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Frontiers in Optics

Linear optical quantum computing

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

Published Jan 1, 2003• 9 citations
Publisher

Abstract

Quantum logic operations can be performed using linear optical elements, additional photons (ancilla), and feed-forward control based on the results of measurements on the ancilla. An experimental demonstration of several logic devices of this kind will be described, including a controlled-NOT logic gate for single photons.
DOI
10.1364/fio.2003.thss2

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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.
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.
Knill, Laflamme, and Milburn [1] have shown that probabilistic quantum logic operations can be implemented using linear optical elements, such as beam splitters and phase shifters, additional photons (ancilla) and post-selection based on the results of measurements on the ancilla. This may seem somewhat surprising, since logic operations are inherently nonlinear. Here the nonlinearity is produced by the quantum measurement process used in the post-selection. Nonlinearities of this kind are inherently quantum-mechanical and have no classical analogy.