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arXiv:2005.00376 · Journal of Optics

Single photon generation and non-locality of perfect W-state

Manoranjan Swain, Amit Rai, M. Karthick Selvan, Prasanta Kumar Panigrahi

Published Jun 25, 2020• 8 citations
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Abstract

Abstract We study the generation of a single photon perfect W-state. An important aspect of this perfect W-state is that it can be used for perfect teleportation and superdense coding, which are not achievable with maximally entangled W-state. Our scheme for generation involves entanglement between various path degrees of freedom of a single photon in a compact and weakly coupled integrated waveguide system, which can be fabricated precisely with a femtosecond laser direct writing technique. These platforms are interferometrically stable, scalable, less sensitive to decoherence and ensure a very low loss factor of 0.1dBcm −1 during photon propagation, and hence are ideal for generation of a perfect W-state. In addition to generation of the single photon perfect W-state, we study its non-local properties using the theory of local elements of reality.

DOI
10.1088/2040-8986/ab9512

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Abstract We study the generation of a single photon perfect W-state. An important aspect of this perfect W-state is that it can be used for perfect teleportation and superdense coding, which are not achievable with maximally entangled W-state. Our scheme for generation involves entanglement between various path degrees of freedom of a single photon in a compact and weakly coupled integrated waveguide system, which can be fabricated precisely with a femtosecond laser direct writing technique. These platforms are interferometrically stable, scalable, less sensitive to decoherence and ensure a very low loss factor of 0.1dBcm −1 during photon propagation, and hence are ideal for generation of a perfect W-state. In addition to generation of the single photon perfect W-state, we study its non-local properties using the theory of local elements of reality.
Abstract W states are maximally entangled states with excellent robustness to loss. They have been studied in ion, photon and waveguide-mode bases, however the entanglement verification has remained challenging. Here, we theoretically study W-state generation by the quantum walk of a single photon through a linearly coupled waveguide array (WGA) that supports self-imaging of light. We design W states in symmetric and asymmetric arrays with even and odd number of waveguides. Bipartite entanglement is formally proven using the von Neumann entropy of the reduced density matrix. We further use the self-imaging to construct an entanglement witness based on the exclusion principle. A key to verification is the interferometer which extends the length of a W-state generator to the revival length where the photon is fully recombined into a single waveguide mode. In addition, W-state coherence is proven by numerically demonstrating the far-field interference. The sensitivities of the proposed W-state generation and verification protocols to the fabrication tolerances are numerically evaluated in glass, silicon nitride and silicon-on-insulator WGAs, indicating the feasibility of their realisation by high-precision e-beam lithography and laser writing.
Relying on weak cross-Kerr nonlinearities, we propose a nearly deterministic generation scheme of the three-photon polarization-entangled perfect WWW state which can be applied to the perfect teleportation of an unknown single-photon state and has robust entanglement against the loss of one photon of them. Three photons entangle together by virtue of the bus function of the coherent state serving as the intermediate among them. In the scheme, three processes are executed successively and two kinds of modules are inserted into the circuit, where the homodyne measurement and the photon number measurement are aptly performed. By means of classical feedforward techniques, single-photon unitary transformation operations are performed on the corresponding photons based on the obtained measurement outcomes, by which the generation efficiency of the perfect WWW state aims to nearly unity. Moreover, some currently available optical elements are applied in the generation process, which offer facilities for the practical implementation.
We report the first experimental realization of perfect W-state in a superconducting qubit based system. In contrast to maximally entangled state, the perfect W state is different in weights and phases of the terms contained in the maximally entangled W-state. The prefect W state finds important applications in quantum information processing tasks such as perfect teleportation, superdense coding, secret sharing etc. The efficiency of generation is quantified by fidelity which is calculated by performing full quantum state tomography. To verify the presence of genuine nonlocality in the generated state, we experimentally perform Mermin's inequality tests. Further, we have also demonstrated splitting and sharing of quantum information using the experimentally generated state.
True tripartite entanglement of the state of a system of three qubits can be classified on the basis of stochastic local operations and classical communications. Such states can be classified into two categories: GHZ states and WWW states. It is known that GHZ states can be used for teleportation and superdense coding, but the prototype WWW state cannot be. However, we show that there is a class of WWW states that can be used for perfect teleportation and superdense coding.
Entanglement represents a fundamental resource for quantum technologies, including quantum communication tasks. In this context, photons are the ideal physical systems due to their capability of carrying information over long distances. Hence, it is fundamental to design reliable sources of entangled photons in the telecom wavelength regime (around 1.55 μm), where optical fiber losses are low. In this paper we review different platforms used to generate telecom entangled photon pairs, and we focus in particular on an integrated source realized by the femtosecond laser writing (FLW) technique, which allows to devise stable and compact optical circuits in glass. We show how this technique can be employed to inscribe waveguides in nonlinear crystals for generation of telecom entangled photon pairs on a chip.
The problem of on-demand generation of entanglement between single-atom qubits via a common photonic channel is examined within the framework of optical interferometry. As expected, for a Mach-Zehnder interferometer with coherent laser beam as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. We show, however, that with a twin-Fock input, useful entanglement can in principle be created without cavity enhancement. Both approaches require single-photon resolving detectors, and best results would be obtained by combining both cavity feedback and twin-Fock inputs. Such an approach may allow a fidelity of 0.99 using a two-photon input and currently available mirror and detector technology. In addition, we study interferometers based on NOON states, i.e., maximally entangled NNN-particle states, and show that they perform similarly to the twin-Fock states, yet without the need for high-precision photodetectors. The present interferometrical approach can serve as a universal, scalable circuit element for quantum information processing, from which fast quantum gates, deterministic teleportation, entanglement swapping, etc., can be realized with the aid of single-qubit operations.