Alok Kumar Pan

Which verification qubits perform best for secure communication in noisy channel

In secure quantum communication protocols, a set of single qubits prepared using 2 or more mutually unbiased bases or a set of n-qubit

Information transfer using a single particle path-spin hybrid entangled state

(n\ge 2)

Contextuality within quantum mechanics manifested in subensemble mean values

(n≥2) entangled states of a particular form are usually used to form a verification string which is subsequently used to detect traces of eavesdropping. The qubits that form a verification string are referred to as decoy qubits, and there exists a large set of different quantum states that can be used as decoy qubits. In the absence of noise, any choice of decoy qubits provides equivalent security. In this paper, we examine such equivalence for noisy environment (e.g., in amplitude damping, phase damping, collective dephasing and collective rotation noise channels) by comparing the decoy-qubit-assisted schemes of secure quantum communication that use single-qubit states as decoy qubits with the schemes that use entangled states as decoy qubits. Our study reveals that the single- qubit-assisted scheme performs better in some noisy environments, while some entangled-qubit-assisted schemes perform better in other noisy environments. Specifically, single-qubit-assisted schemes perform better in amplitude damping and phase damping noisy channels, whereas a few Bell-state-based decoy schemes are found to perform better in the presence of the collective noise. Thus, if the kind of noise present in a communication channel (i.e., the characteristics of the channel) is known or measured, then the present study can provide the best choice of decoy qubits required for implementation of schemes of secure quantum communication through that channel.

On the quantum analogue of Galileo's leaning tower experiment

Based on a scheme that produces an entanglement between the spin and the path variables of a single spin-(1/2) particle (qubit) using a beam-splitter and a spin-flipper, we formulate a procedure for transferring this intraparticle hybrid entanglement to an interparticle entanglement between the spin variables of two other spatially separated spin-(1/2) particles which never interact with each other during the entire process. This procedure of entanglement swapping is accomplished by a Mach-Zehnder setup in conjunction with the Stern-Gerlach measuring device and by using suitable unitary operations. The proposed protocol, thus, enables the use of intraparticle entanglement as a resource —a feature that has remained unexplored.

Observability of the arrival time distribution using spin-rotator as a quantum clock

The path-spin entangled state of a single spin-1/2 particle is considered which is generated by using a beam-spitter and a spin-flipper. Using this hybrid entanglement at the level of a single particle as a resource, we formulate a protocol for transferring of the state of an unknown qubit to a distant location. Our scheme is implemented by a sequence of unitary operations along with suitable spin-measurements, as well as by using classical communication between the two spatially separated parties. This protocol, thus, demonstrates the possibility of using intraparticle entanglement as a physical resource for performing information theoretic tasks.

A variant of Peres-Mermin proof for testing noncontextual realist models

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 from the use of non-orthogonal states compares to a similar loss of fidelity when noisy or non-maximally entangled states are used as the teleportation resource. Our analysis leads to some interesting results on the teleportation efficiency of both pure and mixed non-orthogonal states compared to that of non-maximally entangled and mixed states.

Quantitative probing of the quantum?classical transition for the arrival time distribution

Abstract For spin-1/2 particles, using a suitable Mach–Zehnder-type setup with a spin-flipper, we argue that it is a direct consequence of the quantum mechanical treatment that an experimentally verifiable subensemble mean of the measured values of an arbitrarily chosen spin variable exhibits dependence on the choice of a comeasurable ‘path’ observable. This, in turn, enables inferring path–spin contextuality at the level of individual measured values of spin that are predetermined using a relevant hidden-variable model applied to our setup.

Toward secure communication using intra-particle entanglement

The quantum analogue of Galileos leaning tower experiment is revisited using wave packets evolving under the gravitational potential. We first calculate the position detection probabilities for particles projected upwards against gravity around the classical turning point and also around the point of initial projection, which exhibit mass dependence at both these points. We then compute the mean arrival time of freely falling particles using the quantum probability current, which also turns out to be mass dependent. The mass dependence of both the position detection probabilities and the mean arrival time vanish in the limit of large mass. Thus, compatibility between the weak equivalence principle and quantum mechanics is recovered in the macroscopic limit of the latter.