arXiv subjects

A. K. Pan

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

65 records · Page 4Linked to original sources

arXiv2017

Joint weak value for all order coupling using continuous variable and qubit probe

The notion of weak measurement in quantum mechanics has gained a significant and wide interest in realizing apparently counterintuitive quantum effects. In recent times, several theoretical and experimental works have been reported for demonstrating the joint weak value of two observables where the coupling strength is restricted to the second order. In this paper, we extend such a formulation by providing a complete treatment of joint weak measurement scenario for all-order-coupling for the observable satisfying $A^2=\mathbb{I}$ and $A^2=A$, which allows us to reveal several hitherto unexplored features. By considering the probe state to be discrete as well as continuous variable, we demonstrate how the joint weak value can be inferred for any given strength of the coupling. A particularly interesting result we pointed out that even if the initial pointer state is uncorrelated, the single pointer displacement can provide the information about the joint weak value, if at least third order of the coupling is taken into account. As an application of our scheme, we provide an all-order-coupling treatment of the well-known Hardy paradox by considering the continuous as well as discrete meter states and show how the negative joint weak probabilities emerge in the quantum paradoxes at the weak coupling limit.

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arXiv2017

Inequivalent Leggett-Garg inequalities

It remains an open question how realist view of macroscopic world emerges from quantum formalism. For testing the macrorealism in quantum domain, an interesting approach was put forward by Leggett and Garg in $1985$, by formulating a suitable inequality valid for any macrorealistic theory. Recently, by following the Wigner idea of local realist inequality, a probabilistic version of standard Leggett-Garg inequalities have also been proposed. While the Wigner form of local realist inequalities are equivalent to the two-party, two-measurements and two outcomes CHSH inequalities, in this paper we provide a generic proof to demonstrate that the Wigner form of Leggett-Garg inequalities are not only inequivalent to the standard ones but also stronger than the later. This is demonstrated by quantifying the amount of disturbance caused by a prior measurement to the subsequent measurements. In this connection, the relation between LGIs and another formulation of macrorealism known as no-signaling in time is examined.

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arXiv2015

Towards secure communication using intra-particle entanglement

We explore the use of the resource of intra-particle entanglement for secure quantum key distribution in the device-independent scenario. By virtue of the local nature of such entanglement, Bell tests must be implemented locally, which leads to a natural decoupling of device errors from channel errors. We consider a side channel attack on the sender's state preparation device, for which the intra-particle-entanglement based scheme is shown to be more secure than the one that uses separable states. Of practical relevance is the fact that such entanglement can be easily generated using linear optics.

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arXiv2013

Comment on "Weak Measurements with Orbital-Angular-Momentum Pointer states"

In a recent Letter (Phys. Rev. Lett. 109, 040401 (2012)), G. Puentes, N. Hermosa and J. P. Torres report a scheme for extracting higher-order weak values by using orbital-angular momentum states as pointer states. They claim that such weak values are inaccessible with a Gaussian pointer state only. In this Comment, we show that the Gaussian pointer state by itself can provide access to the higher-order weak value, if suitable pointer displacement is observed.

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arXiv2013

Three-box paradox and "Cheshire cat grin": the case of spin-1 atoms

We propose in this work a definite theoretical implementation of the three-box paradox - a scheme in which a single quantum particle appears to be present with certainty in two separate boxes - with spin-1 atoms. We further show how our setup can give rise to a "Cheshire cat grin" type of situation, in which an atom can apparently be found with certainty in one of the boxes while one of its properties (the angular momentum projection along a specifically chosen axis) appears to be in a different box. The significance of our findings are discussed relative to the status of the properties of a system obtained from weak measurements.

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arXiv2013

Cat state, sub-Planck structure and weak measurement

Heisenberg-limited and weak measurements are the two intriguing notions, used in recent times for enhancing the sensitivity of measurements in quantum metrology. Using a quantum cat state, endowed with sub-Planck structure, we connect these two novel concepts. It is demonstrated that these two phenomena manifest in complementary regimes, depending upon the degree of overlap between the mesoscopic states constituting the cat state under consideration. In particular, we find that when sub-Planck structure manifests, the imaginary weak value is obscured and vice-versa.

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arXiv2013

Quantum key distribution using intra-particle entanglement

We propose the use of intra-particle entanglement to enhance the security of a practical implementation of the Bennett-Brassard-1984 (BB84) quantum key distribution scheme. Intra-particle entanglement is an attractive resource since it can be easily generated using only linear optics. Security is studied under a simple model of incoherent attack for protocols involving two or all five mutually unbiased bases. In terms of efficiency of secret key generation and tolerable error rate, the latter is found to be superior to the former. We find that states that allow secrecy distillation are necessarily entangled, though they may be local. Since more powerful attacks by Eve obviously exist, our result implies that security is a strictly stronger condition than entanglement for these protocols.

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arXiv2013

Quantum violation of entropic non-contextual inequality in four-dimension

Using joint Shannon entropy, we propose an inequality for a four-level system, which is satisfied in any non-contextual realist hidden variable model. We show that this entropic inequality is violated by quantum mechanics for a range of entangled, as well as product states. Our inequality can be experimentally tested using the existing technology.

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arXiv2012

Weak and semiweak values in non-ideal spin measurements: an exact treatment beyond the asymptotic regime

We consider weak measurements (WM) of a spin observable in quantum mechanics beyond the usual asymptotic regime. This is done by obtaining the exact time-dependent wave functions of the measuring apparatus for general non-ideal measurements. Ideal strong measurements and the usual WM regime are obtained as the two extreme limiting cases of our exact treatment. We show that in the intermediate regime non-ideal measurements lead to "semiweak" eccentric values that differ from the usual weak values. We further show that even in the WM regime the exact treatment leads to a meter behavior that can be markedly distinct from the one predicted by the usual WM formalism. We give several illustrations and discuss an application to the distinction of different realizations of the same density matrix.

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arXiv2010

Quantum teleportation using non-orthogonal entangled channels

We study quantum teleportation with the resource of non-orthogonal qubit states. We first extend the standard teleportation protocol to the case of such states. We investigate how the loss of teleportation fidelity resulting for the use of non-orthogonal states compares to a similar loss of fidelity when noisy or non-maximally entangled states as used as teleportation resource. Our analysis leads to certain interesting results on the teleportation efficiency of both pure and mixed non-orthgonal states compared to that of non-maximally entangled and mixed states.

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arXiv2009

Superdense coding using the quantum superposition principle

By sending a classical two-level system, one can transfer information about only \emph{two} distinguishable outcomes. Here we show that in quantum mechanics, using both the spin and path degrees of freedom of a spin-1/2 particle, and a Mach-Zehnder type interferometric arrangement with two suitable Stern-Gerlach detectors, it is possible to transfer information about \emph{four} distinguishable outcomes. This procedure does \emph{not} require using quantum entanglement as a resource as in the well-known protocol of dense coding, but instead hinges entirely on the quantum superposition principle. We also study probabilistic dense coding using our set-up and show that the dense coding scheme using quantum superposition cannot be optimized any further by extending the interferometric arrangement with more beam splitters.

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