arXiv subjects

A. K. Pan

Publications and source records attributed to A. K. Pan.

At least 37 records · Page 2Linked to original sources

arXiv2023

Device-independent self-testing of unsharp measurements

Semi-device-independent certification of an unsharp instrument has recently been demonstrated [New J. Phys. 21, 083034 (2019)] based on the sequential sharing of quantum advantages in a prepare-measure communication game by assuming the system to be qubit. In this work, we provide device-independent (DI) self-testing of the unsharp instrument through the quantum violation of two Bell inequalities where the devices are uncharacterized and the dimension of the system remains unspecified. We introduce an elegant sum-of-squares approach to derive the dimension-independent optimal quantum violation of Bell inequalities which plays a crucial role. Note that the standard Bell test cannot self-test the post-measurement states and consequently cannot self-test unsharp instrument. The sequential Bell test possess the potential to self-test an unsharp instrument. We demonstrate that there exists a trade-off between the maximum sequential quantum violations of the Clauser-Horne-Shimony-Holt inequality, and they form an optimal pair that enables the DI self-testing of the entangled state, the observables, and the unsharpness parameter. Further, we extend our study to the case of elegant Bell inequality and we argue that it has two classical bounds - the local bound and the non-trivial preparation non-contextual bound, lower than the local bound. Based on the sharing of preparation contextuality by three independent sequential observers, we demonstrate the DI self-testing of two unsharpness parameters. Since an actual experimental scenario involves losses and imperfection, we demonstrate robustness of our certification to noise.

quant-ph↗

arXiv2023

Device-independent certification of degeneracy-breaking measurements

In a device-independent Bell test, the devices are considered to be black boxes and the dimension of the system remains unspecified. The dichotomic observables involved in such a Bell test can be degenerate and one may invoke a suitable measurement scheme to lift the degeneracy. However, the standard Bell test cannot account for whether or up to what extent the degeneracy is lifted, as the effect of lifting the degeneracy can only be reflected in the post-measurement states, which the standard Bell tests do not certify. In this work, we demonstrate the device-independent certification of degeneracy-breaking measurement based on the sequential Bell test by multiple observers who perform degeneracy-breaking unsharp measurements characterized by positive-operator-valued measures (POVMs) - the noisy variants of projectors. The optimal quantum violation of Clauser-Horne-Shimony-Holt inequality by multiple sequential observers eventually enables us to certify up to what extent the degeneracy has been lifted. In particular, our protocol certifies the upper bound on the number of POVMs used for performing such measurements along with the entangled state and measurement observables. We use an elegant sum-of-squares approach that powers such certification of degeneracy-breaking measurements.

quant-ph↗

arXiv2023

Interplay of nonlocality and incompatibility breaking qubit channels

Incompatibility and nonlocality are not only of foundational interest but also act as important resources for quantum information theory. In the Clauser-Horne-Shimony-Holt (CHSH) scenario, the incompatibility of a pair of observables is known to be equivalent to Bell nonlocality. Here, we investigate these notions in the context of qubit channels. The Bell-CHSH inequality has a greater perspective--compared to any genuine tripartite nonlocality scenario--while determining the interplay between nonlocality breaking qubit channels and incompatibility breaking qubit channels. In the Bell-CHSH scenario, we prove that if the conjugate of a channel is incompatibility breaking, then the channel is itself nonlocality breaking and vice versa. However, this equivalence is not straightforwardly generalized to multipartite systems, due to the absence of an equivalence relation between incompatibility and nonlocality in the multipartite scenario. We investigate this relation in the tripartite scenario by considering some well-known states like Greenberger-Horne-Zeilinger and $W$ states and using the notion of Mermin and Svetlichny nonlocality. By subjecting the parties in question to unital qubit channels, we identify the range of state and channel parameters for which incompatibility coexists with nonlocality. Further, we identify the set of unital qubit channels that is Mermin or Svetlichny nonlocality breaking irrespective of the input state.

quant-ph↗

arXiv2022

Quantum violation of trivial and non-trivial preparation non-contextuality: Nonlocality and Steering

This paper illustrates a direct connection between quantum steering and non-trivial preparation contextuality. In two party-two measurement per party-two outcomes per measurement $(2-2-2)$ Bell scenario, any argument of Bell nonlocality is a proof of trivial preparation contextuality; however, the converse may not hold. If one of the parties (say, Alice) performs the measurements of more than two dichotomic observables, then it is possible to find a set of non-trivial functional relations between Alice's observables. We argue that the existence of a suitable set of such non-trivial relations between Alice's observables may warrant the unsteerability of quantum states at the end of another spatially separated party (say, Bob). Interestingly, such constraints can be read as non-trivial preparation non-contextuality assumptions in an ontological model. We further demonstrate two types of Bell inequalities that can be converted into linear steering inequalities using the aforementioned non-trivial conditions on Alice's observables. Such steering inequalities can also be considered as non-trivial preparation noncontextual inequalities. Since the local bound of the family of Bell expression gets reduced under the additional non-trivial conditions, it provides a test of quantum steering and nonlocality from the same family of Bell expressions depending upon its violation of the non-trivial preparation non-contextual or the local bound, thereby establishing a direct connection between quantum steering and non-trivial preparation contextuality.

quant-ph↗

arXiv2022

Robust certification of unsharp instruments through sequential quantum advantages in a prepare-measure communication game

Communication games are one of the widely used tools that are designed to demonstrate quantum supremacy over classical resources. In that, two or more parties collaborate to perform an information processing task to achieve the highest success probability of winning the game. We propose a specific two-party communication game in the prepare-measure scenario that relies on an encoding-decoding task of specific information. We first demonstrate that quantum theory outperforms the classical preparation non-contextual theory, and the optimal quantum success probability of such a communication game enables the semi-device-independent certification of qubit states and measurements. Further, we consider the sequential sharing of quantum preparation contextuality and show that, at most, two sequential observers can share the quantum advantage. The sub-optimal quantum advantages for two sequential observers form an optimal pair that certifies a unique value of the unsharpness parameter of the first observer. Since the practical implementation inevitably introduces noise, we devised a scheme to demonstrate the robust certification of the states and unsharp measurement instruments of both the sequential observers.

quant-ph↗

arXiv2022

Generalized $n$-locality inequalities in linear-chain network for arbitrary inputs scenario and their quantum violations

Multipartite nonlocality in a network is conceptually different from standard multipartite Bell nonlocality. In recent times, network nonlocality has been studied for various topologies. We consider a linear-chain topology of the network and demonstrate the quantum nonlocality (the non-$n$-locality). Such a network scenario involves $n$ number of independent sources and $n+1$ parties, two edge parties (Alice and Charlie), and $n-1$ central parties (Bobs). It is commonly assumed that each party receives only two inputs. In this work, we consider a generalized scenario where the edge parties receive an arbitrary $n$ number of inputs (equals to a number of independent sources), and each of the central parties receives two inputs. We derive a family of generalized $n$-locality inequalities for a linear-chain network for arbitrary $n$ and demonstrate the optimal quantum violation of the inequalities. We introduce an elegant sum-of-squares approach enabling the derivation of the optimal quantum violation of aforesaid inequalities \emph{without} assuming the dimension of the system. We show that the optimal quantum violation requires the observables of edge parties to mutually anticommuting. For $n=2$ and $3$, the optimal quantum violation can be obtained when each edge party shares a two-qubit entangled state with central parties. We further argue that for $n\geq 2$, a single copy of a two-qubit-entangled state may not be enough to exhibit the violation of $n$-locality inequality, but multiple copies of it can activate the quantum violation.

quant-ph↗

arXiv2022

Device-independent randomness certification using multiple copies of entangled states

We demonstrate to what extent many copies of maximally entangled two-qubit states enable for generating a greater amount of certified randomness than that can be certified from a single copy. Although it appears that greater the dimension of the system implies a higher amount of randomness, the non-triviality lies in the device-independent simultaneous certification of generated randomness from many copies of entangled states. This is because, most of the two-outcome Bell inequalities (viz., Clauser-Horne-Shimony-Holt, Elegant, or Chain Bell inequality) are optimized for a single copy of two-qubit entangled state. Thus, such Bell inequalities can certify neither many copies of entangled states nor a higher amount of randomness. In this work, we suitably invoke a family of $n$-settings Bell inequalities which is optimized for $\lfloor n/2 \rfloor$ copies of maximally entangled two-qubit states, thereby, possess the ability to certify more randomness from many copies of two-qubit entangled state.

quant-ph↗

arXiv2022

Sharing nonlocality in quantum network by unbounded sequential observers

Of late, there has been an upsurge of interest in studying the sequential sharing of various forms of quantum correlations, viz., nonlocality, preparation contextuality, coherence, and entanglement. In this work, we explore the sequential sharing of nonlocality in a quantum network. We first consider the simplest case of the two-input bilocality scenario that features two independent sources and three parties, including two edge parties and a central party. We demonstrate that in the symmetric case when the sharing is considered for both the edge parties, the nonlocality can be shared by at most two sequential observers per edge party. However, in the asymmetric case, when the sharing across one edge party is considered, we show that at most, six sequential observers can share the nonlocality in the network. We extend our investigation to the two-input $n$-local scenario in the star-network configuration that features an arbitrary $n$ number of edge parties and one central party. In the asymmetric case, we demonstrate that the network nonlocality can be shared by an unbounded number of sequential observers across one edge party for a suitably large value of $n$. Further, we generalize our study for an arbitrary $m$ input $n$-local scenario in the star-network configuration. We show that even for an arbitrary $m$ input scenario, the nonlocality can be shared by an unbounded number of sequential observers. However, increasing the input $m$, one has to employ more number of edge parties $n$ than that of the two-input case to demonstrate the sharing of an unbounded number of sequential observers.

quant-ph↗

arXiv2021

Semi-device independent certification of multiple unsharpness parameters through sequential measurements

Based on a sequential communication game, semi-device independent certification of an unsharp instrument has recently been demonstrated [\href{https://iopscience.iop.org/article/10.1088/1367-2630/ab3773}{New J. Phys. 21 083034 (2019), }\href{https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.033014}{ Phys. Rev. Research 2, 033014 (2020)}]. In this paper, we provide semi-device independent self-testing protocols in the prepare-measure scenario to certify multiple unsharpness parameters along with the states and the measurement settings. This is achieved through the sequential quantum advantage shared by multiple independent observers in a suitable communication game known as parity-oblivious random-access-code. We demonstrate that in 3-bit parity-oblivious random-access-code, at most three independent observers can sequentially share quantum advantage. The optimal pair (triple) of quantum advantages enables us to uniquely certify the qubit states, the measurement settings, and the unsharpness parameter(s). The practical implementation of a given protocol involves inevitable losses. In a sub-optimal scenario, we derive a certified interval within which a specific unsharpness parameter has to be confined. We extend our treatment to the 4-bit case and show that at most two observers can share quantum advantage for the qubit system. Further, we provide a sketch to argue that four sequential observers can share the quantum advantage for the two-qubit system, thereby enabling the certification of three unsharpness parameters.

quant-ph↗

arXiv2021

Lüders bounds of Leggett-Garg inequalities, $\mathcal{PT}$- symmetric evolution and arrow-of-time

Leggett Garg inequalities (LGIs) test the incompatibility between the notion of macrorealism and quantum mechanics. For unitary dynamics, the optimal quantum violation of an LGI is constrained by the Lüders bound. However, the LGIs does not provide the necessary and sufficient for macrorealism. A suitably formulated set of no-signaling in time (NSIT) conditions along with the arrow-of-time (AOT) condition provides the same. In this paper, we study two formulations in the three-time LG scenario, viz., the standard LGIs and the recently formulated variant of LGIs when the system evolves under $\mathcal{PT}$-symmetric Hamiltonian. We first demonstrate that the quantum violations of both forms of LGIs exceed their respective Lüders bounds and can even reach their algebraic maximum. We further show that for the case of standard LGI, the violation of Lüders bound can be obtained when both NSIT and AOT conditions are violated. Interestingly, for the case of a variant of LGI, for suitable choices of relevant parameters, the quantum violation can even be obtained when only the AOT is violated but all NSIT conditions are satisfied. This feature has not hitherto been demonstrated. We discuss the further implication of our study.

quant-ph↗

arXiv2021

Quantum violations of Lüders bound Leggett-Garg inequalities for quantum channel

Leggett Garg inequalities (LGIs) provide an elegant way for probing the incompatibility between the notion of macrorealism and quantum mechanics. For unitary dynamics, the optimal quantum violation of a LGI is constrained by the Lüders bound. In this paper, we have studied two formulations of LGIs in a three-time LG scenarios, viz., the standard LGIs and third-order LGIs both for unbiased and biased measurement settings. We show that if the system evolves under a non-unitary quantum channel between two measurements, the quantum violations of both forms of LGIs exceed their respective Lüders bounds and can even reach their algebraic maximum in sharp measurement settings. We found that when the measurement is unsharp the quantum violations of both standard and third-order LGIs for non-unitary quantum channels can be obtained for the lower value of the unsharpness parameter compared to the unitary dynamics. We critically examined the violation of Lüders bound of LGIs and its relation to the violation of various no-signaling in time conditions, another formalism for testing macrorealism. It is shown that mere violations of no-signaling conditions are not enough to warrant the violation of standard LGIs, an interplay between the violations of various NSIT condition along with a threshold value play an important role. On the other hand violation of third-order LGI is obtained when the degree of violation of a specific no-signaling in time condition reaches a different threshold value.

quant-ph↗

arXiv2021

Oblivious communication game, self-testing of projective and non-projective measurements and certification of randomness

We provide an interesting two-party parity oblivious communication game whose success probability is solely determined by the Bell expression. The parity-oblivious condition in an operational quantum theory implies the preparation non-contextuality in an ontological model of it. We find that the aforementioned Bell expression has two upper bounds in an ontological model; the usual local bound and a non-trivial preparation non-contextual bound arising from the non-trivial parity-oblivious condition, which is smaller that the local bound. We first demonstrate the communication game when both Alice and Bob perform three measurements of dichotomic observables in their respective sites. The optimal quantum value of the Bell expression in this scenario enables us to device-independently self-test the maximally entangled state and trine-set of observables, three-outcome qubit positive-operator-valued-measures (POVMs) and 1.58 bit of local randomness. Further, we generalize the above communication game in that both Alice and Bob perform the same but arbitrary (odd) number ($n> 3$) of measurements. Based on the optimal quantum value of the relevant Bell expression for any arbitrary $n$, we have also demonstrated device-independent self-testing of state and measurements.

quant-ph↗

arXiv2021

Generalized $n$-locality inequalities in star-network configuration and their optimal quantum violations

Standard multiparty Bell experiments involve a single source shared by a set of observers. In contrast, network Bell experiments feature multiple independent sources, and each of them may distribute physical systems to a set of observers who perform randomly chosen measurements. The $n$-locality scenario in star-network configuration involves $n$ number of edge observers (Alices), a central observer (Bob), and $n$ number of independent sources having no prior correlation. Each Alice shares an independent state with the central observer Bob. Usually, in network Bell experiments, one considers that each party measures only two observables. In this work, we propose a non-trivial generalization of $n$-locality scenario in star-network configuration, where each Alice performs some integer $m$ number of binary-outcome measurements, and the central party Bob performs $2^{m-1}$ binary-outcome measurements. We derive a family of generalized $n$-locality inequalities for any arbitrary $m$. Using {blue}{an elegant} sum-of-squares approach, we derive the optimal quantum violation of the aforementioned inequalities can be attained when each and every Alice measures $m$ number of mutually anticommuting observables. For $m=2$ and $3$, one obtains the optimal quantum value {blue}{for qubit system local to each Alice, and it is sufficient to consider the sharing of} a two-qubit entangled state between each Alice and Bob. We further demonstrate that the optimal quantum violation of $n$-locality inequality for any arbitrary $m$ can be obtained when every Alice shares $\lfloor m/2\rfloor$ copies of two-qubit maximally entangled state with the central party Bob. We also argue that for $m>3$, a single copy of a two-qubit entangled state may not be enough to exhibit the violation of $n$-locality inequality but multiple copies of it can activate the quantum violation.

quant-ph↗

arXiv2021

Characterizing nonlocal correlations through various $n$-locality inequlities in quantum network

The multipartite quantum networks feature multiple independent sources, in contrast to the conventional multipartite Bell experiment involving a single source. So far, network nonlocality has been explored when each source produces a two-qubit entangled state. In this work, we demonstrate the network nonlocality when each party performs a black-box measurement, and the dimension of the system remains unspecified. In an interesting work, by considering each source produces two-qubit entangled states in the conventional bilocal scenario, Gisin \emph{et. al.} in https://doi.org/10.1103/PhysRevA.96.020304] demonstrated a correspondence between the violations of bipartite Clauser-Horne-Shimony-Halt inequality and the bilocality inequality. We introduce a variant of the sum-of-squares approach to reproduce their results without assuming the dimension of the system. We then generalize the argument for network nonlocality in star-network topology. Further, we propose a new set of $n$-locality inequalities in star-network configuration where each of the $n$ parties performs an arbitrary number of dichotomic measurements and demonstrate the above correspondence between the quantum violations of the $n$-locality inequalities and the chained Bell inequalities. A similar correspondence is demonstrated based on a recently formulated family of $n$-locality inequalities whose optimal quantum violation cannot be obtained when each source emits a two-qubit entangled state and requires multiple copies of two-qubit entangled states. Throughout this paper, each party in the network performs black-box measurements, and the dimension of the system remains unspecified.

quant-ph↗

arXiv2021

Discriminating mirror symmetric states with restricted contextual advantage

The generalized notion of noncontextuality provides an avenue to explore the fundamental departure of quantum theory from a classical explanation. Recently, extracting a different form of quantum advantage in various information processing tasks has received an upsurge of interest. In a recent work [D. Schmid and R. W. Spekkens, Phys. Rev. X 8, 011015 (2018)] it has been demonstrated that discrimination of two nonorthogonal pure quantum states entails contextual advantage when the states are supplied with equal prior probabilities. We generalized the work to arbitrary prior probabilities as well as to three arbitrary mirror-symmetric states. We show that the contextual advantage can be obtained for any value of prior probability when only two quantum states are present in the task. But surprisingly, in the case of three mirror-symmetric states, the contextual advantage is available only for a restrictive range of prior probabilities with which the states are prepared.

quant-ph↗

arXiv2021

$\mathcal{PT}$ symmetric evolution, coherence and violation of Leggett-Garg inequalities

We report an unusual buildup of the quantum coherence in a qubit subjected to non-Hermitian evolution generated by a Parity-Time ($\mathcal{PT}$) symmetric Hamiltonian, which is reinterpreted as a Hermitian system in a higher dimensional space using Naimark dilation. The coherence is found to be maximum about the exceptional points (EPs), i.e., the points of coalescence of the eigenvalues as well as the eigenvectors. The nontrivial physics about EPs has been observed in various systems, particularly in photonic systems. As a consequence of enhancement in coherence, the various formulations of Leggett-Garg inequality tests show maximal violation about the EPs.

quant-ph↗

arXiv2021

Projective measurements under qubit quantum channels

The action of qubit channels on projective measurements on a qubit state is used to establish an equivalence between channels and properties of generalized measurements characterized by bias and sharpness parameters. This can be interpreted as shifting the description of measurement dynamics from the Schrodinger to the Heisenberg picture. In particular, unital quantum channels are shown to induce unbiased measurements. The Markovian channels are found to be equivalent to measurements for which sharpness is a monotonically decreasing function of time. These results are illustrated by considering various noise channels. Further, the effect of bias and sharpness parameters on the energy cost of a measurement and its interplay with non-Markovianity of dynamics is also discussed

quant-ph↗

arXiv2020

L$\ddot{u}$der rule, von Neumann rule and Cirelson's bound of Bell CHSH inequality

In [PRL, 113, 050401 (2014)] the authors have shown that instead of L$\ddot{u}$der rule, if degeneracy breaking von Neumann projection rule is adopted for state reduction, the quantum value of three-time Leggett-Garg inequality can exceed it's L$\ddot{u}$ders bound. Such violation of L$\ddot{u}$ders bound may even approach algebraic maximum of the inequality in the asymptotic limit of system size. They also claim that for Clauser-Horne-Shimony-Holt (CHSH) inequality such violation of L$\ddot{u}$ders bound (known as Cirelson's bound) cannot be obtained even when the measurement is performed sequentially first by Alice followed by Bob. In this paper, we have shown that if von Neumann projection rule is used, quantum bound of CHSH inequality exceeds it's Cirelson's bound and may also reach its algebraic maximum four. This thus provide a strong objection regarding the viability of von Neumann rule as a valid state reduction rule. Further, we pointed out that the violation of Cirelson's bound occurs due to the injection of additional quantum non-locality by the act of implementing von Neumann measurement rule.

quant-ph↗