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Physical Review A

Experimental probabilistic manipulation of down-converted photon pairs using unbalanced interferometers

Kaoru Sanaka, Karin Kawahara, Takahiro Kuga

Published Oct 11, 2002• 31 citations
Publisher

Abstract

We experimentally demonstrate the transformation from two-photon time entanglement to polarization entanglement using unbalanced interferometers. The polarization states of down-converted photon pairs from the interferometer depend on the path they have experienced. Therefore, the polarization entanglement is achieved by the coincidence measurement. We also show that our scheme can be used for the probabilistic manipulation of down-converted photon pairs and experimentally simulate an operation of quantum logic gate known as controlled-NOT gate. Availability of our method to the quantum information and the relation to the local realism are discussed.

DOI
10.1103/physreva.66.040301

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We experimentally demonstrate the transformation from two-photon time entanglement to polarization entanglement using unbalanced interferometers. The polarization states of down-converted photon pairs from the interferometer depend on the path they have experienced. Therefore, the polarization entanglement is achieved by the coincidence measurement. We also show that our scheme can be used for the probabilistic manipulation of down-converted photon pairs and experimentally simulate an operation of quantum logic gate known as controlled-NOT gate. Availability of our method to the quantum information and the relation to the local realism are discussed.
We propose probabilistic controlled-NOT and controlled-phase gates for qubits stored in the polarization of photons. The gates are composed of linear optics and photon detectors, and consume polarization entangled photon pairs. The fraction of the successful operation is only limited by the efficiency of the Bell-state measurement. The gates work correctly under the use of imperfect detectors and lossy transmission of photons. Combined with single-qubit gates, they can be used for producing arbitrary polarization states and for designing various quantum measurements.
An ideal controlled-NOT gate followed by projective measurements can be used to identify specific Bell states of its two input qubits. When the input qubits are each members of independent Bell states, these projective measurements can be used to swap the post-selected entanglement onto the remaining two qubits. Here we apply this strategy to produce heralded two-photon polarization entanglement using Bell states that originate from independent parametric down-conversion sources, and a particular probabilistic controlled-NOT gate that is constructed from linear optical elements. The resulting implementation is closely related to an earlier proposal by Sliwa and Banaszek, and can be intuitively understood in terms of familiar quantum information protocols. The possibility of producing a "pseudo-demand" source of two-photon entanglement by storing and releasing these heralded pairs from independent cyclical quantum memory devices is also discussed.
A SWAP operation between different types of qubits of single photons is essential for manipulating hyperentangled photons for a variety of applications. We have implemented an efficient SWAP gate for the momentum and polarization degrees of freedom of a single photon. The SWAP gate was utilized in a single-photon two-qubit quantum logic circuit to deterministically transfer momentum entanglement between a pair of down-converted photons to polarization entanglement. The polarization entanglement thus obtained violates Bell's inequality by more than 150 standard deviations.
Summary form only given. Photon pairs entangled in different degrees of freedom are of great interest for quantum communication. In long distance fiber based communications, time-energy entanglement [1] is preferred over polarization entanglement due to its robustness against polarization mode dispersion due to the fiber propagation. On the other hand, the measurement of time-energy entanglement is more complex than that of polarization. In this work we convert the quantum correlation of time -energy entangled photons into polarization entanglement using unbalanced Michelson interferometers. Time correlated photons are generated thorough spontaneous parametric down conversion of a laser of wavelength 405nm in a second order nonlinear crystal (β-barium borate). Correlated photons carrying horizontal polarization are converted to diagonal polarization and send to a Michelson interferometer (Fig. la) with a polarizing beam splitter. In both arms of the interferometer, one quarter -wave plate oriented at 45° is kept so that the returning photons are converted from horizontal to vertical and vice -versa. The output of the interferometer yields co -polarized photons (HUI and VV) with the same relative time of arrival. The anti -correlated photons (HV and VH) exit the interferometer with different relative arrival time due to the unbalanced interferometer arms. A coincidence window less than the photon travel time for the path length difference will fi lter out the anti -correlated photons. We ensure that the coherence length of the pump laser is larger than the path length difference such that the co -polarized bi-photon state coherently adds up to the so called Bell state.
This paper presents a method to convert position correlation of photon pairs into polarization entanglement. This is achieved by individually manipulating the polarization state of photons generated in different parts of a nonlinear medium and coherently superposing them. This concept is experimentally demonstrated using photon pairs produced by spontaneous parametric down-conversion. The method is used to implement a compact source producing an observed photon-pair rate of $120\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{4.pt}{0ex}}{\text{s}}^{\ensuremath{-}1}/\text{mW}$ with an entanglement fidelity of $0.991\ifmmode\pm\else\textpm\fi{}0.013$. This method can be extended to any photon-pair generation process with initial position correlation.