arXiv subjects

Ujjwal Sen

Publications and source records attributed to Ujjwal Sen.

At least 145 records · Page 8Linked to original sources

arXiv2021

Delayed choice of paths selected by grin and snarl of quantum Cheshire Cat

The so-called quantum Cheshire Cat is a scenario where a photon, identified with a cat, and a component of its polarization, identified with the grin of that cat, are separated. We observe that the same techniques can be used to separate two orthogonal components of polarization of a photon, on an average. We identify these polarization components of the photon as the grin and snarl of the cat for ease of comprehension. A gedanken experiment is presented in which we simultaneously tune the input polarizations of the photon in the two arms of a Mach-Zehnder interferometer. It is noted that for two particular choices of photon polarization, the presence of the two components gets reversed in the two arms. This reversal of the grin and the snarl occurs before the polarization components even interact with the tuners, i.e., before the choice of which arm each should be in is made.

quant-ph↗

arXiv2020

Noisy quantum input loophole in measurement-device-independent entanglement witnesses

Entanglement witnesses form an effective method to locally detect entanglement in the laboratory without having the prior knowledge of the full density matrix. However, separable states can be erroneously indicated as entangled in such detections in the presence of wrong measurements or loss in detectors. Measurement-device-independent entanglement witnesses (MDI-EWs) never detect fake entanglement even under wrong measurements and for a particular kind of lossy detectors. A crucial assumption in the case of faithful detection of entanglement employing MDI-EWs is that the preparation devices producing "quantum inputs" - which are inputs additional to the quantum state whose entanglement is to be detected - are perfect and there is no noise during their transmission. Here, we relax these assumptions and provide a general framework for studying the effect of noise on the quantum inputs, invoking uniform and non-uniform noise models. We derive sufficient conditions on the uniform noisy map for retaining the characteristic of MDI-EWs. We find that in the context of non-uniform and entangling noise, fake entanglement detection is possible even by MDI-EWs. We also investigate various paradigmatic models of local noise and find conditions of revealing entanglement in the class of Werner states.

quant-ph↗

arXiv2020

Scaling of non-adiabaticity in disordered quench of quantum Rabi model close to phase transition

Dynamics of a system exhibits non-adiabaticity even for slow quenches near critical points. We analyze the response to disorder in quenches on a non-adiabaticity quantifier for the quantum Rabi model, which possesses a phase transition between normal and superradiant phases. We consider a disordered version of a quench in the Rabi model, in which the system residing in the ground state of an initial Hamiltonian of the normal phase is quenched to the final Hamiltonian corresponding to the critical point. The disorder is inserted either in the total time the quench or in the quench parameter itself. We solve the corresponding quantum dynamics numerically, and find that the non-adiabatic effects are unaffected by the presence of disorder in the total time of the quench. This result is then independently confirmed by the application of adiabatic perturbation theory and the Kibble- Zurek mechanism. For the disorder in the quench parameter, we report a monotonic increase in the adiabaticity with the strength of the disorder. Lastly, we consider a quench where the final Hamiltonian is chosen as the average over the disordered final Hamiltonians, and show that this quench is more adiabatic than the average of the quenches with the disorder in final Hamiltonian.

quant-ph↗

arXiv2020

Equilibrium and dynamical phase transitions in fully connected quantum Ising model: Approximate energy eigenstates and critical time

We study equilibrium as well as dynamical properties of the finite-size fully connected Ising model with a transverse field at the zero temperature. In relation to the equilibrium, we present approximate ground and first excited states that have large overlap -- except near the phase transition point -- with the exact energy eigenstates. For both the approximate and exact eigenstates, we compute the energy gap, concurrence, and geometric measure of quantum entanglement. We observe a good match in the case of energy gap and geometric entanglement between the approximate and exact eigenstates. Whereas, when the system size is large, the concurrence shows a nice agreement only in the paramagnetic phase. In a quench dynamics, we study the time period and the first critical time, which play important roles in the dynamical phase transitions, based on a dynamical order parameter and the Loschmidt rate, respectively. When all the spins are initially polarized in the direction of their mutual interaction, both the time period and critical time diverges logarithmically with the system size at the dynamical critical point. When all the spins are initially in the direction of transverse field, both the time period and critical time exhibit logarithmic or power-law divergences depending on the final field strength. In the case of convergence, we provide estimates for the finite-size scaling and converged value.

cond-mat.stat-mech↗

arXiv2020

How efficient is transport of quantum cargo through multiple highways?

Quantum states can be efficiently transferred over a long distance if the entire quantum channel can be divided into several small blocks. We consider a scenario in which each block consists of two copies of a multiparty state -- one is used for distributing an arbitrary quantum state to multiple parties while the other channel is required to concentrate it back to a single party. Both in noiseless and local noisy scenarios, we find one-shot quantum capacities of these channels in terms of fidelity, when the initial shared states in each block are the generalized Greenberger-Horne-Zeilinger and the generalized W states. We also consider a situation where optimal local measurements transform multipartite states to bipartite ones which can then be used as single-path channels for quantum state transmission in each segment. We show that in some parameter ranges, the former protocol provides strictly better fidelities than that of the latter, thereby establishing the importance of distributing and concentrating arbitrary quantum states via multipartite entangled states in long distance quantum communication, over the local measurement based protocol. Moreover, we show that in presence of bit flip or bit-phase flip noise, shared generalized Greenberger-Horne-Zeilinger states possess an inherent noise detection and correction mechanism, leading to the same fidelity as in the noiseless case. We consider further noise models also, which do not enjoy the same mechanism. In addition to the fidelity based advantages, the multipath scheme is shown to be useful when one considers a situation in which the completion of the teleportation needs to be delayed. We also find the efficiencies of a quantum channel when a quantum state is transferred over long distances and the entire channel is divided into several small blocks.

quant-ph↗

arXiv2020

Witnessing nonseparability of bipartite quantum operations

We provide a method for witnessing nonseparability of quantum processes on shared systems, which uses the channel-state duality. The method uses a maximally entangled state as a resource. We show that using the resource provides significant advantage over the corresponding protocol for nonseparability detection without the resource.

quant-ph↗

arXiv2020

Local preservation of no-signaling in multiparty PT-symmetric evolutions

Violation of the no-signaling principle may occur in PT-symmetric evolutions, that is, evolutions that stem from Hamiltonians that are symmetric with respect to parity and time-reversal, of bipartite entangled states. The violation can be avoided by using a non-conventional inner product. We show that even within the formalism that utilizes the conventional Dirac inner product between physical state vectors, it is possible to obtain instances of local preservation of the no-signaling principle for evolutions corresponding to PT-symmetric non-hermitian Hamiltonians with real eigenvalues, of multiparty entangled states, whose bipartite versions still violate the principle. The phenomenon can be witnessed already by using the Greenberger-Horne-Zeilinger state. Interestingly, the generalized W states do not support such a local preservation of no-signaling.

quant-ph↗

arXiv2020

Frequentist-approach inspired theory of quantum random phenomena predicts signaling

Different ensembles of the same density matrix are indistinguishable within the modern Kolmogorov probability measure theory of quantum random phenomena. We find that changing the framework from the Kolmogorov one to a frequentist-inspired theory of quantum random phenomena -- a la von Mises -- would lift the indistinguishability, and potentially cost us the no-signaling principle (i.e., lead to superluminal communication). We believe that this adds to the recent works on the search for a suitable representation of the state of a quantum system. While erstwhile arguments for potential modifications in the representation of the quantum state were restricted to possible variations in the formalism of the quantum theory, we indicate a possible fallout of altering the underlying theory of random processes.

quant-ph↗

arXiv2020

Signaling versus distinguishing different superpositions of same pure quantum state

Different ensembles of quantum states can have the same average nonpure state. Distinguishing between such constructions, via different mixing procedures of the same nonpure quantum state, is known to entail signaling. In parallel, different superpositions of pure quantum states can lead to the same pure state. We show that the possibility of distinguishing between such preparations, via different interferometric setups leading to the same pure quantum state, also implies signaling. The implication holds irrespective of whether the distinguishing procedure is deterministic or probabilistic.

physics.gen-ph↗

arXiv2020

Multipartite entanglement at dynamical quantum phase transitions with non-uniformly spaced criticalities

We report dynamical quantum phase transition portrait in the alternating field transverse XY spin chain with Dzyaloshinskii-Moriya interaction by investigating singularities in the Loschmidt echo and the corresponding rate function after a sudden quench of system parameters. Unlike the Ising model, the analysis of Loschmidt echo yields non-uniformly spaced transition times in this model. Comparative study between the equilibrium and the dynamical quantum phase transitions in this case reveals that there are quenches where one occurs without the other, and the regimes where they co-exist. However, such transitions happen only when quenching is performed across at least a single gapless or critical line. Contrary to equilibrium phase transitions, bipartite entanglement measures do not turn out to be useful for the detection, while multipartite entanglement emerges as a good identifier of this transition when the quench is done from a disordered phase of this model.

quant-ph↗

arXiv2020

Resource theory of quantum coherence with probabilistically non-distinguishable pointers and corresponding wave-particle duality

One of the fundamental features of quantum mechanics is the superposition principle, a manifestation of which is embodied in quantum coherence. Coherence of a quantum state is invariably defined with respect to a preferred set of pointer states, and there exist quantum coherence measures with respect to deterministically as well as probabilistically distinguishable sets of quantum state vectors. Here we study the resource theory of quantum coherence with respect to an arbitrary set of quantum state vectors, that may not even be probabilistically distinguishable. Geometrically, a probabilistically indistinguishable set of quantum state vectors forms a linearly dependent set. We find the free states of the resource theory, and analyze the corresponding free operations, obtaining a necessary condition for an arbitrary quantum operation to be free. We identify a class of measures of the quantum coherence, and in particular establish a monotonicity property of the measures. We find a connection of an arbitrary set of quantum state vectors with positive operator valued measurements with respect to the resource theory being considered, which paves the way for an alternate definition of the free states. We notice that the resource theory of magic can be looked upon as a resource theory of quantum coherence with respect to a set of quantum state vectors that are probabilistically indistinguishable. We subsequently examine the wave-particle duality in a double-slit set-up in which superposition of probabilistically indistinguishable quantum state vectors is possible. Specifically, we report a complementary relation between quantum coherence and path distinguishability in such a set-up.

quant-ph↗

arXiv2020

How many runs ensure quantum fidelity in teleportation experiment?

The strong law of large numbers asserts that experimentally obtained mean values, in the limit of number of repetitions of the experiment going to infinity, converges almost surely to the theoretical predictions which are based on a priori assumed constant values for probabilities of the random events. Hence in most theoretical calculations, we implicitly neglect fluctuations around the mean. However in practice, we can repeat the experiment only finitely many times, and hence fluctuations are inevitable, and may lead to erroneous judgments. It is theoretically possible to teleport an unknown quantum state, using entanglement, with unit fidelity. The experimentally achieved values are however sub-unit, and often, significantly so. We show that when the number of repetitions of the experiment is small, there is significant probability of achieving a sub-unit experimentally achieved quantum teleportation fidelity that uses entanglement, even classically, i.e., without using entanglement. We further show that only when the number of repetitions of the experiment is of the order of a few thousands, the probability of a classical teleportation process to reach the currently achieved experimental quantum teleportation fidelities becomes negligibly small, and hence ensure that the experimentally obtained fidelities are due to genuine use of the shared entanglements.

quant-ph↗

arXiv2020

Detection loophole in measurement-device-independent entanglement witness

There always exists an entanglement witness for every entangled quantum state. Negativity of the expectation value of an entanglement witness operator guarantees entanglement of the corresponding state, given that the measurement devices involved are perfect, i.e., the performed measurements actually constitute the witness operator for the state under consideration. In a realistic situation, there are two possible ways of measurements to drive the process away from the ideal one. Firstly, wrong measurements may be performed, and secondly, while the measurement operators are implemented correctly, the detection process is noisy. Entanglement witnesses are prone to both of these imperfections. The concept of measurement-device-independent entanglement witnesses was introduced to remove the first problem. We analyze the "detection loophole" in the context of measurement-device-independent entanglement witnesses, which deal with the second problem of imprecise measurements. We obtain an upper bound on the entanglement witness function in the measurement-device-independent entanglement witness scenario, below which entanglement is guaranteed for given non-ideal detector efficiencies, that can involve both lost events and dark counts.

quant-ph↗

arXiv2020

Coherence as witness for quantumness of gravity

We propose an interferometric set-up that utilizes the concept of quantum coherence to provide quantum signatures of gravity. The gravitational force comes into nontrivial play due to the existence of an extra mass in the set-up that transforms an incoherent state to a coherent state. The implication uses the fact that quantum coherence at a certain site cannot be altered by local actions at a separate site. The ability to transform an incoherent state to a coherent one in the presence of gravitational field provides a signature of quantumness of gravity. We also observe that the results remain unaltered in presence of a nontrivial quantity of depolarising noise.

quant-ph↗

arXiv2020

Noncommutative coherence and quantum phase estimation algorithm

We present a measure of quantum coherence by employing the concept of noncommutativity of operators in quantum mechanics. We analyse the behaviour of this noncommutative coherence and underline its similarities and differences with the conventional measures of quantum coherence. The maximally noncommutative coherent states turn out to be far removed from the conventionally considered maximally coherent states. We argue that the efficiency of the quantum phase estimation algorithm, an important rung in the Shor factorisation algorithm, is potentially related to the measure of noncommutative coherence.

quant-ph↗

arXiv2020

Wave-particle duality employing quantum coherence in superposition with non-orthogonal pointers

We propose the notion of quantum coherence for superpositions over states which are not necessarily mutually orthogonal. This anticipatedly leads to a resource theory of non-orthogonal coherence. We characterize free states and free operations in this theory, and connect the latter with free operations in the resource theory of quantum coherence for orthogonal bases. We show that the concept of non-orthogonal coherence naturally furnishes us with a wave-particle duality in quantum double-slit experiments where the channels beyond the slits are leaky between them. Furthermore, we demonstrate existence of a unique maximally coherent qubit state corresponding to any given purity. In addition, and in contradistinction with the case of orthogonal bases, there appears a non-trivial minimally coherent qubit state for a given purity. We also study the behavior of quantum coherence for some typical configurations of non-orthogonal bases which have no analogs for orthogonal bases. We further investigate the problem of determining the energy cost of creating non-orthogonal coherence, and find that it scales linearly with the non-orthogonal coherence created.

quant-ph↗

arXiv2019

Local passivity and entanglement in shared quantum batteries

We identify the conditions for local passivity for shared quantum batteries with local Hamiltonians. For locally passive states of two-qubit batteries, we find the relation of their entanglement content with the amount of energy that can be globally extracted from them. Moreover, we obtain that the deficit in work extraction from pure battery states due to the restriction to local unitaries is equal to the amount of optimal global work extractable from the corresponding pure locally passive battery state, for the same entanglement supply. Furthermore, the pure battery state for which globally extractable work attains a maximum, among the set of all pure states with a fixed value of entanglement, also provides the maximum locally extractable work.

quant-ph↗

arXiv2019

Sequential measurement-device-independent entanglement detection by multiple observers

Violation of a Bell inequality certifies that the underlying state must be entangled in a device-independent way, although there may exist some entangled states which do not violate such an inequality. On the other hand, for every entangled state, it is possible to find a hermitian operator called entanglement witness that can detect entanglement through some local measurements in a device-dependent method. The methods are significantly fragile to lossy detectors. To avoid such difficulties, measurement-device-independent entanglement witness based on a semi-quantum nonlocal game was proposed which turns out to be robust against lossy detectors. We employ here such a measurement-device-independent entanglement witness to detect entanglement in a scenario where half of an entangled pair is possessed by a single observer while the other half is with multiple observers performing measurements sequentially, independently, and preserving entanglement as much as possible. Interestingly, we find that the numbers of successful observers who can detect entanglement measurement device-independently, both with equal and unequal sharpness quotients, are higher than that obtained with standard and Bell inequality-based entanglement detection methods, reflecting its robustness. The entanglement contents of the sequentially shared states are also analyzed. Unlike other scenarios, our investigations also reveal that in this measurement-device-independent situation, states having entanglement in proximity to maximal, remains entangled until two sequential observers even if they measure sharply.

quant-ph↗