1,521 research outputs found
Effective Pure States for Bulk Quantum Computation
In bulk quantum computation one can manipulate a large number of
indistinguishable quantum computers by parallel unitary operations and measure
expectation values of certain observables with limited sensitivity. The initial
state of each computer in the ensemble is known but not pure. Methods for
obtaining effective pure input states by a series of manipulations have been
described by Gershenfeld and Chuang (logical labeling) and Cory et al. (spatial
averaging) for the case of quantum computation with nuclear magnetic resonance.
We give a different technique called temporal averaging. This method is based
on classical randomization, requires no ancilla qubits and can be implemented
in nuclear magnetic resonance without using gradient fields. We introduce
several temporal averaging algorithms suitable for both high temperature and
low temperature bulk quantum computing and analyze the signal to noise behavior
of each.Comment: 24 pages in LaTex, 14 figures, the paper is also avalaible at
http://qso.lanl.gov/qc
Multiqubit Spin
It is proposed that the state space of a quantum object with a complicated
discrete spectrum can be used as a basis for multiqubit recording and
processing of information in a quantum computer. As an example, nuclear spin
3/2 is considered. The possibilities of writing and reading two quantum bits of
information, preparation of the initial state, implementation of the "rotation"
and "controlled negation" operations, which are sufficient for constructing any
algorithms, are demonstrated.Comment: 7 pages, PostScript, no figures; translation of Pis'ma Zh. Eksp.
Teor. Fiz. 70, No. 1, pp. 59-63, 10 July 1999; (Submitted 29 April 1999;
resubmitted 2 June 1999
Experimental Demonstration of Greenberger-Horne-Zeilinger Correlations Using Nuclear Magnetic Resonance
The Greenberger-Horne-Zeilinger (GHZ) effect provides an example of quantum
correlations that cannot be explained by classical local hidden variables. This
paper reports on the experimental realization of GHZ correlations using nuclear
magnetic resonance (NMR). The NMR experiment differs from the originally
proposed GHZ experiment in several ways: it is performed on mixed states rather
than pure states; and instead of being widely separated, the spins on which it
is performed are all located in the same molecule. As a result, the NMR version
of the GHZ experiment cannot entirely rule out classical local hidden
variables. It nonetheless provides an unambiguous demonstration of the
"paradoxical" GHZ correlations, and shows that any classical hidden variables
must communicate by non-standard and previously undetected forces. The NMR
demonstration of GHZ correlations shows the power of NMR quantum information
processing techniques for demonstrating fundamental effects in quantum
mechanics.Comment: Latex2.09, 8 pages, 1 eps figur
NMR quantum computation with indirectly coupled gates
An NMR realization of a two-qubit quantum gate which processes quantum
information indirectly via couplings to a spectator qubit is presented in the
context of the Deutsch-Jozsa algorithm. This enables a successful comprehensive
NMR implementation of the Deutsch-Jozsa algorithm for functions with three
argument bits and demonstrates a technique essential for multi-qubit quantum
computation.Comment: 9 pages, 2 figures. 10 additional figures illustrating output spectr
Separability of very noisy mixed states and implications for NMR quantum computing
We give a constructive proof that all mixed states of N qubits in a
sufficiently small neighborhood of the maximally mixed state are separable. The
construction provides an explicit representation of any such state as a mixture
of product states. We give upper and lower bounds on the size of the
neighborhood, which show that its extent decreases exponentially with the
number of qubits. We also discuss the implications of the bounds for NMR
quantum computing.Comment: 4 pages, extensively revised, references adde
Quantum Simulations on a Quantum Computer
We present a general scheme for performing a simulation of the dynamics of
one quantum system using another. This scheme is used to experimentally
simulate the dynamics of truncated quantum harmonic and anharmonic oscillators
using nuclear magnetic resonance. We believe this to be the first explicit
physical realization of such a simulation.Comment: 4 pages, 2 figures (\documentstyle[prl,aps,epsfig,amscd]{revtex}); to
appear in Phys. Rev. Let
Complete quantum teleportation using nuclear magnetic resonance
Quantum mechanics provides spectacular new information processing abilities
(Bennett 1995, Preskill 1998). One of the most unexpected is a procedure called
quantum teleportation (Bennett et al 1993) that allows the quantum state of a
system to be transported from one location to another, without moving through
the intervening space. Partial implementations of teleportation (Bouwmeester et
al 1997, Boschi et al 1998) over macroscopic distances have been achieved using
optical systems, but omit the final stage of the teleportation procedure. Here
we report an experimental implementation of the full quantum teleportation
operation over inter-atomic distances using liquid state nuclear magnetic
resonance (NMR). The inclusion of the final stage enables for the first time a
teleportation implementation which may be used as a subroutine in larger
quantum computations, or for quantum communication. Our experiment also
demonstrates the use of quantum process tomography, a procedure to completely
characterize the dynamics of a quantum system. Finally, we demonstrate a
controlled exploitation of decoherence as a tool to assist in the performance
of an experiment.Comment: 15 pages, 2 figures. Minor differences between this and the published
versio
Benchmarking quantum control methods on a 12-qubit system
In this letter, we present an experimental benchmark of operational control
methods in quantum information processors extended up to 12 qubits. We
implement universal control of this large Hilbert space using two complementary
approaches and discuss their accuracy and scalability. Despite decoherence, we
were able to reach a 12-coherence state (or 12-qubits pseudo-pure cat state),
and decode it into an 11 qubit plus one qutrit labeled observable pseudo-pure
state using liquid state nuclear magnetic resonance quantum information
processors.Comment: 11 pages, 4 figures, to be published in PR
Rapid solution of problems by nuclear-magnetic-resonance quantum computation
We offer an improved method for using a nuclear-magnetic-resonance quantum
computer (NMRQC) to solve the Deutsch-Jozsa problem. Two known obstacles to the
application of the NMRQC are exponential diminishment of density-matrix
elements with the number of bits, threatening weak signal levels, and the high
cost of preparing a suitable starting state. A third obstacle is a heretofore
unnoticed restriction on measurement operators available for use by an NMRQC.
Variations on the function classes of the Deutsch-Jozsa problem are introduced,
both to extend the range of problems advantageous for quantum computation and
to escape all three obstacles to use of an NMRQC. By adapting it to one such
function class, the Deutsch-Jozsa problem is made solvable without exponential
loss of signal. The method involves an extra work bit and a polynomially more
involved Oracle; it uses the thermal-equilibrium density matrix systematically
for an arbitrary number of spins, thereby avoiding both the preparation of a
pseudopure state and temporal averaging.Comment: 19 page
Local Realistic Model for the Dynamics of Bulk-Ensemble NMR Information Processing
We construct a local realistic hidden-variable model that describes the
states and dynamics of bulk-ensemble NMR information processing up to about 12
nuclear spins. The existence of such a model rules out violation of any Bell
inequality, temporal or otherwise, in present high-temperature, liquid-state
NMR experiments. The model does not provide an efficient description in that
the number of hidden variables grows exponentially with the number of nuclear
spins.Comment: REVTEX, 7 page
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