Implementations of Quantum Gates

   With the advent of theoretically powerful algorithms such as ShorÕs algorithm, as well as the development of quantum analogues to classical computation, implementation of quantum gates has become crucial to the theoretical aspects of gates systems. For example, the gate system that is chosen as the processing method of choice will inevitably receive the lionÕs share of attention from the theoretical quantum computing community as far as future algorithms go. There are two implementations that I would like to take a brief look at as examples: Nuclear Magnetic Resonance (NMR) and Cavity Quantum Electrodynamics (CQE).
   NMR spectroscopy, the most popular genre of quantum computers, is based on the use of magnetic fields. The qubits are spin-1/2 atomic nuclei that are manipulated via radio frequency fields (in the single qubit case) and the naturally occurring spin-spin coupling interaction (for more complex gates) [9]. Currently, NMR computers have been able to implement simple quantum search algorithms but are encountering difficulty in addressing the spins of individual nuclei without affecting neighbors in the quantum molecule of six or more qubits. A couple of example implementations are Chaung and his colleagues at IBMÕs Alamaden Research Center that have developed a computer based on 1H and 13C nuclei in isotopically labeled chloroform and Jonathan JonesÕs groupÕs work in Oxford using two 1H nuclei in cytosine [9].
   At the other end of development, CQE quantum computers are still in its infancy. In this scheme, a set of ions are placed inside an optical resonator (with each atom at a distinct site) and each atom serves as qubits with information encoded in the internal atomic state. Computation proceeds by shooting photons into the intracavity field and thereby cause one or two-bit interactions between atoms. One advantage that the creator (H.J. Kimble) notes is that it is easy to observe directly the level of dissipation through the decay of the cavity field. So far, they have been able to implement a 2-bit phase gate (matrix below). The major problem with the method is with fluctuations inherent in the proton beam but development of more precise light beams is expected to solve that problem [10].

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Created by Brian Patterson
Last Modified 11/22/00