Michelle M. Donegan

High-fidelity quantum logic operations using linear optical elements.

Knill, Laflamme, and Milburn [Nature (London) 409, 46 ((2001))]] have shown that quantum logic operations can be performed using linear optical elements and additional ancilla photons. Their approach is probabilistic in the sense that the logic devices fail to produce an output with a failure rate that scales as 1/n, where n is the number of ancilla. Here we present an alternative approach in which the logic devices always produce an output with an intrinsic error rate that scales as 1/n(2), which may have several advantages in quantum computing applications.

Generation of entangled ancilla states for use in linear optics quantum computing

Quantum logic operations can be performed using linear optical elements, additional photons (ancilla photons), and postselection based on measurements made on the ancilla. Here we describe a method for generating the required entangled state of

Perturbation theory for quantum-mechanical observables

n

Improved single-photon detector performance

ancilla photons using elementary logic gates and postselection. This approach is capable of generating the ancilla states required for either the original proposal by Knill, Laflamme, and Milburn [E. Knill, R. Laflamme, and G. J. Milburn, Nature 409, 46 (2001)] or those required for the more general high-fidelity approach [J. D. Franson, M. M. Donegan, M. J. Fitch, B. C. Jacobs, and T. B. Pittman, Phys. Rev. Lett. 89, 137901 (2002)]. We also show that the entangled ancilla photons could be generated using a series of quantum wells coupled with tunnel junctions.

Single-photon source and quantum memory

The quantum-mechanical state vector is not directly observable even though it is the fundamental variable that appears in Schrodingers equation. In conventional time-dependent perturbation theory, the state vector mustbe calculated before the experimentally observable expectation values of relevant operators can be computed. We discuss an alternative form of time-dependent perturbation theory in which the observable expectation values are calculated directly and expressed in the form of nested commutators. This result is consistent with the fact that the commutation relations determine the properties of a quantum system, while the commutators often have a form that simplifies the calculation and avoids canceling terms. The usefulness of this method is illustrated using several problems of interest in quantum optics and quantum information processing.

High-fidelity quantum logic operations and entangled ancilla states

High efficiency single-photon detectors are needed for many applications including quantum cryptography and linear-optics quantum computing. We present experimental results showing improved detection efficiency of silicon avalanche photodiodes by reducing reflective losses.

Quantum computation with linear optics

We have experimentally demonstrated a new kind of single-photon source and a cyclical quantum memory device for photonic qubits. These initial experiments involved storage loops, active switching, and parametric down-conversion photon pairs.

Techniques for high fidelity quantum teleportation and computing

We describe a high-fidelity approach to linear optics quantum computing in which the quantum logic operations always produce an output. A method for generating the required entangled states is also described.

Heralded two-photon entanglement from probabilistic quantum logic operations on multiple parametric down-conversion sources

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.