10 research outputs found
Optimizing inhomogeneous spin ensembles for quantum memory
We propose a general method to maximize the fidelity of writing, storage and
reading of quantum information (QI) in a spectrally inhomogeneous spin ensemble
used as quantum memory. The method is based on preselecting the optimal
spectral portion of the ensemble by a judiciously designed pulse. It allows
drastic improvement of quantum memory realized by spin ensembles that store QI
from a resonator or an optical beam.Comment: Corrected m
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 channels spin-spin couplings. It may also facilitate transfer in the presence of diagonal disorder (on site energy noise) in the channel.Fil: Zwick, Analía Elizabeth. Weizmann Institute Of Science; Israel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Alvarez, Gonzalo Agustin. Weizmann Institute Of Science; Israel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Bensky, Guy. Weizmann Institute Of Science; IsraelFil: Kurizki, Gershon. Weizmann Institute Of Science; Israe
Task-dependent control of open quantum systems
We develop a general optimization strategy for performing a chosen unitary or
non-unitary task on an open quantum system. The goal is to design a controlled
time-dependent system Hamiltonian by variationally minimizing or maximizing a
chosen function of the system state, which quantifies the task success (score),
such as fidelity, purity, or entanglement. If the time-dependence of the system
Hamiltonian is fast enough to be comparable or shorter than the response-time
of the bath, then the resulting non-Markovian dynamics is shown to optimize the
chosen task score to second order in the coupling to the bath. This strategy
can protect a desired unitary system evolution from bath-induced decoherence,
but ca also take advantage of the system-bath coupling so as to realize a
desired non-unitary effect on the system.Comment: 9 pages, 7 figures, presented at DPG Spring Meeting, Dresden, 201
Non-Markovian control of qubit thermodynamics by frequent quantum measurements
We explore the effects of frequent, impulsive quantum nondemolition
measurements of the energy of two-level systems (TLS), alias qubits, in contact
with a thermal bath. The resulting entropy and temperature of both the system
and the bath are found to be completely determined by the measurement rate, and
unrelated to what is expected by standard thermodynamical rules that hold for
Markovian baths. These anomalies allow for very fast control of heating,
cooling and state-purification (entropy reduction) of qubits, much sooner than
their thermal equilibration time.Comment: 8 pages, 9 figure
Direct Measurement of the System-Environment Coupling as a Tool For Understanding Decoherence and Dynamical Decoupling
Decoherence is a major obstacle to any practical implementation of quantum
information processing. One of the leading strategies to reduce decoherence is
dynamical decoupling --- the use of an external field to average out the effect
of the environment. The decoherence rate under any control field can be
calculated if the spectrum of the coupling to the environment is known. We
present a direct measurement of the bath coupling spectrum in an ensemble of
optically trapped ultracold atoms, by applying a spectrally narrow-band control
field. The measured spectrum follows a Lorentzian shape at low frequencies, but
exhibits non-monotonic features at higher frequencies due to the oscillatory
motion of the atoms in the trap. These features agree with our analytical
models and numerical Monte-Carlo simulations of the collisional bath. From the
inferred bath-coupling spectrum, we predict the performance of well-known
dynamical decoupling sequences: CPMG, UDD and CDD. We then apply these
sequences in experiment and compare the results to predictions, finding good
agreement in the weak-coupling limit. Thus, our work establishes experimentally
the validity of the overlap integral formalism, and is an important step
towards the implementation of an optimal dynamical decoupling sequence for a
given measured bath spectrum.Comment: 9 pages, 6 figure