arXiv subjects

Ahana Ghoshal

Publications and source records attributed to Ahana Ghoshal.

At least 19 recordsLinked to original sources

arXiv2026

Non-Markovian and Thermodynamic Signatures in the Classicality Assessment via Kolmogorov Consistency

The Kolmogorov consistency condition (KCC) defines the statistical boundary between classical and quantum dynamics. Its violation signifies the breakdown of a classical Markov description of temporal correlations. In this work, we establish a direct analytical connection between KCC violation and non-Markovianity in open quantum dynamics, revealing how memory effects manifest as departures from classical probabilistic consistency. Within a generic two-level open quantum system framework, we establish quantitative connections between the magnitude of KCC violation and key information-theoretic and thermodynamic quantities, such as mutual information, the Fano factor, heat exchange, and entropy production rate, thereby enabling a thermodynamic interpretation of temporal quantum correlations. Furthermore, we uncover formal correspondences between KCC violation, the Leggett-Garg inequality, and the negativity of the Kirkwood-Dirac quasi-distribution, identifying them as complementary witnesses of temporal quantum non-classicality. Our results thus provide a unified framework linking information-theoretic, thermodynamic, and temporal indicators of quantumness in open quantum systems.

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arXiv2026

Quantum Heat Transformers

We propose a quantum heat transformer (QHT), a quantum thermodynamic device that modulates temperature gradients between two thermal junctions in quantum systems. Functionally, the QHT is analogous to classical absorption heat transformers in its ability to redistribute thermal energy without external work input. Moreover, we show that its performance ratio mirrors that of classical voltage transformers, where the intrinsic parameters of the system play a role similar to the coil turn ratios. We initially design the device for a three-qubit system, representing the smallest possible self-contained heat transformer model. Subsequently we extend to four-qubit systems, with a specific emphasis on exploring the step-down mode as the primary focus. We showcase the versatility and adaptability of the models by illustrating that a variety of self-contained setups can be constructed, each corresponding to different configurations of the interaction Hamiltonian and their associated self-contained conditions. An important effect in this study is the proof of existence of a necessarily transient step-down quantum heat transformer, that has a dual-mode characteristic, wherein the desired step-down mode can be realized within the transient regime of an originally designed step-up mode of the QHT. We also investigate how to control this transient domain up to which the necessarily transient mode can be achieved, by regulating the initial temperature of the qubits in the four-qubit settings. Therefore, this quantum heat transformer model not only acts as an analog to the classical transformers, but also enjoys advanced characteristics, enabling it to function in both step-up and step-down modes within the same setup, unattainable for classical transformers.

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arXiv2025

Qubits in second quantisation in fermionic simulators

Simulating many-body fermionic systems in conventional qubit-based quantum computers poses significant challenges due to the overheads associated with the encoding of fermionic statistics in qubits, leading to the proposal of native fermionic simulators as an alternative. While allowing for fermionic problems to be simulated efficiently, this class of fermionic simulators carries also specific constraints with them and poses other challenges unfamiliar to qubit systems. Here, we propose to pair fermionic modes to form a so-called qubit in second quantisation representation. This allows fermionic gates to be represented as rotations of these second quantised qubits, enabling adaptation of methods for qubit systems. As an application, we use this pairing scheme to represent the measurement of two- and four-point correlators in fermionic simulators with its native gates as a graph problem. Optimising measurement settings is then analysed with various analytical and algorithmic methods.

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arXiv2025

All multipartite entanglements are quantum coherences in locally distinguishable bases

We find that the m-separability and k-partite entanglement of a multipartite quantum system is correlated with quantum coherence of the same with respect to complete orthonormal bases, distinguishable under local operations and classical communication in certain partitions. In particular, we show that the geometric measure of m-inseparable entanglement of a multipartite quantum state is equal to the square of minimum fidelity-based quantum coherence of the state with respect to complete orthonormal bases, that are locally distinguishable in a partition into m-parties.

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arXiv2025

Violation of the thermodynamic uncertainty relation in quantum collisional models

The thermodynamic uncertainty relation (TUR) is a fundamental principle in non-equilibrium thermodynamics that relates entropy production to fluctuations in a system, establishing a trade-off between the precision of an observable and the thermodynamic cost. Investigating TUR violations challenges classical thermodynamic limits, offering the potential for improved precision-entropy trade-offs, which is crucial for enhancing performance and optimization in quantum technologies. In this work, we investigate the thermodynamic uncertainty relation within a quantum collisional model, which offers the advantage of discretizing interactions into successive collisions with auxiliaries, allowing for precise tracking of dynamics and the incorporation of memory effects and non-Markovian behavior. We consider three types of dynamics in the collisional model: one is Markovian evolution, achieved by taking the continuous time limit and imposing the stability condition, while the other two are non-Markovian dynamics-one arising from increasing the collision time between the system and the auxiliaries, and the other from incorporating interactions between the auxiliaries. For the Markovian dynamics, we examine the classical and quantum TUR bounds in the non-equilibrium steady-state regime, and also the finite-time TUR bound. We identify two distinct regimes of classical TUR violation: in some cases, the maximum violation occurs in the steady state, while in others, it is necessarily transient-appearing only at early times and vanishing with further evolution. For the two non-Markovian approaches, we find that both the degree and type of non-Markovianity crucially affect TUR violations. The second approach shows more pronounced violations during transient times, while the first approach has much stronger violations in the steady-state regime for a certain parameter window.

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arXiv2025

Role of phase of optimal probe in noncommutativity vs coherence in quantum multiparameter estimation

Quantum multiparameter estimation offers a framework for the simultaneous estimation of multiple parameters, pertaining to possibly noncommutating observables. While the optimal probe for estimating a single unitary phase is well understood - being a pure state that is an equal superposition of the eigenvectors of the encoding Hamiltonian corresponding to its maximum and minimum eigenvalues - the structure of optimal probes in the multiparameter setting remains more intricate. We investigate the simultaneous estimation of two phases, each encoded through arbitrary qubit Hamiltonians, using arbitrary weight matrices, and considering single-qubit probes. We also consider single-qutrit probes, for which the encoding Hamiltonians are chosen as SU(2) generators. We find that in both the qubit and qutrit scenarios, the optimal probe is a coherent superposition of the eigenstates corresponding to the largest and smallest eigenvalues of the total encoding Hamiltonian, with a fixed - and not arbitrary - relative phase. Remarkably, this optimal probe is independent of the specific choice of weight matrix, making it a universally optimal input state for the estimation of any pair of SU(2) parameters, which can be reparameterized to the phase estimation problem. Furthermore, we show that this probe also maximizes the determinant of the quantum Fisher information matrix, providing a handy tool for identifying the optimal probe state. We also examine the role of commutativity between the generators of the unitary encodings. Our results demonstrate that a high degree of commutativity degrades the achievable precision, rendering estimation infeasible in the extreme case. However, maximal noncommutativity does not necessarily yield optimal precision.

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arXiv2025

Transient effects in quantum refrigerators with finite environments

We explore a small quantum refrigerator consisting of three qubits, each of which is kept in contact with an environment. We consider two settings: one is when there is necessarily transient cooling and the other is when both steady-state and transient coolings prevail. Our primary focus, however, is on the transient cooling phenomena. We show that in the transient regime, the temperature of the cold qubit can decrease further compared to the case where all qubits are connected to Markovian environments, by replacing the environment attached to the cold qubit with a finite-size spin environment, modeled by a few quantum spins interacting with the cold qubit. We also consider refrigeration with more than one finite-size spin environments of the three-qubit refrigerating device. As expected, a steady temperature is reached only if there are at least two Markovian baths, regardless of whether the cold qubit is attached to an environment. We investigate the effects of the finite-size environments on the cooling of the cold qubit when one, two, or all Markovian baths are replaced by finite-size environments. Additionally, we examine this effect in two- and single-qubit self-sustained devices connected to one or more finite-size environments. In deriving the dynamical equations for the qubits connected to finite-size environments, we made no assumptions about the Markovian nature of the environment. As a result, these finite-size environments inherently exhibit information backflow from the environment to the system, a hallmark of non-Markovianity. Hence, we propose a witness to detect non-Markovianity in such systems. Finally, the cooling processes are studied in presence of Markovian noise, and we analyse the response on the refrigeration of the noise strength. In particular, we find the noise strength until which refrigeration remains possible.

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arXiv2024

Modified Landauer's principle: How much can the Maxwell's demon gain by using general system-environment quantum state?

The Landauer principle states that decrease in entropy of a system, inevitably leads to a dissipation of heat to the environment. This statement is usually established by considering the system to be in contact with an environment that is initially in a thermal state, with the system-environment initial state being in a product state. Here we show that a modified Landauer principle, with correction terms, still holds even if the system and environment are initially correlated and the environment is in an athermal state. This is the most general quantum mechanically allowed operation in the Maxwell demon's arsenal, and, in particular, includes non-completely positive but physically realizable maps on the system. The correction terms provide an advantage: they reduce the work required by the Maxwell's demon to erase its memory. The modified principle also incorporates the possibility of arbitrary charge flows, including the usual heat flow, between system and environment. Furthermore, we consider a case where the system is in contact with a large initially-decoupled athermal environment, and we derive the finite-time modified Landauer's bound for the same.

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arXiv2024

Enhancing precision of atomic clocks by tuning disorder in accessories

We find that a quantum device having an accessory involving precision measurement can have an enhancement of its metrological precision in estimating an unknown parameter of the quantum system by insertion of glassy disorder, accidental or engineered. We clearly mention how an unbiased estimator can also be identified in a disordered situation, and how the precision thereof can be bounded by the quantum Cr{á}mer-Rao inequality. We compare the Fisher information-based lower bound of the minimum standard deviation of an unbiased estimator, in presence of glassy disorder in the system, with the same of an ideal, viz. disorder-free, situation. The phenomenon can boost the efficiency of certain measuring devices, such as atomic clocks. The precision of these clocks, when measuring time, hinges on the precise determination of the frequency of a two-level atom. In cases where impurities are present in the atom, and can be modeled as a disorder parameter, it is possible for the measurement of frequency to be more accurate than in an ideal, disorder-free scenario. Moreover, disorder insertion can reduce the requirement of entanglement content of the initial probes, which are copies of two-qubit states, along with providing a disorder-induced enhancement.

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arXiv2024

Multiparty Spohn's theorem for a combination of local Markovian and non-Markovian quantum dynamics

We obtain a Gorini-Kossakowski-Sudarshan-Lindblad -like master equation for two or more quantum systems connected locally to a combination of Markovian and non-Markovian heat baths. The master equation was originally formulated for multiparty systems with either exclusively Markovian or non-Markovian environments. We extend it to encompass the case of multiple quantum systems connected to a mixture of Markovian and non-Markovian heat baths. The coexistence of both non-Markovian and Markovian environments is a plausible scenario, particularly when studying hybrid physical systems such as atom-photon arrangements. We analyze the thermodynamic quantities for such a set of local environments, and derive a modified form of the Spohn's theorem for the setup. The modification of the theorem naturally leads to a witness as well as an easily computable quantifier of non-Markovianity. Expectedly, we find that for multiparty situations, where a combination of Markovian and non-Markovian heat baths are active, the response in thermodynamic system characteristics due to non-Markovian baths is prominent at times close to the initial time of evolution, whereas the long-time behavior is predominantly controlled by the Markovian ones.

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arXiv2024

Restoring metrological quantum advantage of measurement precision in noisy scenario

We show that in presence of a local and uncorrelated dephasing noise, quantum advantage can be obtained in the Fisher information-based lower bound of the minimum uncertainty in estimating parameters of the system Hamiltonian. The quantum advantage refers here to the benefit of initiating with a maximally entangled state instead of a product one. This quantum advantage was known to vanish in the same noisy scenario for a frequency estimation protocol. Restoration of the better precision in frequency estimation with maximally entangled probes can be obtained by incorporating an interaction between the system particles. The interaction examined here is Ising in nature, and is considered with or without a transverse magnetic field. There are instances, e.g. where frequency estimation in presence of a transverse field is considered and quantum advantage is not restored. A quantum advantage can also be obtained while estimating the strength of the introduced magnetic field along the transverse direction, whereas for the instances considered, using uncorrelated probes is better in measuring the coupling parameter of the Ising interaction. We also investigate the dependence of measurement precision on the entanglement content, which is not necessarily maximal, of the initial state. The precision in estimation of coupling constant decreases monotonically with the increase of entanglement content of the initial state, while the same for frequency estimation is independent of the entanglement content of the inputs.

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arXiv2024

Availing non-Markovian dynamics in effective negative temperature-based transient quantum Otto engines

We demonstrate that the efficiency of effective negative temperature-based quantum Otto engines, already known to outperform their traditional counterparts operating with positive-temperature thermal reservoirs, can be further improved by terminating the isochoric strokes before the working substance reaches perfect equilibrium with its environment. Our investigation encompasses both Markovian and non-Markovian dynamics during these finite-time isochoric processes while considering a weak coupling between the working substance and the reservoirs. We assess the performance of these engines as they undergo a transition from the Markovian to the non-Markovian regime using two figures of merit: maximum achievable efficiency at a certain finite time during the isochoric heating stroke, and overall performance of the engine over an extended period during the transient phase of this stroke. We show that the maximum efficiency increases with the increase of non-Markovianity. However, the overall engine performance decreases as non-Markovianity increases. Additionally, we discover the existence of effective negative temperature-based necessarily transient quantum Otto engines. These engines operate within an extended operational domain, reaching into temperature ranges where conventional effective negative temperature-based quantum Otto engines, which rely on perfect thermalization during the isochoric strokes, are unable to function. Furthermore, this extended operational domain of an effective negative temperature-based necessarily transient quantum Otto engine increases as non-Markovianity becomes more pronounced.

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arXiv2023

Estimating phase transition of perturbed J1-J2 Heisenberg quantum chain in mixtures of ground and first excited states

We show that the nearest neighbour entanglement in a mixture of ground and first excited states - a subjacent state - of the J1-J2 Heisenberg quantum spin chain can be used as an order parameter to detect the phase transition of the chain from a gapless spin fluid to a gapped dimer phase. We study the effectiveness of the order parameter for varying relative mixing probabilities between the ground and first excited states in the subjacent state for different system sizes, and extrapolate the results to the thermodynamic limit. We observe that the nearest neighbour concurrence can play a role of a good order parameter even if the system is in the ground state, but with a small finite probability of leaking into the first excited state. Moreover, we apply the order parameter of the subjacent state to investigate the response to separate introductions of anisotropy and of glassy disorder on the phase diagram of the model, and analyse the corresponding finite-size scale exponents and the emergent tricritical point in the former case. The anisotropic J1-J2 chain has a richer phase diagram which is also clearly visible by using the same order parameter.

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arXiv2023

Kerr-type nonlinear baths enhance cooling in quantum refrigerators

We study the self-contained three-qubit quantum refrigerator, with a three-body interaction enabling cooling of the target qubit, in presence of baths composed of anharmonic quantum oscillators with Kerr-type nonlinearity. We show that such baths, locally connected to the three qubits, opens up the opportunity to implement superior steady-state cooling compared to using harmonic oscillator baths, aiding in access to the free energy required for empowering the refrigerator function autonomously. We find that in spite of providing significant primacy in steady-state cooling, such anharmonic baths do not impart much edge over using harmonic oscillator baths if one targets transient cooling. However, we gain access to steady-state cooling in the parameter region where only transient cooling could be achieved by using harmonic baths. Subsequently, we also study the scaling of steady-state cooling advantage and the minimum attainable temperature for varying levels of anharmonicity present in the bath oscillators. Finally, we analyse heat currents and coefficients of performance of quantum refrigerators using bath modes involving Kerr-type nonlinearity, and present a comparison with the case of using bosonic baths made of simple harmonic oscillators. On the way, we derive the decay rates in the Gorini-Kossakowski-Sudarshan-Lindblad quantum master equation for Kerr-type anharmonic oscillator baths.

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arXiv2023

Temperature- and interaction-tweaked efficiency boost of finite-time robust quantum Otto engines

We demonstrate that under specific conditions, a finite-time quantum Otto engine, employing a spin-1/2 particle as the working substance, despite undergoing incomplete Otto cycles, can achieve higher efficiency than an ideal quantum Otto engine. A finite-time quantum Otto engine refers to an Otto engine where the two isochoric strokes are prematurely terminated before reaching thermal equilibrium with their respective hot and cold baths. We observe that the enhancement of efficiency of a finite-time quantum Otto engine over the ideal one can be realized by adjusting the initial temperature of the working substance within the temperature range of the hot and cold baths. We also find that incorporating an auxiliary qubit, and activating specific interactions between the single-qubit working substance and the auxiliary one, can enhance the efficiency of a finite-time as well as an ideal quantum Otto engine. Furthermore, we analyze the impact of glassy disorder within the system-bath coupling during the two isochoric strokes on the efficiency of a finite-time quantum Otto engine. We find that as strength of disorder increases, efficiency of a finite-time quantum Otto engine tends to decrease, albeit with relatively modest reduction even for strong disorder. However, the advantage in efficiency of the finite-time quantum Otto engine over the ideal one, obtained by tuning the initial state temperature, and the efficiency enhancement obtained by incorporating an auxiliary over the without-auxiliary scenario, persists even in presence of substantial disorder. Additionally, we show that while this disorder does not affect the ideal efficiency, it does influence the duration of isochoric strokes needed for an Otto engine to reach ideal efficiency. This stroke duration remains nearly constant until a specific disorder strength, beyond which it increases rapidly.

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arXiv2023

Isolating noise and amplifying signal with quantum Cheshire cat

The so-called quantum Cheshire cat is a phenomenon in which an object, identified with a "cat", is dissociated from a property of the object, identified with the "grin" of the cat. We propose a thought experiment, similar to this phenomenon, with an interferometric setup, where a property (a component of polarization) of an object (photon) can be separated from the object itself and can simultaneously be amplified when it is already decoupled from its object. We further show that this setup can be used to dissociate two complementary properties, e.g., two orthogonal components of polarization of a photon and identified with the grin and the snarl of a cat, from each other and one of them can be amplified while being detached from the other. Moreover, we extend the work to a noisy scenario, effected by a spin-orbit-coupling -like additional interaction term in the Hamiltonian for the measurement process, with the object in this scenario being identified with a so-called confused Cheshire cat. We devise a gedanken experiment in which such a "confusion" can be successfully dissociated from the system, and we find that the dissociation helps in the amplification of signals.

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arXiv2023

Optimal quantum resource generation in coupled transmons immersed in Markovian baths

We analyze the quantum resource generation of capacitively-coupled multilevel transmon circuits surrounded by bosonic baths, within the Markovian limit. In practice, the superconducting circuit elements are usually part of a larger circuit, constructed with many other linear circuit elements, which along with their environment is assumed to be mimicked by the baths. We study the response to variation of the coupling strength of resource generation for thee system prepared in zero-resource initial states. We focus, in particular, on entanglement and quantum coherence as resources. We quantify the entanglement generation power of coupled transmon qutrits, taking into account the maximum entanglement the system can generate and the time-scale over which the system can sustain a significant entanglement. We identify the optimal initial separable states leading to maximum entanglement generating power.

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arXiv2023

Invariance of success probability in Grover's quantum search under local noise with memory

We analyze the robustness of Grover's quantum search algorithm performed by a quantum register under a possibly time-correlated noise acting locally on the qubits. We model the noise as originating from an arbitrary but fixed unitary evolution, $U$, of some noisy qubits. The noise can occur with some probability in the interval between any pair of consecutive noiseless Grover evolutions. Although each run of the algorithm is a unitary process, the noise model leads to decoherence when all possible runs are considered. We derive a set of unitary $U$'s, called the 'good noises,' for which the success probability of the algorithm at any given time remains unchanged with varying the non-trivial total number ($m$) of noisy qubits in the register. The result holds irrespective of the presence of any time-correlations in the noise. We show that only when $U$ is either of the Pauli matrices $σ_x$ and $σ_z$ (which give rise to $m$-qubit bit-flip and phase-damping channels respectively in the time-correlation-less case), the algorithm's success probability stays unchanged when increasing or decreasing $m$. In contrast, when $U$ is the Pauli matrix $σ_y$ (giving rise to $m$-qubit bit-phase flip channel in the time-correlation-less case), the success probability at all times stays unaltered as long as the parity (even or odd) of the total number $m$ remains the same. This asymmetry between the Pauli operators stems from the inherent symmetry-breaking existing within the Grover circuit. We further show that the positions of the noisy sites are irrelevant in case of any of the Pauli noises. The results are illustrated in the cases of time-correlated and time-correlation-less noise. We find that the former case leads to a better performance of the noisy algorithm. We also discuss physical scenarios where our chosen noise model is of relevance.

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