130 research outputs found
Superadiabatic driving of a three-level quantum system
We study superadiabatic quantum control of a three-level quantum system whose
energy spectrum exhibits multiple avoided crossings. In particular, we
investigate the possibility of treating the full control task in terms of
independent two-level Landau-Zener problems. We first show that the time
profiles of the elements of the full control Hamiltonian are characterized by
peaks centered around the crossing times. These peaks decay algebraically for
large times. In principle, such a power-law scaling invalidates the hypothesis
of perfect separability. Nonetheless, we address the problem from a pragmatic
point of view by studying the fidelity obtained through separate control as a
function of the intercrossing separation. This procedure may be a good approach
to achieve approximate adiabatic driving of a specific instantaneous eigenstate
in realistic implementations.Comment: 11 pages, 7 figure
Counterintuitive transitions between crossing energy levels
We calculate analytically the probabilities for intuitive and
counterintuitive transitions in a three-state system, in which two parallel
energies are crossed by a third, tilted energy. The state with the tilted
energy is coupled to the other two states in a chainwise linkage pattern with
constant couplings of finite duration. The probability for a counterintuitive
transition is found to increase with the square of the coupling and decrease
with the squares of the interaction duration, the energy splitting between the
parallel energies, and the tilt (chirp) rate. Physical examples of this model
can be found in coherent atomic excitation and optical shielding in cold atomic
collisions
Amplitude Spectroscopy of a Solid-State Artificial Atom
The energy-level structure of a quantum system plays a fundamental role in
determining its behavior and manifests itself in a discrete absorption and
emission spectrum. Conventionally, spectra are probed via frequency
spectroscopy whereby the frequency \nu of a harmonic driving field is varied to
fulfill the conditions \Delta E = h \nu, where the driving field is resonant
with the level separation \Delta E (h is Planck's constant). Although this
technique has been successfully employed in a variety of physical systems,
including natural and artificial atoms and molecules, its application is not
universally straightforward, and becomes extremely challenging for frequencies
in the range of 10's and 100's of gigahertz. Here we demonstrate an alternative
approach, whereby a harmonic driving field sweeps the atom through its
energy-level avoided crossings at a fixed frequency, surmounting many of the
limitations of the conventional approach. Spectroscopic information is obtained
from the amplitude dependence of the system response. The resulting
``spectroscopy diamonds'' contain interference patterns and population
inversion that serve as a fingerprint of the atom's spectrum. By analyzing
these features, we determine the energy spectrum of a manifold of states with
energies from 0.01 to 120 GHz \times h in a superconducting artificial atom,
using a driving frequency near 0.1 GHz. This approach provides a means to
manipulate and characterize systems over a broad bandwidth, using only a single
driving frequency that may be orders of magnitude smaller than the energy
scales being probed.Comment: 12 pages, 13 figure
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