246 research outputs found
Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits
Fault tolerant quantum computing relies on the ability to detect and correct
errors, which in quantum error correction codes is typically achieved by
projectively measuring multi-qubit parity operators and by conditioning
operations on the observed error syndromes. Here, we experimentally demonstrate
the use of an ancillary qubit to repeatedly measure the and parity
operators of two data qubits and to thereby project their joint state into the
respective parity subspaces. By applying feedback operations conditioned on the
outcomes of individual parity measurements, we demonstrate the real-time
stabilization of a Bell state with a fidelity of in up to 12
cycles of the feedback loop. We also perform the protocol using Pauli frame
updating and, in contrast to the case of real-time stabilization, observe a
steady decrease in fidelity from cycle to cycle. The ability to stabilize
parity over multiple feedback rounds with no reduction in fidelity provides
strong evidence for the feasibility of executing stabilizer codes on timescales
much longer than the intrinsic coherence times of the constituent qubits.Comment: 12 pages, 10 figures. Update: Fig. 5 correcte
Quantum control theory and applications: A survey
This paper presents a survey on quantum control theory and applications from
a control systems perspective. Some of the basic concepts and main developments
(including open-loop control and closed-loop control) in quantum control theory
are reviewed. In the area of open-loop quantum control, the paper surveys the
notion of controllability for quantum systems and presents several control
design strategies including optimal control, Lyapunov-based methodologies,
variable structure control and quantum incoherent control. In the area of
closed-loop quantum control, the paper reviews closed-loop learning control and
several important issues related to quantum feedback control including quantum
filtering, feedback stabilization, LQG control and robust quantum control.Comment: 38 pages, invited survey paper from a control systems perspective,
some references are added, published versio
Sampled-data design for robust control of a single qubit
This paper presents a sampled-data approach for the robust control of a
single qubit (quantum bit). The required robustness is defined using a sliding
mode domain and the control law is designed offline and then utilized online
with a single qubit having bounded uncertainties. Two classes of uncertainties
are considered involving the system Hamiltonian and the coupling strength of
the system-environment interaction. Four cases are analyzed in detail including
without decoherence, with amplitude damping decoherence, phase damping
decoherence and depolarizing decoherence. Sampling periods are specifically
designed for these cases to guarantee the required robustness. Two sufficient
conditions are presented for guiding the design of unitary control for the
cases without decoherence and with amplitude damping decoherence. The proposed
approach has potential applications in quantum error-correction and in
constructing robust quantum gates.Comment: 33 pages, 5 figures, minor correction
Stabilizing two-qubit entanglement with dynamically decoupled active feedback
We propose and analyze a protocol for stabilizing a maximally entangled state
of two noninteracting qubits using active state-dependent feedback from a
continuous two-qubit half-parity measurement in coordination with a concurrent,
non-commuting dynamical decoupling drive. We demonstrate the surprising result
that such a drive be simultaneous with the measurement and feedback, and can
also be part of the feedback protocol itself. We show that robust stabilization
with near-unit fidelity can be achieved even in the presence of realistic
nonidealities, such as time delay in the feedback loop, imperfect
state-tracking, inefficient measurements, and dephasing from 1/f-distributed
qubit-frequency noise. We mitigate feedback-delay error by introducing a
forward-state-estimation strategy in the feedback controller that tracks the
effects of control signals already in transit.Comment: 27 pages, 9 figure
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