96 research outputs found
Trapped-Ion Quantum Logic Utilizing Position-Dependent ac Stark Shifts
We present a scheme utilizing position-dependent ac Stark shifts for doing
quantum logic with trapped ions. By a proper choice of direction, position and
size, as well as power and frequency of a far-off-resonant Gaussian laser beam,
specific ac Stark shifts can be assigned to the individual ions, making them
distinguishable in frequency-space. In contrast to previous all-optical based
quantum gates with trapped ions, the present scheme enables individual
addressing of single ions and selective addressing of any pair of ions for
two-ion quantum gates, without using tightly focused laser beams. Furthermore,
the decoherence rate due to off-resonant excitations can be made negligible as
compared with other sources of decoherence.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter
Nonperturbative and perturbative treatments of parametric heating in atom traps
We study the quantum description of parametric heating in harmonic potentials
both nonperturbatively and perturbatively, having in mind atom traps. The first
approach establishes an explicit connection between classical and quantum
descriptions; it also gives analytic expressions for properties such as the
width of fractional frequency parametric resonances. The second approach gives
an alternative insight into the problem and can be directly extended to take
into account nonlinear effects. This is specially important for shallow traps.Comment: 12 pages, 2 figure
The Hidden Subgroup Problem and Eigenvalue Estimation on a Quantum Computer
A quantum computer can efficiently find the order of an element in a group,
factors of composite integers, discrete logarithms, stabilisers in Abelian
groups, and `hidden' or `unknown' subgroups of Abelian groups. It is already
known how to phrase the first four problems as the estimation of eigenvalues of
certain unitary operators. Here we show how the solution to the more general
Abelian `hidden subgroup problem' can also be described and analysed as such.
We then point out how certain instances of these problems can be solved with
only one control qubit, or `flying qubits', instead of entire registers of
control qubits.Comment: 16 pages, 3 figures, LaTeX2e, to appear in Proceedings of the 1st
NASA International Conference on Quantum Computing and Quantum Communication
(Springer-Verlag
Sympathetic Cooling of Trapped Cd+ Isotopes
We sympathetically cool a trapped 112Cd+ ion by directly Doppler-cooling a
114Cd+ ion in the same trap. This is the first demonstration of optically
addressing a single trapped ion being sympathetically cooled by a different
species ion. Notably, the experiment uses a single laser source, and does not
require strong focusing. This paves the way toward reducing decoherence in an
ion trap quantum computer based on Cd+ isotopes.Comment: 4 figure
Quantum teleportation with squeezed vacuum states
We show how the partial entanglement inherent in a two mode squeezed vacuum
state admits two different teleportation protocols. These two protocols refer
to the different kinds of joint measurements that may be made by the sender.
One protocol is the recently implemented quadrature phase approach of
Braunstein and Kimble[Phys. Rev. Lett.{\bf 80}, 869 (1998)]. The other is based
on recognising that a two mode squeezed vacuum state is also entangled with
respect to photon number difference and phase sum. We show that this protocol
can also realise teleportation, however limitations can arise due to the fact
that the photon number spectrum is bounded from below by zero. Our examples
show that a given entanglement resource may admit more than a single
teleportation protocol and the question then arises as to what is the optimum
protocol in the general case
Effect of an External Field on Decoherence
"Decoherence of quantum superpositions through coupling to engineered
reservoirs" is the topic of a recent article by Myatt et al. [Nature
{\underline{403}}, 269 (2000)] which has attracted much interest because of its
relevance to current research in fundamental quantum theory, quantum
computation, teleportation, entanglement and the quantum-classical interface.
However, the preponderance of theoretical work on decoherence does not consider
the effect of an {\underline{external field}}. Here, we present an analysis of
such an effect in the case of the random delta-correlated force discussed by
Myatt et al
Individual addressing and state readout of trapped ions utilizing rf- micromotion
A new scheme for the individual addressing of ions in a trap is described
that does not rely on light beams tightly focused onto only one ion. The scheme
utilizes ion micromotion that may be induced in a linear trap by dc offset
potentials. Thus coupling an individual ion to the globally applied light
fields corresponds to a mere switching of voltages on a suitable set of
compensation electrodes. The proposed scheme is especially suitable for
miniaturized rf (Paul) traps with typical dimensions of about 20-40 microns.Comment: 3 pages, 5 figure
Polariton Analysis of a Four-Level Atom Strongly Coupled to a Cavity Mode
We present a complete analytical solution for a single four-level atom
strongly coupled to a cavity field mode and driven by external coherent laser
fields. The four-level atomic system consists of a three-level subsystem in an
EIT configuration, plus an additional atomic level; this system has been
predicted to exhibit a photon blockade effect. The solution is presented in
terms of polaritons. An effective Hamiltonian obtained by this procedure is
analyzed from the viewpoint of an effective two-level system, and the dynamic
Stark splitting of dressed states is discussed. The fluorescence spectrum of
light exiting the cavity mode is analyzed and relevant transitions identified.Comment: 12 pages, 9 figure
Measurement of conditional phase shifts for quantum logic
Measurements of the birefringence of a single atom strongly coupled to a
high-finesse optical resonator are reported, with nonlinear phase shifts
observed for intracavity photon number much less than one. A proposal to
utilize the measured conditional phase shifts for implementing quantum logic
via a quantum-phase gate (QPG) is considered. Within the context of a simple
model for the field transformation, the parameters of the "truth table" for the
QPG are determined.Comment: 4 pages in Postscript format, including 4 figures (attached as
uuencoded version of a gzip-file
Error Prevention Scheme with Four Particles
It is shown that a simplified version of the error correction code recently
suggested by Shor exhibits manifestation of the quantum Zeno effect. Thus,
under certain conditions, protection of an unknown quantum state is achieved.
Error prevention procedures based on four-particle and two-particle encoding
are proposed and it is argued that they have feasible practical
implementations.Comment: 4 pages, RevTeX, references updated and improved protocol adde
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