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Quantum Information and Computation

Experimental Progress in Linear Optics Quantum Computing

J. D. Franson, Michelle M. Donegan, Michael J. Fitch, B. C. Jacobs, T. B. Pittman

Published Oct 1, 2003• 1 citation
Publisher

Abstract

Probabilistic quantum logic operations can be performed using linear optical elements and post-selection based on the results of measurements on ancilla photons. We review the results of a number of recent experiments in this area, including the demonstration of several quantum logic gates, the use of feed-forward control, a new source of single photons, and a quantum memory device for single photons. A high-fidelity approach in which the logic gates always produce an output will also be discussed.

DOI
10.26421/qic3.s-7

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Probabilistic quantum logic operations can be performed using linear optical elements and post-selection based on the results of measurements on ancilla photons. We review the results of a number of recent experiments in this area, including the demonstration of several quantum logic gates, the use of feed-forward control, a new source of single photons, and a quantum memory device for single photons. A high-fidelity approach in which the logic gates always produce an output will also be discussed.
Summary form only given. Knill, Laflamme, and Milburn (2001) showed that probabilistic quantum logic devices could be implemented using only linear optical elements, additional ancilla photons, and post-selection based on the output of single-photon detectors. These devices produce the desired result with certainty when a specific output from the detectors is obtained, but that will only occur for some fraction of the events. Here we report the experimental demonstration of two logic devices of this kind, a destructive controlled-NOT gate and a quantum parity check. In each of these devices, two polarization-encoded qubits in the form of two single photons from a parametric down-conversion source are incident on a polarizing beam splitter.
One of the main requirements in linear optics quantum computing is the ability to perform single-qubit operations that are controlled by classical information fed forward from the output of single-photon detectors. These operations correspond to predetermined combinations of phase corrections and bit flips that are applied to the postselected output modes of nondeterministic quantum logic devices. Corrections of this kind are required in order to obtain the correct logical output for certain detection events, and their use can increase the overall success probability of the devices. In this paper, we report on the experimental demonstration of the use of this type of feed-forward system to increase the probability of success of a simple nondeterministic quantum logic operation from approximately 14\frac{1}{4}41​ to 12.\frac{1}{2}.21​. This logic operation involves the use of one target qubit and one ancilla qubit which, in this experiment, are derived from a parametric down-conversion photon pair. Classical information describing the detection of the ancilla photon is fed forward in real time and used to alter the quantum state of the output photon. A fiber-optic delay line is used to store the output photon until a polarization-dependent phase shift can be applied using a high-speed Pockels cell.
Ever since Knill, Laflamme and Milburn [Nature (London) 409, 46 (2001)] showed that nondeterministic quantum logic operations could be performed with linear optical elements, additional photons (ancilla) and projective measurements, the idea of linear-optics quantum computation has attracted considerable interest. Our group has recently demonstrated several devices of this kind. We give an overview of recent experimental results, including the quantum parity check, the destructive controlled-NOT, and a cyclical quantum memory. The need for high-efficiency detection of single photons, and for detectors capable of distinguishing photon number will be discussed. Some experimental improvements towards meeting that need will be presented.
Abstract Probabilistic quantum logic operations can be implemented using linear optical elements, additional photons (ancilla), and post‐selection based on the results of measurements made on the ancilla photons. Here we describe the experimental demonstration of several quantum logic devices of that kind, including a quantum parity check and a destructive controlled‐NOT (CNOT) gate. An experimental demonstration of feed‐forward control for use in quantum logic operations is also reported. The implementation of a conventional CNOT gate using polarizing beam splitters is described, along with a new “high‐fidelity” approach in which the logic gates always produce an output.