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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Proposal to produce two and four qubits with spatial modes of two photons

Enrique J. Galvez

Published Feb 7, 2012• 5 citations
Publisher

Abstract

This article presents a proposal for preparing photon pairs in states that are entangled in their spatial modes. The method allows the encoding of any pair of spatial modes that is desired, without restrictions; a procedure that up to now has been elusive. This method consists of three steps: preparing photon pairs in polarization-entangled states, filtering the spatial mode, and use of polarization interferometers with diffractive mode-encoding elements to effect entanglement swapping between polarization and spatial modes. An extension of the method consists of entangling polarization and spatial mode, allowing the preparation of a 4-qubit cluster state of two photons.

DOI
10.1117/12.909439

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This article presents a proposal for preparing photon pairs in states that are entangled in their spatial modes. The method allows the encoding of any pair of spatial modes that is desired, without restrictions; a procedure that up to now has been elusive. This method consists of three steps: preparing photon pairs in polarization-entangled states, filtering the spatial mode, and use of polarization interferometers with diffractive mode-encoding elements to effect entanglement swapping between polarization and spatial modes. An extension of the method consists of entangling polarization and spatial mode, allowing the preparation of a 4-qubit cluster state of two photons.
Advancing quantum information processing is predicated on the preparation of ever-larger multiqubit states. Photonic realizations of such states may be achieved by increasing the number of photons populating the state or the number of qubits encoded per photon. Typical approaches to the latter strategy utilize distinct degrees of freedom of the photon field. We present here an approach that encodes two qubits per photon in the spatial parity of its transverse spatial profile. Simple linear optical devices transform each parity qubit separately or the two qubits jointly. Furthermore, we demonstrate that entangled photon pairs produced by spontaneous parametric down-conversion may be used to prepare arbitrary four-qubit states through sculpting the spatial profile of the classical optical pump. Two examples are highlighted---the preparation of two-photon four-qubit Greenberger-Horne-Zeilinger and WWW states, whose encoding in a photon pair has thus far eluded other approaches.
We implement experimentally a deterministic method to prepare and measure the so-called single-photon two-qubit entangled states or single-photon Bell states, in which the polarization and the spatial modes of a single photon each represent a quantum bit. All four single-photon Bell states can be easily prepared and measured deterministically using linear optical elements alone. We also discuss how this method can be used for the recently proposed single-photon two-qubit quantum cryptography scheme.
We report an experimental implementation of deterministic preparation and measurement of single-photon two-qubit entangled states, in which the polarization and the spatial modes of a single-photon each represent a quantum bit. It is demonstrated that all four single-photon two-qubit entangled states (Bell states) can be easily prepared and measured deterministically using linear optical elements alone. We also discuss how this scheme can be used for implementing recently proposed single-photon two-qubit quantum cryptography.