1,506 research outputs found
Optical pumping of quantum dot nuclear spins
An all-optical scheme to polarize nuclear spins in a single quantum dot is
analyzed. The hyperfine interaction with randomly oriented nuclear spins
presents a fundamental limit for electron spin coherence in a quantum dot; by
cooling the nuclear spins, this decoherence mechanism could be suppressed. The
proposed scheme is inspired by laser cooling methods of atomic physics and
implements a "controlled Overhauser effect" in a zero-dimensional structure
Noise effect on Grover algorithm
The decoherence effect on Grover algorithm has been studied numerically
through a noise modelled by a depolarizing channel. Two types of error are
introduced characterizing the qubit time evolution and gate application, so the
noise is directly related to the quantum network construction. The numerical
simulation concludes an exponential damping law for the successive probability
of the maxima as time increases. We have obtained an allowed-error law for the
algorithm: the error threshold for the allowed noise behaves as Eth(N) ~ 1/N1.1
(N being the size of the data set). As the power of N is almost one, we
consider the Grover algorithm as robust to a certain extent against
decoherence. This law also provides an absolute threshold: if the free
evolution error is greater than 0.043, Grover algorithm does not work for any
number of qubits affected by the present error model. The improvement in the
probability of success, in the case of two qubits has been illustrated by using
a fault-tolerant encoding of the initial state by means of the [[7,1,3]]
quantum code.Comment: Accepted to be published in Eur. Phys. J. D (2008
On Protected Realizations of Quantum Information
There are two complementary approaches to realizing quantum information so
that it is protected from a given set of error operators. Both involve encoding
information by means of subsystems. One is initialization-based error
protection, which involves a quantum operation that is applied before error
events occur. The other is operator quantum error correction, which uses a
recovery operation applied after the errors. Together, the two approaches make
it clear how quantum information can be stored at all stages of a process
involving alternating error and quantum operations. In particular, there is
always a subsystem that faithfully represents the desired quantum information.
We give a definition of faithful realization of quantum information and show
that it always involves subsystems. This justifies the "subsystems principle"
for realizing quantum information. In the presence of errors, one can make use
of noiseless, (initialization) protectable, or error-correcting subsystems. We
give an explicit algorithm for finding optimal noiseless subsystems. Finding
optimal protectable or error-correcting subsystems is in general difficult.
Verifying that a subsystem is error-correcting involves only linear algebra. We
discuss the verification problem for protectable subsystems and reduce it to a
simpler version of the problem of finding error-detecting codes.Comment: 17 page
Toward fault-tolerant quantum computation without concatenation
It has been known that quantum error correction via concatenated codes can be
done with exponentially small failure rate if the error rate for physical
qubits is below a certain accuracy threshold. Other, unconcatenated codes with
their own attractive features-improved accuracy threshold, local
operations-have also been studied. By iteratively distilling a certain
two-qubit entangled state it is shown how to perform an encoded Toffoli gate,
important for universal computation, on CSS codes that are either
unconcatenated or, for a range of very large block sizes, singly concatenated.Comment: 12 pages, 2 figures, replaced: new stuff on error models, numerical
example for concatenation criteri
Dynamical Generation of Noiseless Quantum Subsystems
We present control schemes for open quantum systems that combine decoupling
and universal control methods with coding procedures. By exploiting a general
algebraic approach, we show how appropriate encodings of quantum states result
in obtaining universal control over dynamically-generated noise-protected
subsystems with limited control resources. In particular, we provide an
efficient scheme for performing universal encoded quantum computation in a wide
class of systems subjected to linear non-Markovian quantum noise and supporting
Heisenberg-type internal Hamiltonians.Comment: 4 pages, no figures; REVTeX styl
Experimental magic state distillation for fault-tolerant quantum computing
Any physical quantum device for quantum information processing is subject to
errors in implementation. In order to be reliable and efficient, quantum
computers will need error correcting or error avoiding methods. Fault-tolerance
achieved through quantum error correction will be an integral part of quantum
computers. Of the many methods that have been discovered to implement it, a
highly successful approach has been to use transversal gates and specific
initial states. A critical element for its implementation is the availability
of high-fidelity initial states such as |0> and the Magic State. Here we report
an experiment, performed in a nuclear magnetic resonance (NMR) quantum
processor, showing sufficient quantum control to improve the fidelity of
imperfect initial magic states by distilling five of them into one with higher
fidelity
Comparison of LOQC C-sign gates with ancilla inefficiency and an improvement to functionality under these conditions
We compare three proposals for non-deterministic C-sign gates implemented
using linear optics and conditional measurements with non-ideal ancilla mode
production and detection. The simplified KLM gate [Ralph et al, Phys.Rev.A {\bf
65}, 012314 (2001)] appears to be the most resilient under these conditions. We
also find that the operation of this gate can be improved by adjusting the
beamsplitter ratios to compensate to some extent for the effects of the
imperfect ancilla.Comment: to appear in PR
Optimal correction of concatenated fault-tolerant quantum codes
We present a method of concatenated quantum error correction in which
improved classical processing is used with existing quantum codes and
fault-tolerant circuits to more reliably correct errors. Rather than correcting
each level of a concatenated code independently, our method uses information
about the likelihood of errors having occurred at lower levels to maximize the
probability of correctly interpreting error syndromes. Results of simulations
of our method applied to the [[4,1,2]] subsystem code indicate that it can
correct a number of discrete errors up to half of the distance of the
concatenated code, which is optimal.Comment: 7 pages, 2 figures, published versio
Relation between two measures of entanglement in spin-1/2 and spinless fermion quantum chain systems
The concepts of concurrence and mode concurrence are the measures of
entanglement for spin-1/2 and spinless fermion systems respectively. Based on
the Jordan-Wigner transformation, any spin-1/2 system is always associated with
a fermion system (called counterpart system). The comparison of concurrence and
mode concurrence can be made with the aid of the Marshall's sign rule for the
ground states of spin-1/2 and spinless fermion chain systems. We observe
that there exists an inequality between concurrence and mode concurrence for
the ground states of the two corresponding systems. The spin-1/2 XY chain
system and its spinless fermion counterpart as a realistic example is discussed
to demonstrate the analytical results.Comment: 7 pages, no figures, publication version, to appear in PR
Improved magic states distillation for quantum universality
Given stabilizer operations and the ability to repeatedly prepare a
single-qubit mixed state rho, can we do universal quantum computation? As
motivation for this question, "magic state" distillation procedures can reduce
the general fault-tolerance problem to that of performing fault-tolerant
stabilizer circuits.
We improve the procedures of Bravyi and Kitaev in the Hadamard "magic"
direction of the Bloch sphere to achieve a sharp threshold between those rho
allowing universal quantum computation, and those for which any calculation can
be efficiently classically simulated. As a corollary, the ability to repeatedly
prepare any pure state which is not a stabilizer state (e.g., any single-qubit
pure state which is not a Pauli eigenstate), together with stabilizer
operations, gives quantum universality. It remains open whether there is also a
tight separation in the so-called T direction.Comment: 6 pages, 5 figure
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