9 research outputs found
A Magnetic Resonance Realization of Decoherence-Free Quantum Computation
We report the realization, using nuclear magnetic resonance techniques, of
the first quantum computer that reliably executes an algorithm in the presence
of strong decoherence. The computer is based on a quantum error avoidance code
that protects against a class of multiple-qubit errors. The code stores two
decoherence-free logical qubits in four noisy physical qubits. The computer
successfully executes Grover's search algorithm in the presence of arbitrarily
strong engineered decoherence. A control computer with no decoherence
protection consistently fails under the same conditions.Comment: 5 pages with 3 figures, revtex4, accepted by Physical Review Letters;
v2 minor revisions to conten
Fetching marked items from an unsorted database in NMR ensemble computing
Searching a marked item or several marked items from an unsorted database is
a very difficult mathematical problem. Using classical computer, it requires
steps to find the target. Using a quantum computer, Grover's
algorithm uses steps. In NMR ensemble computing,
Brushweiler's algorithm uses steps. In this Letter, we propose an
algorithm that fetches marked items in an unsorted database directly. It
requires only a single query. It can find a single marked item or multiple
number of items.Comment: 4 pages and 1 figur
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