819 research outputs found
Gate-controlled nuclear magnetic resonance in an AlGaAs/GaAs quantum Hall device
We study the resistively detected nuclear magnetic resonance (NMR) in an
AlGaAs/GaAs quantum Hall device with a side gate. The strength of the hyperfine
interaction between electron and nuclear spins is modulated by tuning a
position of the two-dimensional electron systems with respect to the polarized
nuclear spins using the side-gate voltages. The NMR frequency is systematically
controlled by the gate-tuned technique in a semiconductor device.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
Concatenated dynamical decoupling in a solid-state spin bath
Concatenated dynamical decoupling (CDD) pulse sequences hold much promise as
a strategy to mitigate decoherence in quantum information processing. It is
important to investigate the actual performance of these dynamical decoupling
strategies in real systems that are promising qubit candidates. In this Rapid
Communication, we compute the echo decay of concatenations of the Hahn echo
sequence for a solid-state electronic spin qubit in a nuclear spin bath using a
cluster expansion technique. We find that each level of concatenation reverses
the effect of successive levels of intrabath fluctuations. On the one hand,
this advances CDD as a versatile and realistic decoupling strategy. On the
other hand, this invalidates, as overly optimistic, results of the simple pair
approximation used previously to study restoration, through CDD, of coherence
lost to a mesoscopic spin bath
Optimized pulse sequences for suppressing unwanted transitions in quantum systems
We investigate the nature of the pulse sequence so that unwanted transitions
in quantum systems can be inhibited optimally. For this purpose we show that
the sequence of pulses proposed by Uhrig [Phys. Rev. Lett. \textbf{98}, 100504
(2007)] in the context of inhibition of environmental dephasing effects is
optimal. We derive exact results for inhibiting the transitions and confirm the
results numerically. We posit a very significant improvement by usage of the
Uhrig sequence over an equidistant sequence in decoupling a quantum system from
unwanted transitions. The physics of inhibition is the destructive interference
between transition amplitudes before and after each pulse.Comment: 5 figure
Looping on the Bloch sphere: Oscillatory effects in dephasing of qubits subject to broad-spectrum noise
For many implementations of quantum computing, 1/f and other types of
broad-spectrum noise are an important source of decoherence. An important step
forward would be the ability to back out the characteristics of this noise from
qubit measurements and to see if it leads to new physical effects. For certain
types of qubits, the working point of the qubit can be varied. Using a new
mathematical method that is suited to treat all working points, we present
theoretical results that show how this degree of freedom can be used to extract
noise parameters and to predict a new effect: noise-induced looping on the
Bloch sphere. We analyze data on superconducting qubits to show that they are
very near the parameter regime where this looping should be observed.Comment: 4 pages, 3 figure
Switchable coupling for superconducting qubits using double resonance in the presence of crosstalk
Several methods have been proposed recently to achieve switchable coupling
between superconducting qubits. We discuss some of the main considerations
regarding the feasibility of implementing one of those proposals: the
double-resonance method. We analyze mainly issues related to the achievable
effective coupling strength and the effects of crosstalk on this coupling
approach. We also find a new, crosstalk-assisted coupling channel that can be
an attractive alternative when implementing the double-resonance coupling
proposal.Comment: 4 pages, 3 figure
Polarization and frequency disentanglement of photons via stochastic polarization mode dispersion
We investigate the quantum decoherence of frequency and polarization
variables of photons via polarization mode dispersion in optical fibers. By
observing the analogy between the propagation equation of the field and the
Schr\"odinger equation, we develop a master equation under Markovian
approximation and analytically solve for the field density matrix. We identify
distinct decay behaviors for the polarization and frequency variables for
single-photon and two-photon states. For the single photon case, purity
functions indicate that complete decoherence for each variable is possible only
for infinite fiber length. For entangled two-photon states passing through
separate fibers, entanglement associated with each variable can be completely
destroyed after characteristic finite propagation distances. In particular, we
show that frequency disentanglement is independent of the initial polarization
status. For propagation of two photons in a common fiber, the evolution of a
polarization singlet state is addressed. We show that while complete
polarization disentanglement occurs at a finite propagation distance, frequency
entanglement could survive at any finite distance for gaussian states.Comment: 2 figure
Non-equilibrium dynamics of a system with Quantum Frustration
Using flow equations, equilibrium and non-equilibrium dynamics of a two-level
system are investigated, which couples via non-commuting components to two
independent oscillator baths. In equilibrium the two-level energy splitting is
protected when the TLS is coupled symmetrically to both bath. A critical
asymmetry angle separates the localized from the delocalized phase.
On the other hand, real-time decoherence of a non-equilibrium initial state
is for a generic initial state faster for a coupling to two baths than for a
single bath.Comment: 22 pages, 9 figure
Spontaneously modulated spin textures in a dipolar spinor Bose-Einstein condensate
Helical spin textures in a Rb F=1 spinor Bose-Einstein condensate are
found to decay spontaneously toward a spatially modulated structure of spin
domains. This evolution is ascribed to magnetic dipolar interactions that
energetically favor the short-wavelength domains over the long-wavelength spin
helix. This is confirmed by eliminating the dipolar interactions by a sequence
of rf pulses and observing a suppression of the formation of the short-range
domains. This study confirms the significance of magnetic dipole interactions
in degenerate Rb F=1 spinor gases
Soft-Pulse Dynamical Decoupling with Markovian Decoherence
We consider the effect of broadband decoherence on the performance of
refocusing sequences, having in mind applications of dynamical decoupling in
concatenation with quantum error correcting codes as the first stage of
coherence protection. Specifically, we construct cumulant expansions of
effective decoherence operators for a qubit driven by a pulse of a generic
symmetric shape, and for several sequences of - and -pulses. While,
in general, the performance of soft pulses in decoupling sequences in the
presence of Markovian decoherence is worse than that of the ideal
-pulses, it can be substantially improved by shaping.Comment: New version contains minor content clarification
Reducing Constraints on Quantum Computer Design by Encoded Selective Recoupling
The requirement of performing both single-qubit and two-qubit operations in
the implementation of universal quantum logic often leads to very demanding
constraints on quantum computer design. We show here how to eliminate the need
for single-qubit operations in a large subset of quantum computer proposals:
those governed by isotropic and XXZ,XY-type anisotropic exchange interactions.
Our method employs an encoding of one logical qubit into two physical qubits,
while logic operations are performed using an analogue of the NMR selective
recoupling method.Comment: 5 pages, 1 table, no figures. Published versio
- …