6,468 research outputs found
Counteracting systems of diabaticities using DRAG controls: The status after 10 years
The task of controlling a quantum system under time and bandwidth limitations
is made difficult by unwanted excitations of spectrally neighboring energy
levels. In this article we review the Derivative Removal by Adiabatic Gate
(DRAG) framework. DRAG is a multi-transition variant of counterdiabatic
driving, where multiple low-lying gapped states in an adiabatic evolution can
be avoided simultaneously, greatly reducing operation times compared to the
adiabatic limit. In its essence, the method corresponds to a convergent version
of the superadiabatic expansion where multiple counterdiabaticity conditions
can be met simultaneously. When transitions are strongly crowded, the system of
equations can instead be favorably solved by an average Hamiltonian (Magnus)
expansion, suggesting the use of additional sideband control. We give some
examples of common systems where DRAG and variants thereof can be applied to
improve performance.Comment: 7 pages, 2 figure
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4
We report magnetization and specific heat measurements in the 2D frustrated
spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 at temperatures down to 0.05 K and
high magnetic fields up to 11.5 T applied along a, b and c-axes. The low-field
susceptibility chi (T) M/B shows a broad maximum around 2.8 K characteristic of
short-range antiferromagnetic correlations and the overall temperature
dependence is well described by high temperature series expansion calculations
for the partially frustrated triangular lattice with J=4.46 K and J'/J=1/3. At
much lower temperatures (< 0.4 K) and in in-plane field (along b and c-axes)
several new intermediate-field ordered phases are observed in-between the
low-field incommensurate spiral and the high-field saturated ferromagnetic
state. The ground state energy extracted from the magnetization curve shows
strong zero-point quantum fluctuations in the ground state at low and
intermediate fields
Crossover from weak to strong coupling regime in dispersive circuit QED
We study the decoherence of a superconducting qubit due to the dispersive
coupling to a damped harmonic oscillator. We go beyond the weak
qubit-oscillator coupling, which we associate with a phase Purcell effect, and
enter into a strong coupling regime, with qualitatively different behavior of
the dephasing rate. We identify and give a physicaly intuitive discussion of
both decoherence mechanisms. Our results can be applied, with small
adaptations, to a large variety of other physical systems, e. g. trapped ions
and cavity QED, boosting theoretical and experimental decoherence studies.Comment: Published versio
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