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Physical Review A

Hong-Ou-Mandel cloning: Quantum copying without an ancilla

Irfan Ali Khan, John C. Howell

Published Jul 19, 2004• 26 citations
Publisher

Abstract

In this paper we report an experimental realization of an ancilla-free $1\ensuremath{\rightarrow}2$ phase-covariant quantum cloner. The cloner is realized by interfering a linearly polarized photon, which we wish to clone with a circularly polarized photon at a beam splitter. The two-photon effect can be understood in light of Hong-Ou-Mandel interference. The fidelity of the cloner was measured as $0.829\ifmmode\pm\else\textpm\fi{}0.008$ for the $0∕90$ basis and $0.835\ifmmode\pm\else\textpm\fi{}0.006$ for the $45∕135$ basis, which is in good agreement with the theoretical prediction of $5∕6$ fidelity. The experimental scheme is straightforward and has a high cloning success rate.

DOI
10.1103/physreva.70.010303

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In this paper we report an experimental realization of an ancilla-free 1\ensuremath→21\ensuremath{\rightarrow}21\ensuremath→2 phase-covariant quantum cloner. The cloner is realized by interfering a linearly polarized photon, which we wish to clone with a circularly polarized photon at a beam splitter. The two-photon effect can be understood in light of Hong-Ou-Mandel interference. The fidelity of the cloner was measured as 0.829\ifmmode±\else\textpm\fi0.0080.829\ifmmode\pm\else\textpm\fi{}0.0080.829\ifmmode±\else\textpm\fi0.008 for the 0∕900∕900∕90 basis and 0.835\ifmmode±\else\textpm\fi0.0060.835\ifmmode\pm\else\textpm\fi{}0.0060.835\ifmmode±\else\textpm\fi0.006 for the 45∕13545∕13545∕135 basis, which is in good agreement with the theoretical prediction of 5∕65∕65∕6 fidelity. The experimental scheme is straightforward and has a high cloning success rate.
We report on two optical realizations of the $1\ensuremath{\rightarrow}2$ asymmetric phase-covariant cloning machines for polarization states of single photons. The experimental setups combine two-photon interference and tunable polarization filtering, which enables us to control the asymmetry of the cloners. The first scheme involves a special unbalanced bulk beam splitter exhibiting different splitting ratios for vertical and horizontal polarizations, respectively. The second implemented scheme consists of a balanced fiber coupler where photon bunching occurs, followed by a free-space part with polarization filters. With this latter approach we were able to demonstrate very high cloning fidelities which are above the universal cloning limit.
We propose experimental schemes to implement ancilla-free $1\ensuremath{\rightarrow}3$ optimal phase covariant quantum cloning machines for $x\text{\ensuremath{-}}y$ and $x\text{\ensuremath{-}}z$ equatorial qubits by interfering a polarized photon, which we wish to clone, with different light resources at a six-port symmetric beam splitter. The scheme requires linear optical elements and three-photon coincidence detection, and is feasible with current experimental technology.
We present the experimental realization of optimal symmetric and asymmetric phase-covariant 1-->2 cloning of qubit states using fiber optics. The state of each qubit is encoded into a single photon which can propagate through two optical fibers. The operation of our device is based on one- and two-photon interference. We have demonstrated the creation of two copies for a wide range of qubit states from the equator of the Bloch sphere. The measured fidelities of both copies are close to the theoretical values and they surpass the theoretical maximum obtainable with the universal cloner.
A linear optical probabilistic scheme for the optimal cloning of a pair of orthogonally polarized photons is devised, based on single- and two-photon interferences. It consists in a partial symmetrization device realized with a modified unbalanced Mach-Zehnder interferometer, followed by two balanced beam splitters where the Hong-Ou-Mandel photon bunching occurs. This scheme has the advantage that it enables quantum cloning without the need for stimulated amplification in a nonlinear medium. It can also be modified so to make an optical two-qubit partial SWAP gate, thereby providing a potentially useful tool to linear optics quantum computing.
We compare several optical implementations of phase-covariant cloning machines. The experiments are based on copying of the polarization state of a single photon in bulk optics by a special unbalanced beam splitter or by a balanced beam splitter accompanied by a state filtering. Also the all-fiber-based setup is discussed, where the information is encoded into spatial modes, i.e., the photon can propagate through two optical fibers. Each of the four implementations possesses some advantages and disadvantages that are discussed.