195 research outputs found
Analytic Controllability of Time-Dependent Quantum Control Systems
The question of controllability is investigated for a quantum control system
in which the Hamiltonian operator components carry explicit time dependence
which is not under the control of an external agent. We consider the general
situation in which the state moves in an infinite-dimensional Hilbert space, a
drift term is present, and the operators driving the state evolution may be
unbounded. However, considerations are restricted by the assumption that there
exists an analytic domain, dense in the state space, on which solutions of the
controlled Schrodinger equation may be expressed globally in exponential form.
The issue of controllability then naturally focuses on the ability to steer the
quantum state on a finite-dimensional submanifold of the unit sphere in Hilbert
space -- and thus on analytic controllability. A relatively straightforward
strategy allows the extension of Lie-algebraic conditions for strong analytic
controllability derived earlier for the simpler, time-independent system in
which the drift Hamiltonian and the interaction Hamiltonia have no intrinsic
time dependence. Enlarging the state space by one dimension corresponding to
the time variable, we construct an augmented control system that can be treated
as time-independent. Methods developed by Kunita can then be implemented to
establish controllability conditions for the one-dimension-reduced system
defined by the original time-dependent Schrodinger control problem. The
applicability of the resulting theorem is illustrated with selected examples.Comment: 13 page
A universal gap for non-spin quantum control systems
We prove the existence of a universal gap for minimum time controllability of
finite dimensional quantum systems, except for some basic representations of
spin groups.
This is equivalent to the existence of a gap in the diameter of orbit spaces
of the corresponding compact connected Lie group unitary actions on the
Hermitian spheres
Effects of uncertainties and errors on Lyapunov control
Lyapunov control (open-loop) is often confronted with uncertainties and
errors in practical applications. In this paper, we analyze the robustness of
Lyapunov control against the uncertainties and errors in quantum control
systems. The analysis is carried out through examinations of uncertainties and
errors, calculations of the control fidelity under influences of the
certainties and errors, as well as discussions on the caused effects. Two
examples, a closed control system and an open control system, are presented to
illustrate the general formulism.Comment: 4 pages, 5 figure
Higher order traps for some strongly degenerate quantum control systems
Quantum control is necessary for a variety of modern quantum technologies as
it allows to optimally manipulate quantum systems. An important problem in
quantum control is to establish whether the control objective functional has
trapping behaviour or no, namely if it has or no traps -- controls from which
it is difficult to escape by local search optimization methods. Higher order
traps were previously introduced in [A. N. Pechen, D. J. Tannor, "Are there
traps in quantum control landscapes?", Phys. Rev. Lett., 106 (2011), 120402],
where 3-rd order traps were found. In this note we show that traps of
arbitrarily high order exist for controllable quantum systems with special
symmetry in the Hamiltonian.Comment: 4 pages, final versio
A system design approach toward integrated cryogenic quantum control systems
In this paper, we provide a system level perspective on the design of control
electronics for large scale quantum systems. Quantum computing systems with
high-fidelity control and readout, coherent coupling, calibrated gates, and
reconfigurable circuits with low error rates are expected to have superior
quantum volumes. Cryogenic CMOS plays a crucial role in the realization of
scalable quantum computers, by minimizing the feature size, lowering the cost,
power consumption, and implementing low latency error correction. Our approach
toward achieving scalable feed-back based control systems includes the design
of memory based arbitrary waveform generators (AWG's), wide band radio
frequency analog to digital converters, integrated amplifier chain, and state
discriminators that can be synchronized with gate sequences. Digitally assisted
designs, when implemented in an advanced CMOS node such as 7 nm can reap the
benefits of low power due to scaling. A qubit readout chain demands several
amplification stages before the digitizer. We propose the co-integration of our
in-house developed InP HEMT LNAs with CMOS LNA stages to achieve the required
gain at the digitizer input with minimal area. Our approach using high
impedance matching between the HEMT LNA and the cryogenic CMOS receiver can
relax the design constraints of an inverter-based CMOS LNA, paving the way
toward a fully integrated qubit readout chain. The qubit state discriminator
consists of a digital signal processor that computes the qubit state from the
digitizer output and a pre-determined threshold. The proposed system realizes
feedback-based optimal control for error mitigation and reduction of the
required data rate through the serial interface to room temperature
electronics
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