46 research outputs found
Fidelity Between Unitary Operators and the Generation of Gates Robust Against Off-Resonance Perturbations
We perform a functional expansion of the fidelity between two unitary
matrices in order to find the necessary conditions for the robust
implementation of a target gate. Comparison of these conditions with those
obtained from the Magnus expansion and Dyson series shows that they are
equivalent in first order. By exploiting techniques from robust design
optimization, we account for issues of experimental feasibility by introducing
an additional criterion to the search for control pulses. This search is
accomplished by exploring the competition between the multiple objectives in
the implementation of the NOT gate by means of evolutionary multi-objective
optimization
Quantum Multiobservable Control
We present deterministic algorithms for the simultaneous control of an
arbitrary number of quantum observables. Unlike optimal control approaches
based on cost function optimization, quantum multiobservable tracking control
(MOTC) is capable of tracking predetermined homotopic trajectories to target
expectation values in the space of multiobservables. The convergence of these
algorithms is facilitated by the favorable critical topology of quantum control
landscapes. Fundamental properties of quantum multiobservable control
landscapes that underlie the efficiency of MOTC, including the multiobservable
controllability Gramian, are introduced. The effects of multiple control
objectives on the structure and complexity of optimal fields are examined. With
minor modifications, the techniques described herein can be applied to general
quantum multiobjective control problems.Comment: To appear in Physical Review
Spatial Non-Locality Induced Non-Markovian EIT in a Single Giant Atom
In recent experiments, electromagnetically induced transparency (EIT) were
observed with giant atoms, but nothing unconventional were found from the
transmission spectra. In this letter, we show that unconventional EIT does
exist in giant atoms, and indicate why it has not been observed so far.
Different from these existing works, this letter presents a consistent theory
including a real space method and a time delayed master equation for observing
unconventional EIT. We discover that this phenomenon is a quantum effect which
cannot be correctly described in a semi-classical way as those in recent works.
Our theory shows that it can be observed when the time delay between two
neighboring coupling points is comparable to the relaxation time of the atom,
which is crucial for a future experimental observation. This new phenomenon
results from inherent non-locality of the giant atom, which physically forces
propagating fields to be standing waves in space and the atom exhibiting
retardations in time. Our theory establishes a framework for application of
nonlocal systems to quantum information processing.Comment: 6 pages, 3 figures, Comments are welcom
General unifying features of controlled quantum phenomena
Many proposals have been put forth for controlling quantum phenomena,
including open-loop, adaptive feedback, and real-time feedback control. Each of
these approaches has been viewed as operationally, and even physically,
distinct from the others. This work shows that all such scenarios inherently
share the same fundamental control features residing in the topology of the
landscape relating the target physical observable to the applied controls. This
unified foundation may provide a basis for development of hybrid control
schemes that would combine the advantages of the existing approaches to achieve
the best overall performance.Comment: The published version (includes the supplementary material