5,030 research outputs found
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
Exploring More-Coherent Quantum Annealing
In the quest to reboot computing, quantum annealing (QA) is an interesting
candidate for a new capability. While it has not demonstrated an advantage over
classical computing on a real-world application, many important regions of the
QA design space have yet to be explored. In IARPA's Quantum Enhanced
Optimization (QEO) program, we have opened some new lines of inquiry to get to
the heart of QA, and are designing testbed superconducting circuits and
conducting key experiments. In this paper, we discuss recent experimental
progress related to one of the key design dimensions: qubit coherence. Using
MIT Lincoln Laboratory's qubit fabrication process and extending recent
progress in flux qubits, we are implementing and measuring QA-capable flux
qubits. Achieving high coherence in a QA context presents significant new
engineering challenges. We report on techniques and preliminary measurement
results addressing two of the challenges: crosstalk calibration and qubit
readout. This groundwork enables exploration of other promising features and
provides a path to understanding the physics and the viability of quantum
annealing as a computing resource.Comment: 7 pages, 3 figures. Accepted by the 2018 IEEE International
Conference on Rebooting Computing (ICRC
Floquet-engineered quantum state manipulation in a noisy qubit
Adiabatic evolution is a common strategy for manipulating quantum states and
has been employed in diverse fields such as quantum simulation, computation and
annealing. However, adiabatic evolution is inherently slow and therefore
susceptible to decoherence. Existing methods for speeding up adiabatic
evolution require complex many-body operators or are difficult to construct for
multi-level systems. Using the tools of Floquet engineering, we design a scheme
for high-fidelity quantum state manipulation, utilizing only the interactions
available in the original Hamiltonian. We apply this approach to a qubit and
experimentally demonstrate its performance with the electronic spin of a
Nitrogen-vacancy center in diamond. Our Floquet-engineered protocol achieves
state preparation fidelity of , on the same level as the
conventional fast-forward protocol, but is more robust to external noise acting
on the qubit. Floquet engineering provides a powerful platform for
high-fidelity quantum state manipulation in complex and noisy quantum systems
Quantum leakage detection using a model-independent dimension witness
Users of quantum computers must be able to confirm they are indeed
functioning as intended, even when the devices are remotely accessed. In
particular, if the Hilbert space dimension of the components are not as
advertised -- for instance if the qubits suffer leakage -- errors can ensue and
protocols may be rendered insecure. We refine the method of delayed vectors,
adapted from classical chaos theory to quantum systems, and apply it remotely
on the IBMQ platform -- a quantum computer composed of transmon qubits. The
method witnesses, in a model-independent fashion, dynamical signatures of
higher-dimensional processes. We present evidence, under mild assumptions, that
the IBMQ transmons suffer state leakage, with a value no larger than
under a single qubit operation. We also estimate the number
of shots necessary for revealing leakage in a two-qubit system.Comment: 11 pages, 5 figure
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