115 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
Perfect Function Transfer in two- and three- dimensions without initialization
We find analytic models that can perfectly transfer, without state
initializati$ or remote collaboration, arbitrary functions in two- and
three-dimensional interacting bosonic and fermionic networks. We elaborate on a
possible implementation of state transfer through bosonic or fermionic atoms
trapped in optical lattices. A significant finding is that the state of a spin
qubit can be perfectly transferred through a fermionic system. Families of
Hamiltonians, both linear and nonlinear, are described which are related to the
linear Boson model and that enable the perfect transfer of arbitrary functions.
This includes entangled states such as decoherence-free subsystems enabling
noise protection of the transferred state.Comment: 4 pages, no figur
Coherent control in a decoherence-free subspace of a collective multi-level system
Decoherence-free subspaces (DFS) in systems of dipole-dipole interacting
multi-level atoms are investigated theoretically. It is shown that the
collective state space of two dipole-dipole interacting four-level atoms
contains a four-dimensional DFS. We describe a method that allows to populate
the antisymmetric states of the DFS by means of a laser field, without the need
of a field gradient between the two atoms. We identify these antisymmetric
states as long-lived entangled states. Further, we show that any single-qubit
operation between two states of the DFS can be induced by means of a microwave
field. Typical operation times of these qubit rotations can be significantly
shorter than for a nuclear spin system.Comment: 15 pages, 11 figure
Tomographic measurements on superconducting qubit states
We propose an approach to reconstruct any superconducting charge qubit state
by using quantum state tomography. This procedure requires a series of
measurements on a large enough number of identically prepared copies of the
quantum system. The experimental feasibility of this procedure is explained and
the time scales for different quantum operations are estimated according to
experimentally accessible parameters. Based on the state tomography, we also
investigate the possibility of the process tomography.Comment: 12 pages, 4 figure
Entanglement and Quantum Phase Transition in Low Dimensional Spin Systems
Entanglement of the ground states in and dimerized Heisenberg spin
chains as well as in a two-leg spin ladder is analyzed by using the spin-spin
concurrence and the entanglement entropy between a selected sublattice of spins
and the rest of the system. In particular, we reveal that quantum phase
transition points/boundaries may be identified based on the analysis on the
local extreme of this entanglement entropy, which is illustrated to be superior
over the concurrence scenario and may enable us to explore quantum phase
transitions in many other systems including higher dimensional ones.Comment: 4 pages, 4 figure
On the Quantum Computational Complexity of the Ising Spin Glass Partition Function and of Knot Invariants
It is shown that the canonical problem of classical statistical
thermodynamics, the computation of the partition function, is in the case of
+/-J Ising spin glasses a particular instance of certain simple sums known as
quadratically signed weight enumerators (QWGTs). On the other hand it is known
that quantum computing is polynomially equivalent to classical probabilistic
computing with an oracle for estimating QWGTs. This suggests a connection
between the partition function estimation problem for spin glasses and quantum
computation. This connection extends to knots and graph theory via the
equivalence of the Kauffman polynomial and the partition function for the Potts
model.Comment: 8 pages, incl. 2 figures. v2: Substantially rewritte
From Davydov solitons to decoherence-free subspaces: self-consistent propagation of coherent-product states
The self-consistent propagation of generalized [coherent-product]
states and of a class of gaussian density matrix generalizations is examined,
at both zero and finite-temperature, for arbitrary interactions between the
localized lattice (electronic or vibronic) excitations and the phonon modes. It
is shown that in all legitimate cases, the evolution of states reduces
to the disentangled evolution of the component states. The
self-consistency conditions for the latter amount to conditions for
decoherence-free propagation, which complement the Davydov soliton
equations in such a way as to lift the nonlinearity of the evolution for the
on-site degrees of freedom. Although it cannot support Davydov solitons, the
coherent-product ansatz does provide a wide class of exact density-matrix
solutions for the joint evolution of the lattice and phonon bath in compatible
systems. Included are solutions for initial states given as a product of a
[largely arbitrary] lattice state and a thermal equilibrium state of the
phonons. It is also shown that external pumping can produce self-consistent
Frohlich-like effects. A few sample cases of coherent, albeit not solitonic,
propagation are briefly discussed.Comment: revtex3, latex2e; 22 pages, no figs.; to appear in Phys.Rev.E
(Nov.2001
High fidelity quantum memory via dynamical decoupling: theory and experiment
Quantum information processing requires overcoming decoherence---the loss of
"quantumness" due to the inevitable interaction between the quantum system and
its environment. One approach towards a solution is quantum dynamical
decoupling---a method employing strong and frequent pulses applied to the
qubits. Here we report on the first experimental test of the concatenated
dynamical decoupling (CDD) scheme, which invokes recursively constructed pulse
sequences. Using nuclear magnetic resonance, we demonstrate a near order of
magnitude improvement in the decay time of stored quantum states. In
conjunction with recent results on high fidelity quantum gates using CDD, our
results suggest that quantum dynamical decoupling should be used as a first
layer of defense against decoherence in quantum information processing
implementations, and can be a stand-alone solution in the right parameter
regime.Comment: 6 pages, 3 figures. Published version. This paper was initially
entitled "Quantum gates via concatenated dynamical decoupling: theory and
experiment", by Jacob R. West, Daniel A. Lidar, Bryan H. Fong, Mark F. Gyure,
Xinhua Peng, and Dieter Suter. That original version split into two papers:
http://arxiv.org/abs/1012.3433 (theory only) and the current pape
Modulated Entanglement Evolution Via Correlated Noises
We study entanglement dynamics in the presence of correlated environmental
noises. Specifically, we investigate the quantum entanglement dynamics of two
spins in the presence of correlated classical white noises, deriving Markov
master equation and obtaining explicit solutions for several interesting
classes of initial states including Bell states and X form density matrices. We
show how entanglement can be enhanced or reduced by the correlation between the
two participating noises.Comment: 9 pages, 4 figures. To be published in Quantum Information
Processing, special issue on Quantum Decoherence and Entanglemen
Completeness of classical spin models and universal quantum computation
We study mappings between distinct classical spin systems that leave the
partition function invariant. As recently shown in [Phys. Rev. Lett. 100,
110501 (2008)], the partition function of the 2D square lattice Ising model in
the presence of an inhomogeneous magnetic field, can specialize to the
partition function of any Ising system on an arbitrary graph. In this sense the
2D Ising model is said to be "complete". However, in order to obtain the above
result, the coupling strengths on the 2D lattice must assume complex values,
and thus do not allow for a physical interpretation. Here we show how a
complete model with real -and, hence, "physical"- couplings can be obtained if
the 3D Ising model is considered. We furthermore show how to map general
q-state systems with possibly many-body interactions to the 2D Ising model with
complex parameters, and give completeness results for these models with real
parameters. We also demonstrate that the computational overhead in these
constructions is in all relevant cases polynomial. These results are proved by
invoking a recently found cross-connection between statistical mechanics and
quantum information theory, where partition functions are expressed as quantum
mechanical amplitudes. Within this framework, there exists a natural
correspondence between many-body quantum states that allow universal quantum
computation via local measurements only, and complete classical spin systems.Comment: 43 pages, 28 figure
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