875 research outputs found
Ensemble Quantum Computation with atoms in periodic potentials
We show how to perform universal quantum computation with atoms confined in
optical lattices which works both in the presence of defects and without
individual addressing. The method is based on using the defects in the lattice,
wherever they are, both to ``mark'' different copies on which ensemble quantum
computation is carried out and to define pointer atoms which perform the
quantum gates. We also show how to overcome the problem of scalability on this
system
Optimal conversion of non--local unitary operations
We study when a non--local unitary operation acting on two --level systems
can probabilistically simulate another one when arbitrary local operations and
classical communication are allowed. We provide necessary and sufficient
conditions for the simulation to be possible. Probabilistic interconvertability
is used to define an equivalence relation between gates. We show that this
relation induces a finite number of classes, that we identify. In the case of
two qubits, two classes of non--local operations exist. We choose the CNOT and
SWAP as representatives of these classes. We show how the CNOT [SWAP] can be
deterministically converted into any operation of its class. We also calculate
the optimal probability of obtaining the CNOT [SWAP] from any operation of the
corresponding class and provide a protocol to achieve this task.Comment: 4 pages, no figure
Entanglement generation via a completely mixed nuclear spin bath
We show that qubits coupled sequentially to a mesoscopic static completely
mixed spin bath via the Heisenberg interaction can become highly entangled.
Straightforward protocols for the generation of multipartite entangled
(Greenberger-Horne-Zeilinger-)states are presented. We show the feasibility of
an experimental realization in a quantum dot by the hyperfine interaction of an
electron with the nuclear spins.Comment: 4+pages, 3 figure
Storing quantum dynamics in quantum states: stochastic programmable gate for U(1) operations
We show how quantum dynamics can be captured in the state of a quantum
system, in such a way that the system can be used to stochastically perform, at
a later time, the stored transformation perfectly on some other quantum system.
Thus programmable quantum gates for quantum information processing are feasible
if some probability of failure -that we show to decrease exponentially with the
size of the storing resources- is allowed.Comment: RevTex, 4 pages, 3 figures. Extension of quant-ph/0012067, including
several results concerning optimality of the scheme for storage of operation
Delocalized Entanglement of Atoms in optical Lattices
We show how to detect and quantify entanglement of atoms in optical lattices
in terms of correlations functions of the momentum distribution. These
distributions can be measured directly in the experiments. We introduce two
kinds of entanglement measures related to the position and the spin of the
atoms
An Error Model for the Cirac-Zoller CNOT gate
In the framework of ion-trap quantum computing, we develop a characterization
of experimentally realistic imperfections which may affect the Cirac-Zoller
implementation of the CNOT gate. The CNOT operation is performed by applying a
protocol of five laser pulses of appropriate frequency and polarization. The
laser-pulse protocol exploits auxiliary levels, and its imperfect
implementation leads to unitary as well as non-unitary errors affecting the
CNOT operation. We provide a characterization of such imperfections, which are
physically realistic and have never been considered before to the best of our
knowledge. Our characterization shows that imperfect laser pulses unavoidably
cause a leak of information from the states which alone should be transformed
by the ideal gate, into the ancillary states exploited by the experimental
implementation.Comment: 10 pages, 1 figure. Accepted as a contributed oral communication in
the QuantumComm 2009 International Conference on Quantum Communication and
Quantum Networking, Vico Equense, Italy, October 26-30, 200
Quantum simulators, continuous-time automata, and translationally invariant systems
The general problem of finding the ground state energy of lattice
Hamiltonians is known to be very hard, even for a quantum computer. We show
here that this is the case even for translationally invariant systems. We also
show that a quantum computer can be built in a 1D chain with a fixed,
translationally invariant Hamitonian consisting of nearest--neighbor
interactions only. The result of the computation is obtained after a prescribed
time with high probability.Comment: partily rewritten and important references include
- …