715 research outputs found
Semiclassical model for a memory dephasing channel
We study a dephasing channel with memory, described by a Hamiltonian model in
which the system-environment interaction is described by a stochastic process.
We propose a useful way to describe the channel uses correlations. Moreover, we
give a general expression for the coherences decay factors as a function of the
number of channel uses and of the stochastic process power spectrum. We also
study the impact of memory on the three qubit code, showing that correlations
among channel uses affect very little the code performance.Comment: 8pages, 3 figures, proceedings of CEWQO 2008 Conferenc
Optimal operating conditions of an entangling two-transmon gate
We identify optimal operating conditions of an entangling two-qubit gate
realized by a capacitive coupling of two superconducting charge qubits in a
transmission line resonator (the so called "transmons"). We demonstrate that
the sensitivity of the optimized gate to 1/f flux and critical current noise is
suppressed to leading order. The procedure only requires a preliminary estimate
of the 1/f noise amplitudes. No additional control or bias line beyond those
used for the manipulation of individual qubits are needed. The proposed
optimization is effective also in the presence of relaxation processes and of
spontaneous emission through the resonator (Purcell effect).Comment: 12 pages, 5 figure
Decoherence due to telegraph and 1/f noise in Josephson qubits
We study decoherence due to random telegraph and 1/f noise in Josephson
qubits. We illustrate differences between gaussian and non gaussian effects at
different working points and for different protocols. Features of the
intrinsically non-gaussian and non-Markovian low-frequency noise may explain
the rich physics observed in the spectroscopy and the dynamics of charge based
devices.Comment: 6 pages, 4 figures. Proceedings of the International Symposium on
Mesoscopic Superconductivity and Spintronics 2004 (MS+S2004), Atsugi, Japa
Classical and quantum capacities of a fully correlated amplitude damping channel
We study information transmission over a fully correlated amplitude damping
channel acting on two qubits. We derive the single-shot classical channel
capacity and show that entanglement is needed to achieve the channel best
performance. We discuss the degradability properties of the channel and
evaluate the quantum capacity for any value of the noise parameter. We finally
compute the entanglement-assisted classical channel capacity.Comment: 16 pages, 9 figure
Information transmission over an amplitude damping channel with an arbitrary degree of memory
We study the performance of a partially correlated amplitude damping channel
acting on two qubits. We derive lower bounds for the single-shot classical
capacity by studying two kinds of quantum ensembles, one which allows to
maximize the Holevo quantity for the memoryless channel and the other allowing
the same task but for the full-memory channel. In these two cases, we also show
the amount of entanglement which is involved in achieving the maximum of the
Holevo quantity. For the single-shot quantum capacity we discuss both a lower
and an upper bound, achieving a good estimate for high values of the channel
transmissivity. We finally compute the entanglement-assisted classical channel
capacity.Comment: 17 pages, 7 figure
Hidden entanglement in the presence of random telegraph dephasing noise
Entanglement dynamics of two noninteracting qubits, locally affected by
random telegraph noise at pure dephasing, exhibits revivals. These revivals are
not due to the action of any nonlocal operation, thus their occurrence may
appear paradoxical since entanglement is by definition a nonlocal resource. We
show that a simple explanation of this phenomenon may be provided by using the
(recently introduced) concept of "hidden" entanglement, which signals the
presence of entanglement that may be recovered with the only help of local
operations.Comment: 8 pages, 1 figure, submitted to Physica Scripta on September 17th
201
Design of a Lambda system for population transfer in superconducting nanocircuits
The implementation of a Lambda scheme in superconducting artificial atoms
could allow detec- tion of stimulated Raman adiabatic passage (STIRAP) and
other quantum manipulations in the microwave regime. However symmetries which
on one hand protect the system against decoherence, yield selection rules which
may cancel coupling to the pump external drive. The tradeoff between efficient
coupling and decoherence due to broad-band colored Noise (BBCN), which is often
the main source of decoherence is addressed, in the class of nanodevices based
on the Cooper pair box (CPB) design. We study transfer efficiency by STIRAP,
showing that substantial efficiency is achieved for off-symmetric bias only in
the charge-phase regime. We find a number of results uniquely due to
non-Markovianity of BBCN, namely: (a) the efficiency for STIRAP depends
essentially on noise channels in the trapped subspace; (b) low-frequency
fluctuations can be analyzed and represented as fictitious correlated
fluctuations of the detunings of the external drives; (c) a simple figure of
merit for design and operating prescriptions allowing the observation of STIRAP
is proposed. The emerging physical picture also applies to other classes of
coherent nanodevices subject to BBCN.Comment: 14 pages, 11 figure
Hidden entanglement, system-environment information flow and non-Markovianity
It is known that entanglement dynamics of two noninteracting qubits, locally
subjected to classical environments, may exhibit revivals. A simple explanation
of this phenomenon may be provided by using the concept of hidden entanglement,
which signals the presence of entanglement that may be recovered without the
help of nonlocal operations. Here we discuss the link between hidden
entanglement and the (non-Markovian) flow of classical information between the
system and the environment.Comment: 9 pages, 2 figures; proceedings of the conference IQIS 2013,
September 24-26 2013, Como, Ital
Broadband noise decoherence in solid-state complex architectures
Broadband noise represents a severe limitation towards the implementation of
a solid-state quantum information processor. Considering common spectral forms,
we propose a classification of noise sources based on the effects produced
instead of on their microscopic origin. We illustrate a multi-stage approach to
broadband noise which systematically includes only the relevant information on
the environment, out of the huge parametrization needed for a microscopic
description. We apply this technique to a solid-state two-qubit gate in a fixed
coupling implementation scheme.Comment: Proceedings of Nobel Symposium 141: Qubits for Future Quantum
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