488 research outputs found
Stabilizing distinguishable qubits against spontaneous decay by detected-jump correcting quantum codes
A new class of error-correcting quantum codes is introduced capable of
stabilizing qubits against spontaneous decay arising from couplings to
statistically independent reservoirs. These quantum codes are based on the idea
of using an embedded quantum code and exploiting the classical information
available about which qubit has been affected by the environment. They are
immediately relevant for quantum computation and information processing using
arrays of trapped ions or nuclear spins. Interesting relations between these
quantum codes and basic notions of design theory are established
Automatic Quantum Error Correction
Criteria are given by which dissipative evolution can transfer populations
and coherences between quantum subspaces, without a loss of coherence. This
results in a form of quantum error correction that is implemented by the joint
evolution of a system and a cold bath. It requires no external intervention
and, in principal, no ancilla. An example of a system that protects a qubit
against spin-flip errors is proposed. It consists of three spin 1/2 magnetic
particles and three modes of a resonator. The qubit is the triple quantum
coherence of the spins, and the photons act as ancilla.Comment: 16 pages 12 fig LaTex uses multicol, graphicx expanded version of
letter submitted to Phys Rev Let
Quantum Logical States and Operators for Josephson-like Systems
We give a formal algebraic description of Josephson-type quantum dynamical
systems, i.e., Hamiltonian systems with a cos theta-like potential term. The
two-boson Heisenberg algebra plays for such systems the role that the h(1)
algebra does for the harmonic oscillator. A single Josephson junction is
selected as a representative of Josephson systems. We construct both logical
states (codewords) and logical (gate) operators in the superconductive regime.
The codewords are the even and odd coherent states of the two-boson algebra:
they are shift-resistant and robust, due to squeezing. The logical operators
acting on the qubit codewords are expressed in terms of operators in the
enveloping of the two-boson algebra. Such a scheme appears to be relevant for
quantum information applications.Comment: 12 pages in RevTex. In press, Journal of Physics A/Letter
Approximate quantum error correction for generalized amplitude damping errors
We present analytic estimates of the performances of various approximate
quantum error correction schemes for the generalized amplitude damping (GAD)
qubit channel. Specifically, we consider both stabilizer and nonadditive
quantum codes. The performance of such error-correcting schemes is quantified
by means of the entanglement fidelity as a function of the damping probability
and the non-zero environmental temperature. The recovery scheme employed
throughout our work applies, in principle, to arbitrary quantum codes and is
the analogue of the perfect Knill-Laflamme recovery scheme adapted to the
approximate quantum error correction framework for the GAD error model. We also
analytically recover and/or clarify some previously known numerical results in
the limiting case of vanishing temperature of the environment, the well-known
traditional amplitude damping channel. In addition, our study suggests that
degenerate stabilizer codes and self-complementary nonadditive codes are
especially suitable for the error correction of the GAD noise model. Finally,
comparing the properly normalized entanglement fidelities of the best
performant stabilizer and nonadditive codes characterized by the same length,
we show that nonadditive codes outperform stabilizer codes not only in terms of
encoded dimension but also in terms of entanglement fidelity.Comment: 44 pages, 8 figures, improved v
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