Showing 1–29 of 29 results for author: Modi, K

Quantum control of spin qubits using SOT-driven nanomagnets

Authors: Aniruddha Chakraborty, Kanishk Modi, Dhritiman Bhattacharya, Uditendu Mukhopadhyay, Suddhasatta Mahapatra, Jayasimha Atulasimha

Abstract: Spin rotation (SR) is an essential capability for realization of single and two-qubit gates in spin quantum computing (SQC) architectures. To perform SR, resonant AC magnetic fields are either generated by microwave current pulses fed to an antenna, or voltage pulses applied to a gate, in presence of an inhomogeneous Zeeman field. While the former approach is limited by gate-speed and site-selecti… ▽ More Spin rotation (SR) is an essential capability for realization of single and two-qubit gates in spin quantum computing (SQC) architectures. To perform SR, resonant AC magnetic fields are either generated by microwave current pulses fed to an antenna, or voltage pulses applied to a gate, in presence of an inhomogeneous Zeeman field. While the former approach is limited by gate-speed and site-selectivity of SR, the latter adds to the decoherence of the spin qubits. Here, we propose an alternative technique for driving high-speed SR without compromising the qubit coherence, by employing spin-orbit-torque (SOT)-driven nanomagnets to produce oscillating magnetic fields, locally at the qubit site. The proposed scheme is highly energy-efficient, scalable, and compatible with the CMOS fabrication technology. △ Less

Submitted 30 May, 2026; originally announced June 2026.

Comments: Aniruddha Chakraborty and Kanishk Modi, both authors contribute equally

Optically programmable and erasable cryogenic flash memory on an undoped Si/SiGe heterostructure

Authors: S Rastogi, S Samanta, V Jangir, L Patra, K Modi, A Jain, G Scappucci, U Mukhopadhyay, S Mahapatra

Abstract: Scalable cryogenic memory is a critical yet unresolved requirement for large-scale quantum computing architectures, particularly for computing-in-memory schemes. We exploit the interplay between optical excitation and gate bias in an undoped Si/SiGe heterojunction field-effect transistor (HFET) to realize non-volatile memory functionality. The device exploits a high interface trap density (… ▽ More Scalable cryogenic memory is a critical yet unresolved requirement for large-scale quantum computing architectures, particularly for computing-in-memory schemes. We exploit the interplay between optical excitation and gate bias in an undoped Si/SiGe heterojunction field-effect transistor (HFET) to realize non-volatile memory functionality. The device exploits a high interface trap density ($D_{it} > 1.6 \times 10^{12}$~eV$^{-1}$cm$^{-2}$), which, in conjunction with the oxide thickness and dielectric constant, enables effective "locking" of the threshold voltage to the applied gate bias over a wide voltage range. Two of these states can be selected for binary operation, while the availability of multiple stable states within the same device enables multibit data storage. Robust cycling endurance ($>~10^3$ cycles) and long-term state retention ($>~10^4$~s) of the memory states at 1.5 K confirm the suitability of this approach for integration into Si/SiGe-based quantum computing architectures. △ Less

Submitted 30 May, 2026; originally announced June 2026.

Comments: S Rastogi, S Samanta: These authors contributed equally

Full Shapiro spectroscopy of current-phase relationships

Authors: Maxim Tjøtta, Devashish Shah, Kanishk Modi, Marco Valentini, Rubén Seoane Souto, Georgios Katsaros, Jeroen Danon

Abstract: Extracting the current-phase relationship (CPR) of a single superconducting junction is challenging in practice and traditionally involves embedding the junction in a larger superconducting circuit containing SQUIDs and/or resonators. Applying ac driving to the junction has proven to be a viable and less invasive way to extract information about the few lowest harmonics of the CPR, by locating the… ▽ More Extracting the current-phase relationship (CPR) of a single superconducting junction is challenging in practice and traditionally involves embedding the junction in a larger superconducting circuit containing SQUIDs and/or resonators. Applying ac driving to the junction has proven to be a viable and less invasive way to extract information about the few lowest harmonics of the CPR, by locating the integer and fractional Shapiro steps in the IV-curve of the driven junction. Here, we present an alternative driving-based method that allows to extract the full harmonic content of a CPR in a non-invasive way, by fitting the measured critical currents of the driven junction as a function of driving power. We test our method, both using numerical simulations and in experiments, and we show that it works very accurately, also in the presence of noise. △ Less

Submitted 18 November, 2025; originally announced November 2025.

High-Throughput Screening of 2D Photocatalyst Heterostructures with Suppressed Electron-Hole Recombination for Solar Water Splitting

Authors: Shivanand Yadav, Jainandan Kumar Modi, Raihan Ahammed, B. S. Bhadoria, Yogesh S. Chauhan, Amit Agarwal, Somnath Bhowmick

Abstract: Efficient and scalable photocatalysts for solar water splitting remain a critical challenge in renewable energy research. The work presents a high-throughput first-principles discovery of two-dimensional (2D) type-II van der Waals heterostructures (vdWHs) optimized for visible-light-driven photocatalytic water splitting. We screened 482 heterostructures constructed from 60 experimentally realizabl… ▽ More Efficient and scalable photocatalysts for solar water splitting remain a critical challenge in renewable energy research. The work presents a high-throughput first-principles discovery of two-dimensional (2D) type-II van der Waals heterostructures (vdWHs) optimized for visible-light-driven photocatalytic water splitting. We screened 482 heterostructures constructed from 60 experimentally realizable 2D monolayers and identified 148 stable type-II vdWHs with spatially separated valence and conduction band edges, out of which 65 satisfy the thermodynamic redox conditions for water splitting over a broad pH range. Among these, the best two, MoTe2/Tl2O and MoSe2/WSe2, exhibit a high visible-light absorption coefficient exceeding 0.6X10^6 cm-1, resulting in a high power conversion efficiency of 2%. Quantum kinetic analysis of the hydrogen evolution reaction (HER) reveals nearly barrierless free energy profiles across multiple adsorption sites. Our study further reveals that intrinsic interlayer electric fields in these vdWHs drive directional charge separation, suppressing carrier recombination. Our results establish a design framework for using type-II 2D heterostructures as tunable and experimentally accessible 2D photocatalysts for efficient hydrogen production. △ Less

Submitted 24 August, 2025; originally announced August 2025.

Comments: 22 pages, 5 figures

Journal ref: ACS Applied Energy Materials (2025)

Diagnosing chaos with projected ensembles of process tensors

Authors: Peter O'Donovan, Neil Dowling, Kavan Modi, Mark T. Mitchison

Abstract: The process tensor provides a general representation of a quantum system evolving under repeated interventions and is fundamental for numerical simulations of local many-body dynamics. In this work, we introduce the projected process ensemble, an ensemble of pure output states of a process tensor in a given basis of local interventions, and use it to define increasingly more fine-grained probes of… ▽ More The process tensor provides a general representation of a quantum system evolving under repeated interventions and is fundamental for numerical simulations of local many-body dynamics. In this work, we introduce the projected process ensemble, an ensemble of pure output states of a process tensor in a given basis of local interventions, and use it to define increasingly more fine-grained probes of quantum chaos. The first moment of this ensemble encapsulates numerous previously studied chaos quantifiers, including the Alicki-Fannes quantum dynamical entropy, butterfly flutter fidelity, and spatiotemporal entanglement. We discover characteristic entanglement structures within the ensemble's higher moments that can sharply distinguish chaotic from integrable dynamics, overcoming deficiencies of the quantum dynamical and spatiotemporal entropies. These conclusions are supported by extensive numerical simulations of many-body dynamics for a range of spin-chain models, including non-interacting, interacting-integrable, chaotic, and many-body localized regimes. Our work elucidates the fingerprints of chaos on spatiotemporal correlations in quantum stochastic processes, and provides a unified framework for analyzing the complexity of unitary and monitored many-body dynamics. △ Less

Submitted 15 May, 2026; v1 submitted 19 February, 2025; originally announced February 2025.

Journal ref: PRX Quantum 7, 020322 (2026)

Quantum master equation from the eigenstate thermalization hypothesis

Authors: Peter O'Donovan, Philipp Strasberg, Kavan Modi, John Goold, Mark T. Mitchison

Abstract: We use the eigenstate thermalization hypothesis to derive a quantum master equation for a system weakly coupled to a chaotic finite-sized bath prepared in a pure state. We show that the emergence of Markovianity is controlled by the spectral function of the ETH and that local detailed balance emerges in the Markovian regime for a broad class of pure bath states. We numerically verify this result b… ▽ More We use the eigenstate thermalization hypothesis to derive a quantum master equation for a system weakly coupled to a chaotic finite-sized bath prepared in a pure state. We show that the emergence of Markovianity is controlled by the spectral function of the ETH and that local detailed balance emerges in the Markovian regime for a broad class of pure bath states. We numerically verify this result by comparing the master equation to dynamics computed using exact diagonalization of a chaotic Hamiltonian. We also compare the master equation to exact dynamics for an integrable bath and find that at finite size they strongly disagree. Our work puts forward eigenstate thermalization as a foundation for open quantum systems theory, thus extending it beyond ensemble bath preparations to chaotic many-body environments in generic pure states. △ Less

Submitted 2 September, 2025; v1 submitted 12 November, 2024; originally announced November 2024.

Journal ref: Phys. Rev. B 112, 014312 (2025)

Adversarial Robustness Guarantees for Quantum Classifiers

Authors: Neil Dowling, Maxwell T. West, Angus Southwell, Azar C. Nakhl, Martin Sevior, Muhammad Usman, Kavan Modi

Abstract: Despite their ever more widespread deployment throughout society, machine learning algorithms remain critically vulnerable to being spoofed by subtle adversarial tampering with their input data. The prospect of near-term quantum computers being capable of running {quantum machine learning} (QML) algorithms has therefore generated intense interest in their adversarial vulnerability. Here we show th… ▽ More Despite their ever more widespread deployment throughout society, machine learning algorithms remain critically vulnerable to being spoofed by subtle adversarial tampering with their input data. The prospect of near-term quantum computers being capable of running {quantum machine learning} (QML) algorithms has therefore generated intense interest in their adversarial vulnerability. Here we show that quantum properties of QML algorithms can confer fundamental protections against such attacks, in certain scenarios guaranteeing robustness against classically-armed adversaries. We leverage tools from many-body physics to identify the quantum sources of this protection. Our results offer a theoretical underpinning of recent evidence which suggest quantum advantages in the search for adversarial robustness. In particular, we prove that quantum classifiers are: (i) protected against weak perturbations of data drawn from the trained distribution, (ii) protected against local attacks if they are insufficiently scrambling, and (iii) show evidence that they are protected against universal adversarial attacks if they are sufficiently chaotic. Our analytic results are supported by numerical evidence demonstrating the applicability of our theorems and the resulting robustness of a quantum classifier in practice. This line of inquiry constitutes a concrete pathway to advantage in QML, orthogonal to the usually sought improvements in model speed or accuracy. △ Less

Submitted 26 January, 2026; v1 submitted 16 May, 2024; originally announced May 2024.

Comments: 22 pages, 2 figures. Comments welcome

Journal ref: npj Quantum Information 12, 16 (2026)

Scrambling is Necessary but Not Sufficient for Chaos

Authors: Neil Dowling, Pavel Kos, Kavan Modi

Abstract: We show that out-of-time-order correlators (OTOCs) constitute a probe for Local-Operator Entanglement (LOE). There is strong evidence that a volumetric growth of LOE is a faithful dynamical indicator of quantum chaos, while OTOC decay corresponds to operator scrambling, often conflated with chaos. We show that rapid OTOC decay is a necessary but not sufficient condition for linear (chaotic) growth… ▽ More We show that out-of-time-order correlators (OTOCs) constitute a probe for Local-Operator Entanglement (LOE). There is strong evidence that a volumetric growth of LOE is a faithful dynamical indicator of quantum chaos, while OTOC decay corresponds to operator scrambling, often conflated with chaos. We show that rapid OTOC decay is a necessary but not sufficient condition for linear (chaotic) growth of the LOE entropy. We analytically support our results through wide classes of local-circuit models of many-body dynamics, including both integrable and non-integrable dual-unitary circuits. We show sufficient conditions under which local dynamics leads to an equivalence of scrambling and chaos. △ Less

Submitted 2 November, 2023; v1 submitted 14 April, 2023; originally announced April 2023.

Comments: 6+16 Pages. Comments welcome

Journal ref: Phys. Rev. Lett. 131, 180403 (2023)

Operational Metric for Quantum Chaos and the Corresponding Spatiotemporal Entanglement Structure

Authors: Neil Dowling, Kavan Modi

Abstract: Chaotic systems are highly sensitive to a small perturbation, and are ubiquitous throughout biological sciences, physical sciences and even social sciences. Taking this as the underlying principle, we construct an operational notion for quantum chaos. Namely, we demand that the future state of a many-body, isolated quantum system is sensitive to past multitime operations on a small subpart of that… ▽ More Chaotic systems are highly sensitive to a small perturbation, and are ubiquitous throughout biological sciences, physical sciences and even social sciences. Taking this as the underlying principle, we construct an operational notion for quantum chaos. Namely, we demand that the future state of a many-body, isolated quantum system is sensitive to past multitime operations on a small subpart of that system. By `sensitive', we mean that the resultant states from two different perturbations cannot easily be transformed into each other. That is, the pertinent quantity is the complexity of the effect of the perturbation within the final state. From this intuitive metric, which we call the Butterfly Flutter Fidelity, we use the language of multitime quantum processes to identify a series of operational conditions on chaos, in particular the scaling of the spatiotemporal entanglement. Our criteria already contain the routine notions, as well as the well-known diagnostics for quantum chaos. This includes the Peres-Loschmidt Echo, Dynamical Entropy, Tripartite Mutual Information, and Local-Operator Entanglement. We hence present a unified framework for these existing diagnostics within a single structure. We also go on to quantify how several mechanisms lead to quantum chaos, such as evolution generated from random circuits. Our work paves the way to systematically study many-body dynamical phenomena like Many-Body Localization, measurement-induced phase transitions, and Floquet dynamics. △ Less

Submitted 5 February, 2024; v1 submitted 26 October, 2022; originally announced October 2022.

Comments: Updated/condensed results and discussion. 18 + 7 pages, 7 figures

Journal ref: PRX Quantum 5, 010314 (2024)

Fisher information of correlated stochastic processes

Authors: Marco Radaelli, Gabriel T. Landi, Kavan Modi, Felix C. Binder

Abstract: Many real-world tasks include some kind of parameter estimation, i.e., determination of a parameter encoded in a probability distribution. Often, such probability distributions arise from stochastic processes. For a stationary stochastic process with temporal correlations, the random variables that constitute it are identically distributed but not independent. This is the case, for instance, for q… ▽ More Many real-world tasks include some kind of parameter estimation, i.e., determination of a parameter encoded in a probability distribution. Often, such probability distributions arise from stochastic processes. For a stationary stochastic process with temporal correlations, the random variables that constitute it are identically distributed but not independent. This is the case, for instance, for quantum continuous measurements. In this paper we prove two fundamental results concerning the estimation of parameters encoded in a memoryful stochastic process. First, we show that for processes with finite Markov order, the Fisher information is always asymptotically linear in the number of outcomes, and determined by the conditional distribution of the process' Markov order. Second, we prove with suitable examples that correlations do not necessarily enhance the metrological precision. In fact, we show that unlike for entropic information quantities, in general nothing can be said about the sub- or super-additivity of the joint Fisher information, in the presence of correlations. We discuss how the type of correlations in the process affects the scaling. We then apply these results to the case of thermometry on a spin chain. △ Less

Submitted 7 June, 2023; v1 submitted 1 June, 2022; originally announced June 2022.

Comments: 16 pages, 6 figures; final version

Journal ref: New J. Phys. 25 053037 (2023)

Equilibration of Multitime Quantum Processes in Finite Time Intervals

Authors: Neil Dowling, Pedro Figueroa-Romero, Felix A. Pollock, Philipp Strasberg, Kavan Modi

Abstract: A generic non-integrable (unitary) out-of-equilibrium quantum process, when interrogated across many times, is shown to yield the same statistics as an (non-unitary) equilibrated process. In particular, using the tools of quantum stochastic processes, we prove that under loose assumptions, quantum processes equilibrate within finite time intervals. Sufficient conditions for this to occur are that… ▽ More A generic non-integrable (unitary) out-of-equilibrium quantum process, when interrogated across many times, is shown to yield the same statistics as an (non-unitary) equilibrated process. In particular, using the tools of quantum stochastic processes, we prove that under loose assumptions, quantum processes equilibrate within finite time intervals. Sufficient conditions for this to occur are that multitime observables are coarse grained in both space and time, and that the initial state overlaps with many different energy eigenstates. These results help bridge the gap between (unitary) quantum and (non-unitary) statistical physics, i.e., when all multitime properties and correlations are well approximated by stationary quantities, which includes non-Markovianity and temporal entanglement. We discuss implications of this result for the emergence of classical stochastic processes from multitime measurements of an underlying genuinely quantum system. △ Less

Submitted 16 March, 2023; v1 submitted 2 December, 2021; originally announced December 2021.

Comments: 12+6 pages, 3 figures

Journal ref: SciPost Phys. Core 6, 043 (2023)

Reduced Dynamics of Full Counting Statistics

Authors: Felix A. Pollock, Emanuel Gull, K. Modi, Guy Cohen

Abstract: We present a theory of modified reduced dynamics in the presence of counting fields. Reduced dynamics techniques are useful for describing open quantum systems at long emergent timescales when the memory timescales are short. However, they can be difficult to formulate for observables spanning the system and its environment, such as those characterizing transport properties. A large variety of mix… ▽ More We present a theory of modified reduced dynamics in the presence of counting fields. Reduced dynamics techniques are useful for describing open quantum systems at long emergent timescales when the memory timescales are short. However, they can be difficult to formulate for observables spanning the system and its environment, such as those characterizing transport properties. A large variety of mixed system--environment observables, as well as their statistical properties, can be evaluated by considering counting fields. Given a numerical method able to simulate the field-modified dynamics over the memory timescale, we show that the long-lived full counting statistics can be efficiently obtained from the reduced dynamics. We demonstrate the utility of the technique by computing the long-time current in the nonequilibrium Anderson impurity model from short-time Monte Carlo simulations. △ Less

Submitted 19 May, 2022; v1 submitted 16 November, 2021; originally announced November 2021.

Journal ref: SciPost Phys. 13, 027 (2022)

Relaxation of Multitime Statistics in Quantum Systems

Authors: Neil Dowling, Pedro Figueroa-Romero, Felix A. Pollock, Philipp Strasberg, Kavan Modi

Abstract: Equilibrium statistical mechanics provides powerful tools to understand physics at the macroscale. Yet, the question remains how this can be justified based on a microscopic quantum description. Here, we extend the ideas of pure state quantum statistical mechanics, which focus on single time statistics, to show the equilibration of isolated quantum processes. Namely, we show that most multitime ob… ▽ More Equilibrium statistical mechanics provides powerful tools to understand physics at the macroscale. Yet, the question remains how this can be justified based on a microscopic quantum description. Here, we extend the ideas of pure state quantum statistical mechanics, which focus on single time statistics, to show the equilibration of isolated quantum processes. Namely, we show that most multitime observables for sufficiently large times cannot distinguish a nonequilibrium process from an equilibrium one, unless the system is probed for an extremely large number of times or the observable is particularly fine-grained. A corollary of our results is that the size of non-Markovianity and other multitime characteristics of a nonequilibrium process also equilibrate. △ Less

Submitted 26 May, 2023; v1 submitted 16 August, 2021; originally announced August 2021.

Comments: 7+13 pages, 2+2 figures

Journal ref: Quantum 7, 1027 (2023)

Memory in quantum dot blinking

Authors: Roberto N. Munoz, Laszlo Frazer, Gangcheng Yuan, Paul Mulvaney, Felix A. Pollock, Kavan Modi

Abstract: The photoluminescence intermittency (blinking) of quantum dots is interesting because it is an easily-measured quantum process whose transition statistics cannot be explained by Fermi's Golden Rule. Commonly, the transition statistics are power-law distributed, implying that quantum dots possess at least trivial memories. By investigating the temporal correlations in the blinking data, we demonstr… ▽ More The photoluminescence intermittency (blinking) of quantum dots is interesting because it is an easily-measured quantum process whose transition statistics cannot be explained by Fermi's Golden Rule. Commonly, the transition statistics are power-law distributed, implying that quantum dots possess at least trivial memories. By investigating the temporal correlations in the blinking data, we demonstrate with high statistical confidence that quantum dot blinking data has non-trivial memory, which we define to be statistical complexity greater than one. We show that this memory cannot be discovered using the transition distribution. We show by simulation that this memory does not arise from standard data manipulations. Finally, we conclude that at least three physical mechanisms can explain the measured non-trivial memory: 1) Storage of state information in the chemical structure of a quantum dot; 2) The existence of more than two intensity levels in a quantum dot; and 3) The overlap in the intensity distributions of the quantum dot states, which arises from fundamental photon statistics. △ Less

Submitted 23 June, 2021; v1 submitted 23 June, 2021; originally announced June 2021.

Comments: Added supplementary quantum dot plots in source directory

Journal ref: Phys. Rev. E 106, 014127 (2022)

Universal Bound on Energy Cost of Bit Reset in Finite Time

Authors: Yi-Zheng Zhen, Dario Egloff, Kavan Modi, Oscar Dahlsten

Abstract: We consider how the energy cost of bit reset scales with the time duration of the protocol. Bit reset necessarily takes place in finite time, where there is an extra penalty on top of the quasistatic work cost derived by Landauer. This extra energy is dissipated as heat in the computer, inducing a fundamental limit on the speed of irreversible computers. We formulate a hardware-independent express… ▽ More We consider how the energy cost of bit reset scales with the time duration of the protocol. Bit reset necessarily takes place in finite time, where there is an extra penalty on top of the quasistatic work cost derived by Landauer. This extra energy is dissipated as heat in the computer, inducing a fundamental limit on the speed of irreversible computers. We formulate a hardware-independent expression for this limit in the framework of stochastic processes. We derive a closed-form lower bound on the work penalty as a function of the time taken for the protocol and bit reset error. It holds for discrete as well as continuous systems, assuming only that the master equation respects detailed balance. △ Less

Submitted 13 October, 2021; v1 submitted 1 June, 2021; originally announced June 2021.

Comments: Discussions about the tightness and the application of the bound are added. References are updated. 7+10 pages, 3+1 figures

Journal ref: Phys. Rev. Lett. 127, 190602 (2022)

Energy-efficient quantum frequency estimation

Authors: Pietro Liuzzo-Scorpo, Luis A. Correa, Felix A. Pollock, Agnieszka Górecka, Kavan Modi, Gerardo Adesso

Abstract: The problem of estimating the frequency of a two-level atom in a noisy environment is studied. Our interest is to minimise both the energetic cost of the protocol and the statistical uncertainty of the estimate. In particular, we prepare a probe in a "GHZ-diagonal" state by means of a sequence of qubit gates applied on an ensemble of $ n $ atoms in thermal equilibrium. Noise is introduced via a ph… ▽ More The problem of estimating the frequency of a two-level atom in a noisy environment is studied. Our interest is to minimise both the energetic cost of the protocol and the statistical uncertainty of the estimate. In particular, we prepare a probe in a "GHZ-diagonal" state by means of a sequence of qubit gates applied on an ensemble of $ n $ atoms in thermal equilibrium. Noise is introduced via a phenomenological time-nonlocal quantum master equation, which gives rise to a phase-covariant dissipative dynamics. After an interval of free evolution, the $ n $-atom probe is globally measured at an interrogation time chosen to minimise the error bars of the final estimate. We model explicitly a measurement scheme which becomes optimal in a suitable parameter range, and are thus able to calculate the total energetic expenditure of the protocol. Interestingly, we observe that scaling up our multipartite entangled probes offers no precision enhancement when the total available energy $ \mathcal{E} $ is limited. This is at stark contrast with standard frequency estimation, where larger probes---more sensitive but also more "expensive" to prepare---are always preferred. Replacing $ \mathcal{E} $ by the resource that places the most stringent limitation on each specific experimental setup, would thus help to formulate more realistic metrological prescriptions. △ Less

Submitted 13 June, 2018; v1 submitted 21 December, 2017; originally announced December 2017.

Comments: 16 pages, 3 figures, published version

Journal ref: New J. Phys. 20 063009 (2018)

Spin-chain model of a many-body quantum battery

Authors: Thao P. Le, Jesper Levinsen, Kavan Modi, Meera Parish, Felix A. Pollock

Abstract: Recently, it has been shown that energy can be deposited on a collection of quantum systems at a rate that scales super-extensively. Some of these schemes for `quantum batteries' rely on the use of global many-body interactions that take the batteries through a correlated short cut in state space. Here, we extend the notion of a quantum battery from a collection of a priori isolated systems to a m… ▽ More Recently, it has been shown that energy can be deposited on a collection of quantum systems at a rate that scales super-extensively. Some of these schemes for `quantum batteries' rely on the use of global many-body interactions that take the batteries through a correlated short cut in state space. Here, we extend the notion of a quantum battery from a collection of a priori isolated systems to a many-body quantum system with intrinsic interactions. Specifically, we consider a one-dimensional spin chain with physically realistic two-body interactions. We find that the spin-spin interactions can yield an advantage in charging power over the non-interacting case, and we demonstrate that this advantage can grow super-extensively when the interactions are long ranged. However, we show that, unlike in previous work, this advantage is a mean-field interaction effect that does not involve correlations and that relies on the interactions being intrinsic to the battery. △ Less

Submitted 10 December, 2017; originally announced December 2017.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. A 97, 022106 (2018)

Enhancing the charging power of quantum batteries

Authors: Francesco Campaioli, Felix A. Pollock, Felix C. Binder, Lucas C. Céleri, John Goold, Sai Vinjanampathy, Kavan Modi

Abstract: Can collective quantum effects make a difference in a meaningful thermodynamic operation? Focusing on energy storage and batteries, we demonstrate that quantum mechanics can lead to an enhancement in the amount of work deposited per unit time, i.e., the charging power, when $N$ batteries are charged collectively. We first derive analytic upper bounds for the collective \emph{quantum advantage} in… ▽ More Can collective quantum effects make a difference in a meaningful thermodynamic operation? Focusing on energy storage and batteries, we demonstrate that quantum mechanics can lead to an enhancement in the amount of work deposited per unit time, i.e., the charging power, when $N$ batteries are charged collectively. We first derive analytic upper bounds for the collective \emph{quantum advantage} in charging power for two choices of constraints on the charging Hamiltonian. We then highlight the importance of entanglement by proving that the quantum advantage vanishes when the collective state of the batteries is restricted to be in the separable ball. Finally, we provide an upper bound to the achievable quantum advantage when the interaction order is restricted, i.e., at most $k$ batteries are interacting. Our result is a fundamental limit on the advantage offered by quantum technologies over their classical counterparts as far as energy deposition is concerned. △ Less

Submitted 19 April, 2017; v1 submitted 15 December, 2016; originally announced December 2016.

Comments: In this new updated version Theorem 1 has been changed with Proposition 1. The paper has been published on PRL, and DOI included accordingly

Journal ref: Phys. Rev. Lett. 118, 150601 (2017)

Non-Markovian quantum processes: complete framework and efficient characterisation

Authors: Felix A. Pollock, César Rodríguez-Rosario, Thomas Frauenheim, Mauro Paternostro, Kavan Modi

Abstract: Currently, there is no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. However, recent technological advances mean we have control over systems at scales where memory effects are non-negligible. The lack of such an operational description has hindered advances in understanding physical, chemical and biological processes, where often… ▽ More Currently, there is no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. However, recent technological advances mean we have control over systems at scales where memory effects are non-negligible. The lack of such an operational description has hindered advances in understanding physical, chemical and biological processes, where often unjustified theoretical assumptions are made to render a dynamical description tractable. This has led to theories plagued with unphysical results and no consensus on what a quantum Markov (memoryless) process is. Here, we develop a universal framework to characterise arbitrary non-Markovian quantum processes. We show how a multi-time non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this state is expected to have an efficient matrix product operator form in many cases. Our framework constitutes a systematic tool for the effective description of memory-bearing open-system evolutions. △ Less

Submitted 31 January, 2018; v1 submitted 2 December, 2015; originally announced December 2015.

Comments: 10+4 pages, 6 figures; split into new version and arXiv:1801.09811

Journal ref: Phys. Rev. A 97, 012127 (2018)

Emergence of a fluctuation relation for heat in nonequilibrium Landauer processes

Authors: Philip Taranto, Kavan Modi, Felix A. Pollock

Abstract: In a generalized framework for the Landauer erasure protocol, we study bounds on the heat dissipated in typical nonequilibrium quantum processes. In contrast to thermodynamic processes, quantum fluctuations are not suppressed in the nonequilibrium regime and cannot be ignored, making such processes difficult to understand and treat. Here we derive an emergent fluctuation relation that virtually gu… ▽ More In a generalized framework for the Landauer erasure protocol, we study bounds on the heat dissipated in typical nonequilibrium quantum processes. In contrast to thermodynamic processes, quantum fluctuations are not suppressed in the nonequilibrium regime and cannot be ignored, making such processes difficult to understand and treat. Here we derive an emergent fluctuation relation that virtually guarantees the average heat produced to be dissipated into the reservoir either when the system or reservoir is large (or both) or when the temperature is high. The implication of our result is that for nonequilibrium processes, heat fluctuations away from its average value are suppressed independently of the underlying dynamics exponentially quickly in the dimension of the larger subsystem and linearly in the inverse temperature. We achieve these results by generalizing a concentration of measure relation for subsystem states to the case where the global state is mixed. △ Less

Submitted 14 May, 2018; v1 submitted 28 October, 2015; originally announced October 2015.

Comments: Rewritten to focus on fluctuation theorem result, new application of Levy's lemma to unitary orbits of mixed states; 4+2.5 pages; 2 figures

Journal ref: Phys. Rev. E 97, 052111 (2018)

Quantacell: Powerful charging of quantum batteries

Authors: Felix C. Binder, Sai Vinjanampathy, Kavan Modi, John Goold

Abstract: We study the problem of charging a quantum battery in finite time. We demonstrate an analytical optimal protocol for the case of a single qubit. Extending this analysis to an array of N qubits, we demonstrate that an N-fold advantage in power per qubit can be achieved when global operations are permitted. The exemplary analytic argument for this quantum advantage in the charging power is backed up… ▽ More We study the problem of charging a quantum battery in finite time. We demonstrate an analytical optimal protocol for the case of a single qubit. Extending this analysis to an array of N qubits, we demonstrate that an N-fold advantage in power per qubit can be achieved when global operations are permitted. The exemplary analytic argument for this quantum advantage in the charging power is backed up by numerical analysis using optimal control techniques. It is demonstrated that the quantum advantage for power holds when, with cyclic operation in mind, initial and final states are required to be separable. △ Less

Submitted 27 July, 2015; v1 submitted 24 March, 2015; originally announced March 2015.

Comments: 11 pages, 3 figures, comments welcome

Journal ref: New J. Phys. 17 (2015) 075015

Correlations, operations and the second law of thermodynamics

Authors: Sai Vinjanampathy, Kavan Modi

Abstract: Completely positive trace preserving maps are essential for the formulation of the second law of thermodynamics. The dynamics of quantum systems, correlated with their environments, are in general not described by such maps. We explore how this issue can be fixed by describing the classical analogue of this problem. We consider correlated probability distributions, whose subsequent system dynamics… ▽ More Completely positive trace preserving maps are essential for the formulation of the second law of thermodynamics. The dynamics of quantum systems, correlated with their environments, are in general not described by such maps. We explore how this issue can be fixed by describing the classical analogue of this problem. We consider correlated probability distributions, whose subsequent system dynamics is ill-described by stochastic maps, and prescribe the correct way to describe the dynamics. We use this prescription to discuss the classical version of the second law, valid for correlated probability distributions. △ Less

Submitted 28 November, 2014; originally announced November 2014.

Journal ref: Int. J. Quantum Inf. 14, 1640033 (2016)

Energetic fluctuations in an open quantum process

Authors: John Goold, Kavan Modi

Abstract: Relations similar to work and exchange fluctuations have been recently derived for open systems dynamically evolving in the presence of an ancilla. Extending these relations and constructing a non-equilibrium Helmholtz equation we derive a general expression for the energetic and entropic changes of an open quantum system undergoing a nontrivial evolution. The expressions depend only on the state… ▽ More Relations similar to work and exchange fluctuations have been recently derived for open systems dynamically evolving in the presence of an ancilla. Extending these relations and constructing a non-equilibrium Helmholtz equation we derive a general expression for the energetic and entropic changes of an open quantum system undergoing a nontrivial evolution. The expressions depend only on the state of the system and the dynamical map generating the evolution. Furthermore our formalism makes no assumption on either the nature or dimension of the ancilla. Our results are expected to find application in understanding the energetics of complex quantum systems undergoing open dynamics. △ Less

Submitted 17 July, 2014; originally announced July 2014.

Comments: 5 pages and 3 figures

Journal ref: AVS Quantum Sci. 3, 045001 (2021)

Quantum thermodynamics of general quantum processes

Authors: Felix C. Binder, Sai Vinjanampathy, Kavan Modi, John Goold

Abstract: Accurately describing work extraction from a quantum system is a central objective for the extension of thermodynamics to individual quantum systems. The concepts of work and heat are surprisingly subtle when generalizations are made to arbitrary quantum states. We formulate an operational thermodynamics suitable for application to an open quantum system undergoing quantum evolution under a genera… ▽ More Accurately describing work extraction from a quantum system is a central objective for the extension of thermodynamics to individual quantum systems. The concepts of work and heat are surprisingly subtle when generalizations are made to arbitrary quantum states. We formulate an operational thermodynamics suitable for application to an open quantum system undergoing quantum evolution under a general quantum process by which we mean a completely-positive and trace-preserving map. We derive an operational first law of thermodynamics for such processes and show consistency with the second law. We show that heat, from the first law, is positive when the input state of the map majorises the output state. Moreover, the change in entropy is also positive for the same majorisation condition. This makes a strong connection between the two operational laws of thermodynamics. △ Less

Submitted 27 March, 2015; v1 submitted 11 June, 2014; originally announced June 2014.

Comments: 6 pages, 1 figure

Journal ref: Phys. Rev. E 91, 032119 (2015)

A non-equilibrium quantum Landauer principle

Authors: John Goold, Mauro Paternostro, Kavan Modi

Abstract: Using the operational framework of completely positive, trace preserving operations and thermodynamic fluctuation relations, we derive a lower bound for the heat exchange in a Landauer erasure process on a quantum system. Our bound comes from a non-phenomenological derivation of the Landauer principle which holds for generic non-equilibrium dynamics. Furthermore the bound depends on the non-unital… ▽ More Using the operational framework of completely positive, trace preserving operations and thermodynamic fluctuation relations, we derive a lower bound for the heat exchange in a Landauer erasure process on a quantum system. Our bound comes from a non-phenomenological derivation of the Landauer principle which holds for generic non-equilibrium dynamics. Furthermore the bound depends on the non-unitality of dynamics, giving it a physical significance that differs from other derivations. We apply our framework to the model of a spin-1/2 system coupled to an interacting spin chain at finite temperature. △ Less

Submitted 9 January, 2015; v1 submitted 18 February, 2014; originally announced February 2014.

Comments: 4 pages, 2 figures, RevTeX4-1; Accepted for publication in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 114, 060602 (2015)

Measuring the heat exchange of a quantum process

Authors: John Goold, Ulrich Poschinger, Kavan Modi

Abstract: Very recently, interferometric methods have been proposed to measure the full statistics of work performed on a driven quantum system [Dorner et al. Phys. Rev. Lett. 110 230601 (2013)] and [Mazzola et al. Phys. Rev. Lett. 110 230602 (2013)]. The advantage of such schemes is that they replace the necessity to make projective measurements by performing phase estimation on an appropriately coupled an… ▽ More Very recently, interferometric methods have been proposed to measure the full statistics of work performed on a driven quantum system [Dorner et al. Phys. Rev. Lett. 110 230601 (2013)] and [Mazzola et al. Phys. Rev. Lett. 110 230602 (2013)]. The advantage of such schemes is that they replace the necessity to make projective measurements by performing phase estimation on an appropriately coupled ancilla qubit. These proposals are one possible route to the tangible experimental exploration of quantum thermodynamics, a subject which is the centre of much current attention due to the current control of mesoscopic quantum systems. In this Letter we demonstrate that a modification of the phase estimation protocols can be used in order to measure the heat distribution of a quantum process. In addition we demonstrate how our scheme may be implemented using ion trap technology. Our scheme should pave the way for the first experimental explorations of the Landauer principle and hence the intricate energy to information conversion in mesoscopic quantum systems. △ Less

Submitted 19 August, 2014; v1 submitted 16 January, 2014; originally announced January 2014.

Comments: An experimental proposal with trapped ions added

Journal ref: Phys. Rev. E 90, 020101 (2014)

Work and Quantum Phase Transitions: Is there Quantum Latency?

Authors: E. Mascarenhas, H. Braganca, R. Dorner, M. Franca Santos, V. Vedral, K. Modi, J. Goold

Abstract: We study the physics of quantum phase transitions from the perspective of non-equilibrium thermodynamics. For first order quantum phase transitions, we find that the average work done per quench in crossing the critical point is discontinuous. This leads us to introduce the quantum latent work in analogy with the classical latent heat of first order classical phase transitions. For second order qu… ▽ More We study the physics of quantum phase transitions from the perspective of non-equilibrium thermodynamics. For first order quantum phase transitions, we find that the average work done per quench in crossing the critical point is discontinuous. This leads us to introduce the quantum latent work in analogy with the classical latent heat of first order classical phase transitions. For second order quantum phase transitions the irreversible work is closely related to the fidelity susceptibility for weak sudden quenches of the system Hamiltonian. We demonstrate our ideas with numerical simulations of first, second, and infinite order phase transitions in various spin chain models. △ Less

Submitted 23 May, 2014; v1 submitted 21 July, 2013; originally announced July 2013.

Comments: accepted in PRE

Journal ref: Phys. Rev. E 89, 062103 (2014)

Quantum discord bounds the amount of distributed entanglement

Authors: T. K. Chuan, J. Maillard, K. Modi, T. Paterek, M. Paternostro, M. Piani

Abstract: The ability to distribute quantum entanglement is a prerequisite for many fundamental tests of quantum theory and numerous quantum information protocols. Two distant parties can increase the amount of entanglement between them by means of quantum communication encoded in a carrier that is sent from one party to the other. Intriguingly, entanglement can be increased even when the exchanged carrier… ▽ More The ability to distribute quantum entanglement is a prerequisite for many fundamental tests of quantum theory and numerous quantum information protocols. Two distant parties can increase the amount of entanglement between them by means of quantum communication encoded in a carrier that is sent from one party to the other. Intriguingly, entanglement can be increased even when the exchanged carrier is not entangled with the parties. However, in light of the defining property of entanglement stating that it cannot increase under classical communication, the carrier must be quantum. Here we show that, in general, the increase of relative entropy of entanglement between two remote parties is bounded by the amount of non-classical correlations of the carrier with the parties as quantified by the relative entropy of discord. We study implications of this bound, provide new examples of entanglement distribution via unentangled states and put further limits on this phenomenon. △ Less

Submitted 31 July, 2012; v1 submitted 6 March, 2012; originally announced March 2012.

Comments: 8 pages, 1 figure, RevTeX4; Accepted for publication in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 109, 070501 (2012)

The classical-quantum boundary for correlations: discord and related measures

Authors: Kavan Modi, Aharon Brodutch, Hugo Cable, Tomasz Paterek, Vlatko Vedral

Abstract: One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are amongst the more actively-studied topics of quantum information theory over the past decade. Entanglement is the most prominent of these correlations, but in many cases unentangle… ▽ More One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are amongst the more actively-studied topics of quantum information theory over the past decade. Entanglement is the most prominent of these correlations, but in many cases unentangled states exhibit nonclassical behavior too. Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states. Here we review different notions of classical and quantum correlations quantified by quantum discord and other related measures. In the first half, we review the mathematical properties of the measures of quantum correlations, relate them to each other, and discuss the classical-quantum division that is common among them. In the second half, we show that the measures identify and quantify the deviation from classicality in various quantum-information-processing tasks, quantum thermodynamics, open-system dynamics, and many-body physics. We show that in many cases quantum correlations indicate an advantage of quantum methods over classical ones. △ Less

Submitted 27 November, 2012; v1 submitted 29 December, 2011; originally announced December 2011.

Comments: Close to the published version

Journal ref: Rev. Mod. Phys. 84, 1655-1707 (2012)