What can we do with a linear optical logic gate?

변영태, 김재헌, Nam‐Gyu Park +2 more

John Rarity, Harald Weinfurter

Optical quantum computation was limited to a few photon applications such as quantum cryptography and partial teleportation until 2001. Then the possibility of performing arbitrary logic conditional on certain detection outcomes was introduced by Knill, Laflamme and Milburn ( Nature 2001 409 46). Simple circuits consisting of single photon sources, entangled pair sources, linear optical elements and high efficiency photon counting detectors can be used to develop probabilistic two-photon logic gates. Initially such gates show a low probability of success but can approach the success rate of deterministic (non-linear) logic at the cost of added complexity. These developments have led to the emergence of a novel field of research aimed at improving the efficiency of linear logic and developing the efficient sources and detectors for its implementation. This topical issue of Journal of Optics B: Quantum and Semiclassical Optics is devoted to recent advances in this field. Key to all linear optics gates is the interference effect between separate photons that have no previous history. In this issue we include the first experiment to show such an effect (Rarity et al ), on which many later experiments are based. These interference effects can then be used to entangle separate pairs of particles into higher order entangled states (Zou et al 2005 J. Opt. B: Quantum Semiclass. Opt. 7 119–121) and also to establish entanglement between matter qubits (Kok et al ). Quantum dot single photon sources suitable for linear optics quantum logic are discussed in the paper by Unitt et al while a waveguide source of pair photons is presented by Ducci et al . The difficulties in scaling linear optics quantum computing arise from the low success probabilities of the gates. Introducing a small amount of non-linearity could allow unit efficiency gates to be designed (Munro et al ) while large non-linearities generated by the collective effects in EIT could be used to implement a wide variety of quantum logic functions (Petrosyan). The development of algorithms suitable for optical implementation is addressed by Ellinas et al . Key contributors to this issue are collaborating in the project IST:38864 RAMBOQ which has a primary goal of developing linear optics computation tools and techniques.

Harvard Scott Hinton, John R. Erickson, Tom J. Cloonan +3 more

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

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.

J. D. Franson, Michael J. Fitch, B. C. Jacobs +1 more

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.

Alkim Bozkurt, Serdar Kocaman