25,319 research outputs found
Control of quantum phenomena: Past, present, and future
Quantum control is concerned with active manipulation of physical and
chemical processes on the atomic and molecular scale. This work presents a
perspective of progress in the field of control over quantum phenomena, tracing
the evolution of theoretical concepts and experimental methods from early
developments to the most recent advances. The current experimental successes
would be impossible without the development of intense femtosecond laser
sources and pulse shapers. The two most critical theoretical insights were (1)
realizing that ultrafast atomic and molecular dynamics can be controlled via
manipulation of quantum interferences and (2) understanding that optimally
shaped ultrafast laser pulses are the most effective means for producing the
desired quantum interference patterns in the controlled system. Finally, these
theoretical and experimental advances were brought together by the crucial
concept of adaptive feedback control, which is a laboratory procedure employing
measurement-driven, closed-loop optimization to identify the best shapes of
femtosecond laser control pulses for steering quantum dynamics towards the
desired objective. Optimization in adaptive feedback control experiments is
guided by a learning algorithm, with stochastic methods proving to be
especially effective. Adaptive feedback control of quantum phenomena has found
numerous applications in many areas of the physical and chemical sciences, and
this paper reviews the extensive experiments. Other subjects discussed include
quantum optimal control theory, quantum control landscapes, the role of
theoretical control designs in experimental realizations, and real-time quantum
feedback control. The paper concludes with a prospective of open research
directions that are likely to attract significant attention in the future.Comment: Review article, final version (significantly updated), 76 pages,
accepted for publication in New J. Phys. (Focus issue: Quantum control
Feedback control of spin systems
The feedback stabilization problem for ensembles of coupled spin 1/2 systems
is discussed from a control theoretic perspective. The noninvasive nature of
the bulk measurement allows for a fully unitary and deterministic closed loop.
The Lyapunov-based feedback design presented does not require spins that are
selectively addressable. With this method, it is possible to obtain control
inputs also for difficult tasks, like suppressing undesired couplings in
identical spin systems.Comment: 16 pages, 15 figure
Sliding Mode Control of Two-Level Quantum Systems
This paper proposes a robust control method based on sliding mode design for
two-level quantum systems with bounded uncertainties. An eigenstate of the
two-level quantum system is identified as a sliding mode. The objective is to
design a control law to steer the system's state into the sliding mode domain
and then maintain it in that domain when bounded uncertainties exist in the
system Hamiltonian. We propose a controller design method using the Lyapunov
methodology and periodic projective measurements. In particular, we give
conditions for designing such a control law, which can guarantee the desired
robustness in the presence of the uncertainties. The sliding mode control
method has potential applications to quantum information processing with
uncertainties.Comment: 29 pages, 4 figures, accepted by Automatic
Cryogenic setup for trapped ion quantum computing
We report on the design of a cryogenic setup for trapped ion quantum
computing containing a segmented surface electrode trap. The heat shield of our
cryostat is designed to attenuate alternating magnetic field noise, resulting
in 120~dB reduction of 50~Hz noise along the magnetic field axis. We combine
this efficient magnetic shielding with high optical access required for single
ion addressing as well as for efficient state detection by placing two lenses
each with numerical aperture 0.23 inside the inner heat shield. The cryostat
design incorporates vibration isolation to avoid decoherence of optical qubits
due to the motion of the cryostat. We measure vibrations of the cryostat of
less than 20~nm over 2~s. In addition to the cryogenic apparatus, we
describe the setup required for an operation with
Ca and Sr ions.
The instability of the laser manipulating the optical qubits in
Ca is characterized yielding a minimum of its
Allan deviation of 2.410 at 0.33~s. To evaluate the
performance of the apparatus, we trapped Ca
ions, obtaining a heating rate of 2.14(16)~phonons/s and a Gaussian decay of
the Ramsey contrast with a 1/e-time of 18.2(8)~ms
Persistent control of a superconducting qubit by stroboscopic measurement feedback
Making a system state follow a prescribed trajectory despite fluctuations and
errors commonly consists in monitoring an observable (temperature,
blood-glucose level...) and reacting on its controllers (heater power, insulin
amount ...). In the quantum domain, there is a change of paradigm in feedback
since measurements modify the state of the system, most dramatically when the
trajectory goes through superpositions of measurement eigenstates. Here, we
demonstrate the stabilization of an arbitrary trajectory of a superconducting
qubit by measurement based feedback. The protocol benefits from the long
coherence time (s) of the 3D transmon qubit, the high efficiency
(82%) of the phase preserving Josephson amplifier, and fast electronics
ensuring less than 500 ns delay. At discrete time intervals, the state of the
qubit is measured and corrected in case an error is detected. For Rabi
oscillations, where the discrete measurements occur when the qubit is supposed
to be in the measurement pointer states, we demonstrate an average fidelity of
85% to the targeted trajectory. For Ramsey oscillations, which does not go
through pointer states, the average fidelity reaches 75%. Incidentally, we
demonstrate a fast reset protocol allowing to cool a 3D transmon qubit down to
0.6% in the excited state.Comment: 7 pages, 3 figures and 1 table. Supplementary information available
as an ancilla fil
Approximate stabilization of an infinite dimensional quantum stochastic system
We propose a feedback scheme for preparation of photon number states in a
microwave cavity. Quantum Non-Demolition (QND) measurements of the cavity field
and a control signal consisting of a microwave pulse injected into the cavity
are used to drive the system towards a desired target photon number state.
Unlike previous work, we do not use the Galerkin approximation of truncating
the infinite-dimensional system Hilbert space into a finite-dimensional
subspace. We use an (unbounded) strict Lyapunov function and prove that a
feedback scheme that minimizes the expectation value of the Lyapunov function
at each time step stabilizes the system at the desired photon number state with
(a pre-specified) arbitrarily high probability. Simulations of this scheme
demonstrate that we improve the performance of the controller by reducing
"leakage" to high photon numbers.Comment: Submitted to CDC 201
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