211 research outputs found
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
Control Paradigms for Quantum Engineering
We give an overview of different paradigms for control of quantum systems and
their applications, illustrated with specific examples. We further discuss the
implications of fault-tolerance requirements for quantum process engineering
using optimal control, and explore the possibilities for architecture
simplification and effective control using a minimum number of simple switch
actuators.Comment: 6 pages, presented at ISCCP08 in Malta, to appear in IEEE Proceeding
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
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
Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits
Amorphous solids show surprisingly universal behaviour at low temperatures.
The prevailing wisdom is that this can be explained by the existence of
two-state defects within the material. The so-called standard tunneling model
has become the established framework to explain these results, yet it still
leaves the central question essentially unanswered -- what are these two-level
defects? This question has recently taken on a new urgency with the rise of
superconducting circuits in quantum computing, circuit quantum electrodynamics,
magnetometry, electrometry and metrology. Superconducting circuits made from
aluminium or niobium are fundamentally limited by losses due to two-level
defects within the amorphous oxide layers encasing them. On the other hand,
these circuits also provide a novel and effective method for studying the very
defects which limit their operation. We can now go beyond ensemble measurements
and probe individual defects -- observing the quantum nature of their dynamics
and studying their formation, their behaviour as a function of applied field,
strain, temperature and other properties. This article reviews the plethora of
recent experimental results in this area and discusses the various theoretical
models which have been used to describe the observations. In doing so, it
summarises the current approaches to solving this fundamentally important
problem in solid-state physics.Comment: 34 pages, 7 figures, 1 tabl
Integrated Photonic Sensing
Loss is a critical roadblock to achieving photonic quantum-enhanced
technologies. We explore a modular platform for implementing integrated
photonics experiments and consider the effects of loss at different stages of
these experiments, including state preparation, manipulation and measurement.
We frame our discussion mainly in the context of quantum sensing and focus
particularly on the use of loss-tolerant Holland-Burnett states for optical
phase estimation. In particular, we discuss spontaneous four-wave mixing in
standard birefringent fibre as a source of pure, heralded single photons and
present methods of optimising such sources. We also outline a route to
programmable circuits which allow the control of photonic interactions even in
the presence of fabrication imperfections and describe a ratiometric
characterisation method for beam splitters which allows the characterisation of
complex circuits without the need for full process tomography. Finally, we
present a framework for performing state tomography on heralded states using
lossy measurement devices. This is motivated by a calculation of the effects of
fabrication imperfections on precision measurement using Holland-Burnett
states.Comment: 19 pages, 7 figure
Experimental study of the quantum phase-slip effect in NbN nanowires
Coherent quantum phase-slip (QPS) in a superconducting nanowire is the dual phenomenon to the well-known Josephson effect. Josephson junctions form the basis of superconducting electronic circuits with a wide range of applications, and each of those circuits has a corresponding dual quantum phase-slip device with a dual purpose. Examples that draw particular attention are a new quantum standard of electric current, and a quantum phase-slip qubit. The aim of this project is to develop methods of design, fabrication, and measurement of quantum phase-slip nanowires, and to demonstrate the potential of these devices for technological application. In our experiments we incorporate NbN nanowires into a superconducting loop and bias the loop with a magnetic flux. The state of the nanowire-embedded loop is then read out by coupling to a high quality coplanar waveguide resonator. In this thesis we present the results of two such experiments. First, we fabricated NbN nanowires using a neon focused-ion-beam, and measured their properties at T=300 mK. Periodic tuning of the resonant frequency of the readout resonator revealed that magnetic flux is transferred to the interior of the loop with flux-quantum-periodicity. Our measurements confirm that the flux-quantum transfer is mediated by incoherent quantum phase-slips occurring in the nanowires, and that these incoherent QPS can be fully controlled with an external bias. In the second experiment, nanowire-embedded NbN loops were fabricated by electron-beam lithography and cooled to T=10 mK. The resonant frequency tuning exhibited avoided crossings, which is evidence of coherent coupling between the resonator and a coherent quantum two-level system. We numerically fit these avoided crossings to the Jaynes-Cummings model to extract the properties of the two-level system, and find a good fit with the design parameters of our nanowire qubit. Finally we discuss whether the observation of coherent dynamics is evidence of coherent QPS in the EBL-fabricated nanowire
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