802175700001702203700002002220856003702240 2024 eng d00aComplexity-constrained quantum thermodynamics0 aComplexityconstrained quantum thermodynamics c3/7/20243 a

Quantum complexity measures the difficulty of realizing a quantum process, such as preparing a state or implementing a unitary. We present an approach to quantifying the thermodynamic resources required to implement a process if the process's complexity is restricted. We focus on the prototypical task of information erasure, or Landauer erasure, wherein an n-qubit memory is reset to the all-zero state. We show that the minimum thermodynamic work required to reset an arbitrary state, via a complexity-constrained process, is quantified by the state's complexity entropy. The complexity entropy therefore quantifies a trade-off between the work cost and complexity cost of resetting a state. If the qubits have a nontrivial (but product) Hamiltonian, the optimal work cost is determined by the complexity relative entropy. The complexity entropy quantifies the amount of randomness a system appears to have to a computationally limited observer. Similarly, the complexity relative entropy quantifies such an observer's ability to distinguish two states. We prove elementary properties of the complexity (relative) entropy and determine the complexity entropy's behavior under random circuits. Also, we identify information-theoretic applications of the complexity entropy. The complexity entropy quantifies the resources required for data compression if the compression algorithm must use a restricted number of gates. We further introduce a complexity conditional entropy, which arises naturally in a complexity-constrained variant of information-theoretic decoupling. Assuming that this entropy obeys a conjectured chain rule, we show that the entropy bounds the number of qubits that one can decouple from a reference system, as judged by a computationally bounded referee. Overall, our framework extends the resource-theoretic approach to thermodynamics to integrate a notion of time, as quantified by complexity.

1 aMunson, Anthony1 aKothakonda, Naga, Bhavya Tej1 aHaferkamp, Jonas1 aHalpern, Nicole, Yunger1 aEisert, Jens1 aFaist, Philippe uhttps://arxiv.org/abs/2403.0482801817nas a2200181 4500008004100000245006100041210006100102260001400163520125300177100003201430700001901462700002201481700002201503700002001525700002801545700002501573856003701598 2024 eng d00aEstimation of Hamiltonian parameters from thermal states0 aEstimation of Hamiltonian parameters from thermal states c1/18/20243 a

We upper- and lower-bound the optimal precision with which one can estimate an unknown Hamiltonian parameter via measurements of Gibbs thermal states with a known temperature. The bounds depend on the uncertainty in the Hamiltonian term that contains the parameter and on the term's degree of noncommutativity with the full Hamiltonian: higher uncertainty and commuting operators lead to better precision. We apply the bounds to show that there exist entangled thermal states such that the parameter can be estimated with an error that decreases faster than 1/n−−√, beating the standard quantum limit. This result governs Hamiltonians where an unknown scalar parameter (e.g. a component of a magnetic field) is coupled locally and identically to n qubit sensors. In the high-temperature regime, our bounds allow for pinpointing the optimal estimation error, up to a constant prefactor. Our bounds generalize to joint estimations of multiple parameters. In this setting, we recover the high-temperature sample scaling derived previously via techniques based on quantum state discrimination and coding theory. In an application, we show that noncommuting conserved quantities hinder the estimation of chemical potentials.

1 aGarcía-Pintos, Luis, Pedro1 aBharti, Kishor1 aBringewatt, Jacob1 aDehghani, Hossein1 aEhrenberg, Adam1 aHalpern, Nicole, Yunger1 aGorshkov, Alexey, V. uhttps://arxiv.org/abs/2401.1034301716nas a2200169 4500008004100000245005800041210005800099260001300157520120700170100002501377700002001402700001801422700002801440700001901468700002201487856003701509 2024 eng d00aIntegrability of Goldilocks quantum cellular automata0 aIntegrability of Goldilocks quantum cellular automata c4/3/20243 a

Goldilocks quantum cellular automata (QCA) have been simulated on quantum hardware and produce emergent small-world correlation networks. In Goldilocks QCA, a single-qubit unitary is applied to each qubit in a one-dimensional chain subject to a balance constraint: a qubit is updated if its neighbors are in opposite basis states. Here, we prove that a subclass of Goldilocks QCA -- including the one implemented experimentally -- map onto free fermions and therefore can be classically simulated efficiently. We support this claim with two independent proofs, one involving a Jordan--Wigner transformation and one mapping the integrable six-vertex model to QCA. We compute local conserved quantities of these QCA and predict experimentally measurable expectation values. These calculations can be applied to test large digital quantum computers against known solutions. In contrast, typical Goldilocks QCA have equilibration properties and quasienergy-level statistics that suggest nonintegrability. Still, the latter QCA conserve one quantity useful for error mitigation. Our work provides a parametric quantum circuit with tunable integrability properties with which to test quantum hardware.

1 aHillberry, Logan, E.1 aPiroli, Lorenzo1 aVernier, Eric1 aHalpern, Nicole, Yunger1 aProsen, Tomaž1 aCarr, Lincoln, D. uhttps://arxiv.org/abs/2404.0299402530nas a2200205 4500008004100000245006700041210006600108260001400174520187400188100003502062700002502097700002302122700002002145700002302165700002702188700002202215700002202237700002802259856003702287 2024 eng d00aProperties and Applications of the Kirkwood-Dirac Distribution0 aProperties and Applications of the KirkwoodDirac Distribution c3/27/20243 a

The most famous quasi-probability distribution, the Wigner function, has played a pivotal role in the development of a continuous-variable quantum theory that has clear analogues of position and momentum. However, the Wigner function is ill-suited for much modern quantum-information research, which is focused on finite-dimensional systems and general observables. Instead, recent years have seen the Kirkwood-Dirac (KD) distribution come to the forefront as a powerful quasi-probability distribution for analysing quantum mechanics. The KD distribution allows tools from statistics and probability theory to be applied to problems in quantum-information processing. A notable difference to the Wigner function is that the KD distribution can represent a quantum state in terms of arbitrary observables. This paper reviews the KD distribution, in three parts. First, we present definitions and basic properties of the KD distribution and its generalisations. Second, we summarise the KD distribution's extensive usage in the study or development of measurement disturbance; quantum metrology; weak values; direct measurements of quantum states; quantum thermodynamics; quantum scrambling and out-of-time-ordered correlators; and the foundations of quantum mechanics, including Leggett-Garg inequalities, the consistent-histories interpretation, and contextuality. We emphasise connections between operational quantum advantages and negative or non-real KD quasi-probabilities. Third, we delve into the KD distribution's mathematical structure. We summarise the current knowledge regarding the geometry of KD-positive states (the states for which the KD distribution is a classical probability distribution), describe how to witness and quantify KD non-positivity, and outline relationships between KD non-positivity and observables' incompatibility.

1 aArvidsson-Shukur, David, R. M.1 aBraasch, William, F.1 aDe Bievre, Stephan1 aDressel, Justin1 aJordan, Andrew, N.1 aLangrenez, Christopher1 aLostaglio, Matteo1 aLundeen, Jeff, S.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2403.1889902067nas a2200169 4500008004100000245008100041210006900122260001300191520151900204100002001723700002701743700002001770700002301790700001901813700002801832856003701860 2024 eng d00aQuantum thermodynamics of nonequilibrium processes in lattice gauge theories0 aQuantum thermodynamics of nonequilibrium processes in lattice ga c4/3/20243 a

A key objective in nuclear and high-energy physics is to describe nonequilibrium dynamics of matter, e.g., in the early universe and in particle colliders, starting from the Standard Model. Classical-computing methods, via the framework of lattice gauge theory, have experienced limited success in this mission. Quantum simulation of lattice gauge theories holds promise for overcoming computational limitations. Because of local constraints (Gauss's laws), lattice gauge theories have an intricate Hilbert-space structure. This structure complicates the definition of thermodynamic properties of systems coupled to reservoirs during equilibrium and nonequilibrium processes. We show how to define thermodynamic quantities such as work and heat using strong-coupling thermodynamics, a framework that has recently burgeoned within the field of quantum thermodynamics. Our definitions suit instantaneous quenches, simple nonequilibrium processes undertaken in quantum simulators. To illustrate our framework, we compute the work and heat exchanged during a quench in a Z2 lattice gauge theory coupled to matter in 1+1 dimensions. The thermodynamic quantities, as functions of the quench parameter, evidence an expected phase transition. For general thermal states, we derive a simple relation between a quantum many-body system's entanglement Hamiltonian, measurable with quantum-information-processing tools, and the Hamiltonian of mean force, used to define strong-coupling thermodynamic quantities.

1 aDavoudi, Zohreh1 aJarzynski, Christopher1 aMueller, Niklas1 aOruganti, Greeshma1 aPowers, Connor1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2404.0296501495nas a2200181 4500008004100000245008500041210006900126260001400195490000800209520092100217100001901138700002101157700002701178700002301205700002001228700002801248856003701276 2023 eng d00aCritical phase and spin sharpening in SU(2)-symmetric monitored quantum circuits0 aCritical phase and spin sharpening in SU2symmetric monitored qua c8/17/20230 v1083 a

Monitored quantum circuits exhibit entanglement transitions at certain measurement rates. Such a transition separates phases characterized by how much information an observer can learn from the measurement outcomes. We study SU(2)-symmetric monitored quantum circuits, using exact numerics and a mapping onto an effective statistical-mechanics model. Due to the symmetry's non-Abelian nature, measuring qubit pairs allows for nontrivial entanglement scaling even in the measurement-only limit. We find a transition between a volume-law entangled phase and a critical phase whose diffusive purification dynamics emerge from the non-Abelian symmetry. Additionally, we numerically identify a "spin-sharpening transition." On one side is a phase in which the measurements can efficiently identify the system's total spin quantum number; on the other side is a phase in which measurements cannot.

1 aMajidy, Shayan1 aAgrawal, Utkarsh1 aGopalakrishnan, Sarang1 aPotter, Andrew, C.1 aVasseur, Romain1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2305.1335601222nas a2200157 4500008004100000245006100041210006000102260001400162520073300176100003100909700001600940700002500956700001800981700002800999856003701027 2023 eng d00aDiVincenzo-like criteria for autonomous quantum machines0 aDiVincenzolike criteria for autonomous quantum machines c7/17/20233 a

Controlled quantum machines have matured significantly. A natural next step is to grant them autonomy, freeing them from timed external control. For example, autonomy could unfetter quantum computers from classical control wires that heat and decohere them; and an autonomous quantum refrigerator recently reset superconducting qubits to near their ground states, as is necessary before a computation. What conditions are necessary for realizing useful autonomous quantum machines? Inspired by recent quantum thermodynamics and chemistry, we posit conditions analogous to DiVincenzo's criteria for quantum computing. Our criteria are intended to foment and guide the development of useful autonomous quantum machines.

1 aGuzmán, José, Antonio Ma1 aErker, Paul1 aGasparinetti, Simone1 aHuber, Marcus1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2307.0873901881nas a2200193 4500008004100000245007300041210006900114260001400183490000600197520129800203100002001501700002201521700002101543700001601564700001801580700002401598700002801622856003701650 2023 eng d00aExperimental Observation of Thermalization with Noncommuting Charges0 aExperimental Observation of Thermalization with Noncommuting Cha c4/28/20230 v43 a

Quantum simulators have recently enabled experimental observations of quantum many-body systems' internal thermalization. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number - in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6-21 spins in an approximate microcanonical subspace, a generalization of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

1 aKranzl, Florian1 aLasek, Aleksander1 aJoshi, Manoj, K.1 aKalev, Amir1 aBlatt, Rainer1 aRoos, Christian, F.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2202.0465202055nas a2200241 4500008004100000245005300041210005200094260001300146490000800159520125800167653002701425653004201452653003901494653003101533653004701564653005201611100002201663700002101685700002201706700002001728700002801748856003701776 2023 eng d00aNon-Abelian eigenstate thermalization hypothesis0 aNonAbelian eigenstate thermalization hypothesis c4/6/20230 v1303 a

The eigenstate thermalization hypothesis (ETH) explains why chaotic quantum many-body systems thermalize internally if the Hamiltonian lacks symmetries. If the Hamiltonian conserves one quantity ("charge"), the ETH implies thermalization within a charge sector -- in a microcanonical subspace. But quantum systems can have charges that fail to commute with each other and so share no eigenbasis; microcanonical subspaces may not exist. Furthermore, the Hamiltonian will have degeneracies, so the ETH need not imply thermalization. We adapt the ETH to noncommuting charges by positing a non-Abelian ETH and invoking the approximate microcanonical subspace introduced in quantum thermodynamics. Illustrating with SU(2) symmetry, we apply the non-Abelian ETH in calculating local observables' time-averaged and thermal expectation values. In many cases, we prove, the time average thermalizes. However, we also find cases in which, under a physically reasonable assumption, the time average converges to the thermal average unusually slowly as a function of the global-system size. This work extends the ETH, a cornerstone of many-body physics, to noncommuting charges, recently a subject of intense activity in quantum thermodynamics.

10aFOS: Physical sciences10aHigh Energy Physics - Theory (hep-th)10aQuantum Gases (cond-mat.quant-gas)10aQuantum Physics (quant-ph)10aStatistical Mechanics (cond-mat.stat-mech)10aStrongly Correlated Electrons (cond-mat.str-el)1 aMurthy, Chaitanya1 aBabakhani, Arman1 aIniguez, Fernando1 aSrednicki, Mark1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2206.0531001491nas a2200157 4500008004100000245005900041210005800100260001300158490000800171520102800179100001901207700002201226700002001248700002801268856003701296 2023 eng d00aNon-Abelian symmetry can increase entanglement entropy0 aNonAbelian symmetry can increase entanglement entropy c1/3/20230 v1073 a

The pillars of quantum theory include entanglement and operators' failure to commute. The Page curve quantifies the bipartite entanglement of a many-body system in a random pure state. This entanglement is known to decrease if one constrains extensive observables that commute with each other (Abelian ``charges''). Non-Abelian charges, which fail to commute with each other, are of current interest in quantum thermodynamics. For example, noncommuting charges were shown to reduce entropy-production rates and may enhance finite-size deviations from eigenstate thermalization. Bridging quantum thermodynamics to many-body physics, we quantify the effects of charges' noncommutation -- of a symmetry's non-Abelian nature -- on Page curves. First, we construct two models that are closely analogous but differ in whether their charges commute. We show analytically and numerically that the noncommuting-charge case has more entanglement. Hence charges' noncommutation can promote entanglement.

1 aMajidy, Shayan1 aLasek, Aleksander1 aHuse, David, A.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2209.1430301518nas a2200169 4500008004100000245007200041210006900113260001300182520098500195100001901180700002501199700002201224700002101246700001601267700002801283856003701311 2023 eng d00aNoncommuting conserved charges in quantum thermodynamics and beyond0 aNoncommuting conserved charges in quantum thermodynamics and bey c9/7/20233 a

Thermodynamic systems typically conserve quantities ("charges") such as energy and particle number. The charges are often assumed implicitly to commute with each other. Yet quantum phenomena such as uncertainty relations rely on observables' failure to commute. How do noncommuting charges affect thermodynamic phenomena? This question, upon arising at the intersection of quantum information theory and thermodynamics, spread recently across many-body physics. Charges' noncommutation has been found to invalidate derivations of the thermal state's form, decrease entropy production, conflict with the eigenstate thermalization hypothesis, and more. This Perspective surveys key results in, opportunities for, and work adjacent to the quantum thermodynamics of noncommuting charges. Open problems include a conceptual puzzle: Evidence suggests that noncommuting charges may hinder thermalization in some ways while enhancing thermalization in others.

1 aMajidy, Shayan1 aBraasch, William, F.1 aLasek, Aleksander1 aUpadhyaya, Twesh1 aKalev, Amir1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2306.0005401735nas a2200145 4500008004100000245007200041210006900113260001400182490000800196520127600204100003501480700002501515700002801540856002101568 2023 eng d00aQuantum simulations of time travel can power nonclassical metrology0 aQuantum simulations of time travel can power nonclassical metrol c11/3/20230 v1313 a

We construct a metrology experiment in which the metrologist can sometimes amend her input state by simulating a closed timelike curve, a worldline that travels backward in time. The existence of closed timelike curves is hypothetical. Nevertheless, they can be simulated probabilistically by quantum-teleportation circuits. We leverage such simulations to pinpoint a counterintuitive nonclassical advantage achievable with entanglement. Our experiment echoes a common information-processing task: A metrologist must prepare probes to input into an unknown quantum interaction. The goal is to infer as much information per probe as possible. If the input is optimal, the information gained per probe can exceed any value achievable classically. The problem is that, only after the interaction does the metrologist learn which input would have been optimal. The metrologist can attempt to change her input by effectively teleporting the optimal input back in time, via entanglement manipulation. The effective time travel sometimes fails but ensures that, summed over trials, the metrologist's winnings are positive. Our Gedankenexperiment demonstrates that entanglement can generate operational advantages forbidden in classical chronology-respecting theories.

1 aArvidsson-Shukur, David, R. M.1 aMcConnell, Aidan, G.1 aHalpern, Nicole, Yunger uarXiv:2207.0766601748nas a2200181 4500008004100000245008400041210006900125260001400194520113500208100002501343700002301368700003101391700002901422700002501451700002801476700002501504856003701529 2023 eng d00aThermally driven quantum refrigerator autonomously resets superconducting qubit0 aThermally driven quantum refrigerator autonomously resets superc c5/26/20233 a

The first thermal machines steered the industrial revolution, but their quantum analogs have yet to prove useful. Here, we demonstrate a useful quantum absorption refrigerator formed from superconducting circuits. We use it to reset a transmon qubit to a temperature lower than that achievable with any one available bath. The process is driven by a thermal gradient and is autonomous -- requires no external control. The refrigerator exploits an engineered three-body interaction between the target qubit and two auxiliary qudits coupled to thermal environments. The environments consist of microwave waveguides populated with synthesized thermal photons. The target qubit, if initially fully excited, reaches a steady-state excited-level population of 5×10−4±5×10−4 (an effective temperature of 23.5~mK) in about 1.6~μs. Our results epitomize how quantum thermal machines can be leveraged for quantum information-processing tasks. They also initiate a path toward experimental studies of quantum thermodynamics with superconducting circuits coupled to propagating thermal microwave fields.

1 aAamir, Mohammed, Ali1 aSuria, Paul, Jamet1 aGuzmán, José, Antonio Ma1 aCastillo-Moreno, Claudia1 aEpstein, Jeffrey, M.1 aHalpern, Nicole, Yunger1 aGasparinetti, Simone uhttps://arxiv.org/abs/2305.1671001580nas a2200145 4500008004100000245009700041210006900138260001400207520107900221100002101300700002501321700002301346700002801369856003701397 2023 eng d00aWhat happens to entropy production when conserved quantities fail to commute with each other0 aWhat happens to entropy production when conserved quantities fai c5/24/20233 a

We extend entropy production to a deeply quantum regime involving noncommuting conserved quantities. Consider a unitary transporting conserved quantities ("charges") between two systems initialized in thermal states. Three common formulae model the entropy produced. They respectively cast entropy as an extensive thermodynamic variable, as an information-theoretic uncertainty measure, and as a quantifier of irreversibility. Often, the charges are assumed to commute with each other (e.g., energy and particle number). Yet quantum charges can fail to commute. Noncommutation invites generalizations, which we posit and justify, of the three formulae. Charges' noncommutation, we find, breaks the formulae's equivalence. Furthermore, different formulae quantify different physical effects of charges' noncommutation on entropy production. For instance, entropy production can signal contextuality - true nonclassicality - by becoming nonreal. This work opens up stochastic thermodynamics to noncommuting - and so particularly quantum - charges.

1 aUpadhyaya, Twesh1 aBraasch, William, F.1 aLandi, Gabriel, T.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2305.1548002003nas a2200217 4500008004100000245007300041210006900114260001300183520129800196653002701494653003101521653004701552100002001599700002201619700002101641700001601662700001801678700002401696700002801720856003701748 2022 eng d00aExperimental observation of thermalisation with noncommuting charges0 aExperimental observation of thermalisation with noncommuting cha c2/9/20223 a

Quantum simulators have recently enabled experimental observations of quantum many-body systems' internal thermalisation. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number - in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6-15 spins in an approximate microcanonical subspace, a generalisation of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

10aFOS: Physical sciences10aQuantum Physics (quant-ph)10aStatistical Mechanics (cond-mat.stat-mech)1 aKranzl, Florian1 aLasek, Aleksander1 aJoshi, Manoj, K.1 aKalev, Amir1 aBlatt, Rainer1 aRoos, Christian, F.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2202.0465201745nas a2200181 4500008004100000245009400041210006900135260001500204300001100219490000800230520117000238100002401408700001901432700002801451700002601479700002101505856003701526 2022 eng d00aExperimentally Measuring Rolling and Sliding in Three-Dimensional Dense Granular Packings0 aExperimentally Measuring Rolling and Sliding in ThreeDimensional c06/18/2022 a0480010 v1293 a

We experimentally measure a three-dimensional (3D) granular system’s reversibility under cyclic compression. We image the grains using a refractive-index-matched fluid, then analyze the images using the artificial intelligence of variational autoencoders. These techniques allow us to track all the grains’ translations and 3D rotations with accuracy sufficient to infer sliding and rolling displacements. Our observations reveal unique roles played by 3D rotational motions in granular flows. We find that rotations and contact-point motion dominate the dynamics in the bulk, far from the perturbation’s source. Furthermore, we determine that 3D rotations are irreversible under cyclic compression. Consequently, contact-point sliding, which is dissipative, accumulates throughout the cycle. Using numerical simulations whose accuracy our experiment supports, we discover that much of the dissipation occurs in the bulk, where grains rotate more than they translate. Our observations suggest that the analysis of 3D rotations is needed for understanding granular materials’ unique and powerful ability to absorb and dissipate energy.

1 aBenson, Zackery, A.1 aPeshkov, Anton1 aHalpern, Nicole, Yunger1 aRichardson, Derek, C.1 aLosert, Wolfgang uhttps://arxiv.org/abs/2108.1197501616nas a2200133 4500008004100000245009200041210006900133260001500202490000600217520117300223100002801396700001901424856003901443 2022 eng d00aHow to build Hamiltonians that transport noncommuting charges in quantum thermodynamics0 aHow to build Hamiltonians that transport noncommuting charges in c01/27/20220 v83 a

Noncommuting conserved quantities have recently launched a subfield of quantum thermodynamics. In conventional thermodynamics, a system of interest and a bath exchange quantities -- energy, particles, electric charge, etc. -- that are globally conserved and are represented by Hermitian operators. These operators were implicitly assumed to commute with each other, until a few years ago. Freeing the operators to fail to commute has enabled many theoretical discoveries -- about reference frames, entropy production, resource-theory models, etc. Little work has bridged these results from abstract theory to experimental reality. This paper provides a methodology for building this bridge systematically: We present an algorithm for constructing Hamiltonians that conserve noncommuting quantities globally while transporting the quantities locally. The Hamiltonians can couple arbitrarily many subsystems together and can be integrable or nonintegrable. Special cases of our construction have appeared in quantum chromodynamics (QCD). Our Hamiltonians may be realized physically with superconducting qudits, with ultracold atoms, with trapped ions, and in QCD.

1 aHalpern, Nicole, Yunger1 aMajidy, Shayan uhttps://arxiv.org/abs/2103.14041v101533nas a2200157 4500008004100000245004800041210004800089260001400137520103000151100002101181700002001202700002801222700001701250700002801267856008001295 2022 eng d00aLinear growth of quantum circuit complexity0 aLinear growth of quantum circuit complexity c3/28/20223 a

The complexity of quantum states has become a key quantity of interest across various subfields of physics, from quantum computing to the theory of black holes. The evolution of generic quantum systems can be modelled by considering a collection of qubits subjected to sequences of random unitary gates. Here we investigate how the complexity of these random quantum circuits increases by considering how to construct a unitary operation from Haar-random two-qubit quantum gates. Implementing the unitary operation exactly requires a minimal number of gates—this is the operation’s exact circuit complexity. We prove a conjecture that this complexity grows linearly, before saturating when the number of applied gates reaches a threshold that grows exponentially with the number of qubits. Our proof overcomes difficulties in establishing lower bounds for the exact circuit complexity by combining differential topology and elementary algebraic geometry with an inductive construction of Clifford circuits.

1 aHaferkamp, Jonas1 aFaist, Philippe1 aKothakonda, Naga, B. T.1 aEisert, Jens1 aHalpern, Nicole, Yunger uhttps://quics.umd.edu/publications/linear-growth-quantum-circuit-complexity02003nas a2200205 4500008004100000245008600041210006900127260001300196300001100209490000800220520132800228100002501556700002001581700003501601700002101636700002401657700002801681700002801709856006001737 2022 eng d00aNegative Quasiprobabilities Enhance Phase Estimation in Quantum-Optics Experiment0 aNegative Quasiprobabilities Enhance Phase Estimation in QuantumO c6/2/2022 a2205040 v1283 a

Operator noncommutation, a hallmark of quantum theory, limits measurement precision, according to uncertainty principles. Wielded correctly, though, noncommutation can boost precision. A recent foundational result relates a metrological advantage with negative quasiprobabilities—quantum extensions of probabilities—engendered by noncommuting operators. We crystallize the relationship in an equation that we prove theoretically and observe experimentally. Our proof-of-principle optical experiment features a filtering technique that we term partially postselected amplification (PPA). Using PPA, we measure a wave plate’s birefringent phase. PPA amplifies, by over two orders of magnitude, the information obtained about the phase per detected photon. In principle, PPA can boost the information obtained from the average filtered photon by an arbitrarily large factor. The filter’s amplification of systematic errors, we find, bounds the theoretically unlimited advantage in practice. PPA can facilitate any phase measurement and mitigates challenges that scale with trial number, such as proportional noise and detector saturation. By quantifying PPA’s metrological advantage with quasiprobabilities, we reveal deep connections between quantum foundations and precision measurement.

1 aLupu-Gladstein, Noah1 aYilmaz, Batuhan1 aArvidsson-Shukur, David, R. M.1 aBrodutch, Aharon1 aPang, Arthur, O. T.1 aSteinberg, Aephraim, M.1 aHalpern, Nicole, Yunger uhttps://link.aps.org/doi/10.1103/PhysRevLett.128.22050401714nas a2200181 4500008004100000245004400041210004400085260001500129490000800144520120900152100002801361700002801389700002101417700002001438700001701458700002001475856003701495 2022 eng d00aResource theory of quantum uncomplexity0 aResource theory of quantum uncomplexity c12/19/20220 v1063 a

Quantum complexity is emerging as a key property of many-body systems, including black holes, topological materials, and early quantum computers. A state's complexity quantifies the number of computational gates required to prepare the state from a simple tensor product. The greater a state's distance from maximal complexity, or "uncomplexity," the more useful the state is as input to a quantum computation. Separately, resource theories -- simple models for agents subject to constraints -- are burgeoning in quantum information theory. We unite the two domains, confirming Brown and Susskind's conjecture that a resource theory of uncomplexity can be defined. The allowed operations, fuzzy operations, are slightly random implementations of two-qubit gates chosen by an agent. We formalize two operational tasks, uncomplexity extraction and expenditure. Their optimal efficiencies depend on an entropy that we engineer to reflect complexity. We also present two monotones, uncomplexity measures that decline monotonically under fuzzy operations, in certain regimes. This work unleashes on many-body complexity the resource-theory toolkit from quantum information theory.

1 aHalpern, Nicole, Yunger1 aKothakonda, Naga, B. T.1 aHaferkamp, Jonas1 aMunson, Anthony1 aEisert, Jens1 aFaist, Philippe uhttps://arxiv.org/abs/2110.1137101588nas a2200241 4500008004100000022001400041245008300055210006900138260001400207300001100221490000600232520086600238100002401104700002201128700002101150700001801171700002801189700001401217700002401231700002301255700002101278856004701299 2021 eng d a2058-956500aEntangled quantum cellular automata, physical complexity, and Goldilocks rules0 aEntangled quantum cellular automata physical complexity and Gold c9/29/2021 a0450170 v63 a

Cellular automata are interacting classical bits that display diverse emergent behaviors, from fractals to random-number generators to Turing-complete computation. We discover that quantum cellular automata (QCA) can exhibit complexity in the sense of the complexity science that describes biology, sociology, and economics. QCA exhibit complexity when evolving under "Goldilocks rules" that we define by balancing activity and stasis. Our Goldilocks rules generate robust dynamical features (entangled breathers), network structure and dynamics consistent with complexity, and persistent entropy fluctuations. Present-day experimental platforms -- Rydberg arrays, trapped ions, and superconducting qubits -- can implement our Goldilocks protocols, making testable the link between complexity science and quantum computation exposed by our QCA.

1 aHillberry, Logan, E1 aJones, Matthew, T1 aVargas, David, L1 aRall, Patrick1 aHalpern, Nicole, Yunger1 aBao, Ning1 aNotarnicola, Simone1 aMontangero, Simone1 aCarr, Lincoln, D uhttp://dx.doi.org/10.1088/2058-9565/ac1c4101677nas a2200181 4500008004100000022001400041245010400055210006900159260001500228490000700243520109900250100001901349700002001368700001801388700002401406700002801430856003701458 2021 eng d a2045-232200aMachine learning outperforms thermodynamics in measuring how well a many-body system learns a drive0 aMachine learning outperforms thermodynamics in measuring how wel c10/22/20210 v113 a

Diverse many-body systems, from soap bubbles to suspensions to polymers, learn and remember patterns in the drives that push them far from equilibrium. This learning may be leveraged for computation, memory, and engineering. Until now, many-body learning has been detected with thermodynamic properties, such as work absorption and strain. We progress beyond these macroscopic properties first defined for equilibrium contexts: We quantify statistical mechanical learning using representation learning, a machine-learning model in which information squeezes through a bottleneck. By calculating properties of the bottleneck, we measure four facets of many-body systems' learning: classification ability, memory capacity, discrimination ability, and novelty detection. Numerical simulations of a classical spin glass illustrate our technique. This toolkit exposes self-organization that eludes detection by thermodynamic measures: Our toolkit more reliably and more precisely detects and quantifies learning by matter while providing a unifying framework for many-body learning. 

1 aZhong, Weishun1 aGold, Jacob, M.1 aMarzen, Sarah1 aEngland, Jeremy, L.1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/2004.0360401694nas a2200169 4500008004100000245004400041210004400085260001500129520120900144100002801353700002801381700002101409700002001430700001701450700002001467856003701487 2021 eng d00aResource theory of quantum uncomplexity0 aResource theory of quantum uncomplexity c10/21/20213 a

Quantum complexity is emerging as a key property of many-body systems, including black holes, topological materials, and early quantum computers. A state's complexity quantifies the number of computational gates required to prepare the state from a simple tensor product. The greater a state's distance from maximal complexity, or ``uncomplexity,'' the more useful the state is as input to a quantum computation. Separately, resource theories -- simple models for agents subject to constraints -- are burgeoning in quantum information theory. We unite the two domains, confirming Brown and Susskind's conjecture that a resource theory of uncomplexity can be defined. The allowed operations, fuzzy operations, are slightly random implementations of two-qubit gates chosen by an agent. We formalize two operational tasks, uncomplexity extraction and expenditure. Their optimal efficiencies depend on an entropy that we engineer to reflect complexity. We also present two monotones, uncomplexity measures that decline monotonically under fuzzy operations, in certain regimes. This work unleashes on many-body complexity the resource-theory toolkit from quantum information theory.

1 aHalpern, Nicole, Yunger1 aKothakonda, Naga, B. T.1 aHaferkamp, Jonas1 aMunson, Anthony1 aEisert, Jens1 aFaist, Philippe uhttps://arxiv.org/abs/2110.1137102031nas a2200145 4500008004100000245007100041210006900112260001400181490000800195520154100203100002801744700002701772700001601799856007001815 2020 eng d00aNoncommuting conserved charges in quantum many-body thermalization0 aNoncommuting conserved charges in quantum manybody thermalizatio c4/15/20200 v1013 a

In statistical mechanics, a small system exchanges conserved quantities—heat, particles, electric charge, etc.—with a bath. The small system thermalizes to the canonical ensemble or the grand canonical ensemble, etc., depending on the quantities. The conserved quantities are represented by operators usually assumed to commute with each other. This assumption was removed within quantum-information-theoretic (QI-theoretic) thermodynamics recently. The small system's long-time state was dubbed “the non-Abelian thermal state (NATS).” We propose an experimental protocol for observing a system thermalize to the NATS. We illustrate with a chain of spins, a subset of which forms the system of interest. The conserved quantities manifest as spin components. Heisenberg interactions push the conserved quantities between the system and the effective bath, the rest of the chain. We predict long-time expectation values, extending the NATS theory from abstract idealization to finite systems that thermalize with finite couplings for finite times. Numerical simulations support the analytics: The system thermalizes to near the NATS, rather than to the canonical prediction. Our proposal can be implemented with ultracold atoms, nitrogen-vacancy centers, trapped ions, quantum dots, and perhaps nuclear magnetic resonance. This work introduces noncommuting conserved quantities from QI-theoretic thermodynamics into quantum many-body physics: atomic, molecular, and optical physics and condensed matter.

1 aHalpern, Nicole, Yunger1 aBeverland, Michael, E.1 aKalev, Amir uhttps://journals.aps.org/pre/abstract/10.1103/PhysRevE.101.04211701930nas a2200133 4500008004100000245008000041210006900121260001400190520148400204100002801688700002701716700001601743856003701759 2019 eng d00aEquilibration to the non-Abelian thermal state in quantum many-body physics0 aEquilibration to the nonAbelian thermal state in quantum manybod c6/21/20193 a

In statistical mechanics, a small system exchanges conserved charges---heat, particles, electric charge, etc.---with a bath. The small system thermalizes to the canonical ensemble, or the grand canonical ensemble, etc., depending on the charges. The charges are usually represented by operators assumed to commute with each other. This assumption was removed within quantum-information-theoretic (QI-theoretic) thermodynamics recently. The small system's long-time state was dubbed "the non-Abelian thermal state (NATS)." We propose an experimental protocol for observing a system thermalize to the NATS. We illustrate with a chain of spins, a subset of which form the system of interest. The conserved charges manifest as spin components. Heisenberg interactions push the charges between the system and the effective bath, the rest of the chain. We predict long-time expectation values, extending the NATS theory from abstract idealization to finite systems that thermalize with finite couplings for finite times. Numerical simulations support the analytics: The system thermalizes to the NATS, rather than to the canonical prediction. Our proposal can be implemented with ultracold atoms, nitrogen-vacancy centers, trapped ions, quantum dots, and perhaps nuclear magnetic resonance. This work introduces noncommuting charges from QI-theoretic thermodynamics into quantum many-body physics: atomic, molecular, and optical physics and condensed matter. 

1 aHalpern, Nicole, Yunger1 aBeverland, Michael, E.1 aKalev, Amir uhttps://arxiv.org/abs/1906.0922702365nas a2200145 4500008004100000245006700041210006000108260001200168490000600180520192900186100002802115700001902143700002002162856003702182 2018 eng d00aThe quasiprobability behind the out-of-time-ordered correlator0 aquasiprobability behind the outoftimeordered correlator c04/20180 vA3 a

Two topics, evolving rapidly in separate fields, were combined recently: The out-of-time-ordered correlator (OTOC) signals quantum-information scrambling in many-body systems. The Kirkwood-Dirac (KD) quasiprobability represents operators in quantum optics. The OTOC has been shown to equal a moment of a summed quasiprobability. That quasiprobability, we argue, is an extension of the KD distribution. We explore the quasiprobability's structure from experimental, numerical, and theoretical perspectives. First, we simplify and analyze the weak-measurement and interference protocols for measuring the OTOC and its quasiprobability. We decrease, exponentially in system size, the number of trials required to infer the OTOC from weak measurements. We also construct a circuit for implementing the weak-measurement scheme. Next, we calculate the quasiprobability (after coarse-graining) numerically and analytically: We simulate a transverse-field Ising model first. Then, we calculate the quasiprobability averaged over random circuits, which model chaotic dynamics. The quasiprobability, we find, distinguishes chaotic from integrable regimes. We observe nonclassical behaviors: The quasiprobability typically has negative components. It becomes nonreal in some regimes. The onset of scrambling breaks a symmetry that bifurcates the quasiprobability, as in classical-chaos pitchforks. Finally, we present mathematical properties. The quasiprobability obeys a Bayes-type theorem, for example, that exponentially decreases the memory required to calculate weak values, in certain cases. A time-ordered correlator analogous to the OTOC, insensitive to quantum-information scrambling, depends on a quasiprobability closer to a classical probability. This work not only illuminates the OTOC's underpinnings, but also generalizes quasiprobability theory and motivates immediate-future weak-measurement challenges.

1 aHalpern, Nicole, Yunger1 aSwingle, Brian1 aDressel, Justin uhttps://arxiv.org/abs/1704.0197101416nas a2200133 4500008004100000245004200041210004200083260001500125490000600140520105200146100001901198700002801217856003701245 2018 eng d00aResilience of scrambling measurements0 aResilience of scrambling measurements c2018/06/180 vA3 a

Most experimental protocols for measuring scrambling require time evolution with a Hamiltonian and with the Hamiltonian's negative counterpart (backwards time evolution). Engineering controllable quantum many-body systems for which such forward and backward evolution is possible is a significant experimental challenge. Furthermore, if the system of interest is quantum-chaotic, one might worry that any small errors in the time reversal will be rapidly amplified, obscuring the physics of scrambling. This paper undermines this expectation: We exhibit a renormalization protocol that extracts nearly ideal out-of-time-ordered-correlator measurements from imperfect experimental measurements. We analytically and numerically demonstrate the protocol's effectiveness, up to the scrambling time, in a variety of models and for sizable imperfections. The scheme extends to errors from decoherence by an environment.

1 aSwingle, Brian1 aHalpern, Nicole, Yunger uhttps://arxiv.org/abs/1802.01587