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[236, 237] King et al. [238], and Turchette et al. [239]) and a group at Los
Alamos (Hughes [240], Hughes et al. [241], and James [242]) have
experimentally demonstrated trapped ion QC. These and other research-
ers have addressed various key issues associated with quantum computa-
tion with trapped ions.
Deterministic entanglement of two trapped ions (Turchette et al. [239])
Decoherence bounds (Hughes et al. [240] and Plenio, Knight [244])
Measurement and state preparation, i.e., initialization of the collective
motion of the trapped ions (Schneider et al. [245] and King et al. [238]),
Coherent quantum-state manipulation of
trapped atomic
ions
(Wineland et al. [236, 237]),
Heating of the quantum ground state of trapped ions (James [246]) and
quantum computation with ''hot'' trapped ions (Schneider et al. [247]).
Cavity QED. A group at Cal Tech (Turchette [248]) have experimentally
demonstrated the use of trapped photons in a cavity QED system to
execute 2-qubit XOR gates and thus in principle can do universal QC. The
qubits are encoded by the circular polarization of photons interacting. The
XOR unitary transitions on superpositions can be executed by resonance
between interacting photons in the cavity; the coupling of qubits is via
resonance between interacting photons using a Cesium atom also in the
cavity, and the coupling is tuned by the spacing of mirrors in the cavity.
Photonics. Various groups (i.e., Chuang et al. [249, 250] and Torma,
Stenholm [251]) have experimentally demonstrated QC using optical
systems where qubits are encoded by photon phases, and universal
quantum gates are implemented by optical components consisting of
beamsplitters and phase shifters as well as nonlinear media (also see the
linear optics QC proposed by Adami, Cerf [252]).
Heteropolymer. This is a polymer consisting of a linear array of atoms,
each of which can be either in a ground or excited energy state. Teich et al.
[253] first proposed classical (without quantium superpositions) molecu-
lar computations using heteropolymer. Later Lloyd [254] extended the use
of heteropolymers to QC using the energy states to store the state of the
qubits. The coupling of qubits may be via electric dipole moments that
cause energy shifts on adjacent atoms. Unitary transitions on super-
positions can be executed via pulses of a laser at particular frequencies;
these induce electric dipole moments that determine the transitions.
Nuclear Spin. DiVincenzo [255] and Wei et al. [256, 257] proposed the use
of nuclear spin to do QC; see the remarks following the discussion of bulk
QC.
Quantum Propagation Delays. Castagnoli [258] proposed a way to do QC
using retarded and advanced propagation of particles through various
media.
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