1,679 research outputs found
Universal dynamical decoherence control of noisy single-and multi-qubit systems
In this article we develop, step by step, the framework for universal
dynamical control of two-level systems (TLS) or qubits experiencing amplitude-
or phase-noise (AN or PN) due to coupling to a thermal bath. A comprehensive
arsenal of modulation schemes is introduced and applied to either AN or PN,
resulting in completely analogous formulae for the decoherence rates, thus
underscoring the unified nature of this universal formalism. We then address
the extension of this formalism to multipartite decoherence control, where
symmetries are exploited to overcome decoherence.Comment: 28 pages, 4 figure
Observation of anomalous decoherence effect in a quantum bath at room temperature
Decoherence of quantum objects is critical to modern quantum sciences and
technologies. It is generally believed that stronger noises cause faster
decoherence. Strikingly, recent theoretical research discovers the opposite
case for spins in quantum baths. Here we report experimental observation of the
anomalous decoherence effect for the electron spin-1 of a nitrogen-vacancy
centre in high-purity diamond at room temperature. We demonstrate that under
dynamical decoupling, the double-transition can have longer coherence time than
the single-transition, even though the former couples to the nuclear spin bath
as twice strongly as the latter does. The excellent agreement between the
experimental and the theoretical results confirms the controllability of the
weakly coupled nuclear spins in the bath, which is useful in quantum
information processing and quantum metrology.Comment: 22 pages, related paper at http://arxiv.org/abs/1102.557
Optimized dynamical control of state transfer through noisy spin chains
We propose a method of optimally controlling the tradeoff of speed and
fidelity of state transfer through a noisy quantum channel (spin-chain). This
process is treated as qubit state-transfer through a fermionic bath. We show
that dynamical modulation of the boundary-qubits levels can ensure state
transfer with the best tradeoff of speed and fidelity. This is achievable by
dynamically optimizing the transmission spectrum of the channel. The resulting
optimal control is robust against both static and fluctuating noise in the
channel's spin-spin couplings. It may also facilitate transfer in the presence
of diagonal disorder (on site energy noise) in the channel.Comment: 20 pages, 5 figures. arXiv admin note: text overlap with
arXiv:1310.162
Quantum Computers and Decoherence: Exorcising the Demon from the Machine
Decoherence is the main obstacle to the realization of quantum computers.
Until recently it was thought that quantum error correcting codes are the only
complete solution to the decoherence problem. Here we present an alternative
that is based on a combination of a decoherence-free subspace encoding and the
application of strong and fast pulses: ``encoded recoupling and decoupling''
(ERD). This alternative has the advantage of lower encoding overhead (as few as
two physical qubits per logical qubit suffice), and direct application to a
number of promising proposals for the experimental realization of quantum
computers.Comment: 15 pages, no figures. Invited contribution to the proceedings of the
SPIE Conference on Fluctuations and Noise. Section 8 contains a new result:
how to eliminate off-resonant transitions induced by generic "bang-bang"
pulses, by using a special type of "bang-bang" pulse
Thermal Baths as Quantum Resources: More Friends than Foes?
In this article we argue that thermal reservoirs (baths) are potentially
useful resources in processes involving atoms interacting with quantized
electromagnetic fields and their applications to quantum technologies. One may
try to suppress the bath effects by means of dynamical control, but such
control does not always yield the desired results. We wish instead to take
advantage of bath effects, that do not obliterate "quantumness" in the
system-bath compound. To this end, three possible approaches have been pursued
by us: (i) Control of a quantum system faster than the correlation time of the
bath to which it couples: Such control allows us to reveal
quasi-reversible/coherent dynamical phenomena of quantum open systems, manifest
by the quantum Zeno or anti-Zeno effects (QZE or AZE, respectively). Dynamical
control methods based on the QZE are aimed not only at protecting the
quantumness of the system, but also diagnosing the bath spectra or transferring
quantum information via noisy media. By contrast, AZE-based control is useful
for fast cooling of thermalized quantum systems. (ii) Engineering the coupling
of quantum systems to selected bath modes: This approach, based on field -atom
coupling control in cavities, waveguides and photonic band structures, allows
to drastically enhance the strength and range of atom-atom coupling through the
mediation of the selected bath modes. More dramatically, it allows us to
achieve bath-induced entanglement that may appear paradoxical if one takes the
conventional view that coupling to baths destroys quantumness. (iii)
Engineering baths with appropriate non-flat spectra: This approach is a
prerequisite for the construction of the simplest and most efficient quantum
heat machines (engines and refrigerators). We may thus conclude that often
thermal baths are "more friends than foes" in quantum technologies.Comment: 27 pages, 17 figure
Weak coupling study of decoherence of a qubit in disordered magnetic environments
We study the decoherence of a qubit weakly coupled to frustrated spin baths.
We focus on spin-baths described by the classical Ising spin glass and the
quantum random transverse Ising model which are known to have complex
thermodynamic phase diagrams as a function of an external magnetic field and
temperature. Using a combination of numerical and analytical methods, we show
that for baths initally in thermal equilibrium, the resulting decoherence is
highly sensitive to the nature of the coupling to the environment and is
qualitatively different in different parts of the phase diagram. We find an
unexpected strong non-Markovian decay of the coherence when the random
transverse Ising model bath is prepared in an initial state characterized by a
finite temperature paramagnet. This is contrary to the usual case of
exponential decay (Markovian) expected for spin baths in finite temperature
paramagnetic phases, thereby illustrating the importance of the underlying
non-trivial dynamics of interacting quantum spinbaths.Comment: 12 pages, 18 figure
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