2,092 research outputs found
Optimum Quantum Error Recovery using Semidefinite Programming
Quantum error correction (QEC) is an essential element of physical quantum
information processing systems. Most QEC efforts focus on extending classical
error correction schemes to the quantum regime. The input to a noisy system is
embedded in a coded subspace, and error recovery is performed via an operation
designed to perfectly correct for a set of errors, presumably a large subset of
the physical noise process. In this paper, we examine the choice of recovery
operation. Rather than seeking perfect correction on a subset of errors, we
seek a recovery operation to maximize the entanglement fidelity for a given
input state and noise model. In this way, the recovery operation is optimum for
the given encoding and noise process. This optimization is shown to be
calculable via a semidefinite program (SDP), a well-established form of convex
optimization with efficient algorithms for its solution. The error recovery
operation may also be interpreted as a combining operation following a quantum
spreading channel, thus providing a quantum analogy to the classical diversity
combining operation.Comment: 7 pages, 3 figure
Quantum information processing via a lossy bus
We describe a method to perform two qubit measurements and logic operations
on pairs of qubits which each interact with a harmonic oscillator degree of
freedom (the \emph{bus}), but do not directly interact with one another. Our
scheme uses only weak interactions between the qubit and the bus, homodyne
measurements, and single qubit operations. In contrast to earlier schemes, the
technique presented here is extremely robust to photon loss in the bus mode,
and can function with high fidelity even when the rate of photon loss is
comparable to the strength of the qubit-bus coupling.Comment: Added more discussion on effects of noise. Typos correcte
Classical Rules in Quantum Games
We consider two aspects of quantum game theory: the extent to which the
quantum solution solves the original classical game, and to what extent the new
solution can be obtained in a classical model.Comment: The previous title, "Quantum games are no fun (yet)", was too
whimsical for Physical Review. This is a comment on most, but not all, papers
on quantum game theor
Quantum theory of the charge stability diagram of semiconductor double quantum dot systems
We complete our recently introduced theoretical framework treating the double
quantum dot system with a generalized form of Hubbard model. The effects of all
quantum parameters involved in our model on the charge stability diagram are
discussed in detail. A general formulation of the microscopic theory is
presented, and truncating at one orbital per site, we study the implication of
different choices of the model confinement potential on the Hubbard parameters
as well as the charge stability diagram. We calculate the charge stability
diagram keeping three orbitals per site and find that the effect of additional
higher-lying orbitals on the subspace with lowest-energy orbitals only can be
regarded as a small renormalization of Hubbard parameters, thereby justifying
our practice of keeping only the lowest-orbital in all other calculations. The
role of the harmonic oscillator frequency in the implementation of the Gaussian
model potential is discussed, and the effect of an external magnetic field is
identified to be similar to choosing a more localized electron wave function in
microscopic calculations. The full matrix form of the Hamiltonian including all
possible exchange terms, and several peculiar charge stability diagrams due to
unphysical parameters are presented in the appendix, thus emphasizing the
critical importance of a reliable microscopic model in obtaining the system
parameters defining the Hamiltonian.Comment: 19 pages, 15 figure
Magnetic qubits as hardware for quantum computers
We propose two potential realisations for quantum bits based on nanometre
scale magnetic particles of large spin S and high anisotropy molecular
clusters. In case (1) the bit-value basis states |0> and |1> are the ground and
first excited spin states Sz = S and S-1, separated by an energy gap given by
the ferromagnetic resonance (FMR) frequency. In case (2), when there is
significant tunnelling through the anisotropy barrier, the qubit states
correspond to the symmetric, |0>, and antisymmetric, |1>, combinations of the
two-fold degenerate ground state Sz = +- S. In each case the temperature of
operation must be low compared to the energy gap, \Delta, between the states
|0> and |1>. The gap \Delta in case (2) can be controlled with an external
magnetic field perpendicular to the easy axis of the molecular cluster. The
states of different molecular clusters and magnetic particles may be entangled
by connecting them by superconducting lines with Josephson switches, leading to
the potential for quantum computing hardware.Comment: 17 pages, 3 figure
Structured Near-Optimal Channel-Adapted Quantum Error Correction
We present a class of numerical algorithms which adapt a quantum error
correction scheme to a channel model. Given an encoding and a channel model, it
was previously shown that the quantum operation that maximizes the average
entanglement fidelity may be calculated by a semidefinite program (SDP), which
is a convex optimization. While optimal, this recovery operation is
computationally difficult for long codes. Furthermore, the optimal recovery
operation has no structure beyond the completely positive trace preserving
(CPTP) constraint. We derive methods to generate structured channel-adapted
error recovery operations. Specifically, each recovery operation begins with a
projective error syndrome measurement. The algorithms to compute the structured
recovery operations are more scalable than the SDP and yield recovery
operations with an intuitive physical form. Using Lagrange duality, we derive
performance bounds to certify near-optimality.Comment: 18 pages, 13 figures Update: typos corrected in Appendi
Reversible quantum teleportation in an optical lattice
We propose a protocol, based on entanglement procedures recently suggested by
[D. Jaksch et al., Phys. Rev. Lett. 82, 1975 (1999)], which allows the
teleportation of an unknown state of a neutral atom in an optical lattice to
another atom in another site of the lattice, without any irreversible
detection.Comment: 8 pages, 3 figure
Quantum correlation games
A new approach to play games quantum mechanically is proposed. We consider two players who perform measurements in an EPR-type setting. The payoff relations are defined as functions of correlations, i.e. without reference to classical or quantum mechanics. Classical bi-matrix games are reproduced if the input states are classical and perfectly anti-correlated, that is, for a classical correlation game. However, for a quantum correlation game, with an entangled singlet state as input, qualitatively different solutions are obtained. For example, the Prisoners' Dilemma acquires a Nash equilibrium if both players apply a mixed strategy. It appears to be conceptually impossible to reproduce the properties of quantum correlation games within the framework of classical games
Quasi-deterministic generation of entangled atoms in a cavity
We present a scheme to generate a maximally entangled state of two
three-level atoms in a cavity. The success or failure of the generation of the
desired entangled state can be determined by detecting the polarization of the
photon leaking out of the cavity. With the use of an automatic feedback, the
success probability of the scheme can be made to approach unity.Comment: 10 pages, 3 figure
Thermally assisted adiabatic quantum computation
We study the effect of a thermal environment on adiabatic quantum computation
using the Bloch-Redfield formalism. We show that in certain cases the
environment can enhance the performance in two different ways: (i) by
introducing a time scale for thermal mixing near the anticrossing that is
smaller than the adiabatic time scale, and (ii) by relaxation after the
anticrossing. The former can enhance the scaling of computation when the
environment is superohmic, while the latter can only provide a prefactor
enhancement. We apply our method to the case of adiabatic Grover search and
show that performance better than classical is possible with a superohmic
environment, with no a priori knowledge of the energy spectrum.Comment: 4 pages, 2 figures, Final version to appear in PR
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