19,265 research outputs found
Adaptive weight estimator for quantum error correction
Quantum error correction of a surface code or repetition code requires the
pairwise matching of error events in a space-time graph of qubit measurements,
such that the total weight of the matching is minimized. The input weights
follow from a physical model of the error processes that affect the qubits.
This approach becomes problematic if the system has sources of error that
change over time. Here we show how the weights can be determined from the
measured data in the absence of an error model. The resulting adaptive decoder
performs well in a time-dependent environment, provided that the characteristic
time scale of the variations is greater than , with the duration of one error-correction cycle and
the typical error probability per qubit in one cycle.Comment: 5 pages, 4 figure
Adaptive quantum metrology under general Markovian noise
We consider a general model of unitary parameter estimation in presence of
Markovian noise, where the parameter to be estimated is associated with the
Hamiltonian part of the dynamics. In absence of noise, unitary parameter can be
estimated with precision scaling as , where is the total probing time.
We provide a simple algebraic condition involving solely the operators
appearing in the quantum Master equation, implying at most scaling
of precision under the most general adaptive quantum estimation strategies. We
also discuss the requirements a quantum error-correction like protocol must
satisfy in order to regain the precision scaling in case the above
mentioned algebraic condition is not satisfied. Furthermore, we apply the
developed methods to understand fundamental precision limits in atomic
interferometry with many-body effects taken into account, shedding new light on
the performance of non-linear metrological models.Comment: 13 pages, see also arXiv:1706.0244
Designing High-Fidelity Single-Shot Three-Qubit Gates: A Machine Learning Approach
Three-qubit quantum gates are key ingredients for quantum error correction
and quantum information processing. We generate quantum-control procedures to
design three types of three-qubit gates, namely Toffoli, Controlled-Not-Not and
Fredkin gates. The design procedures are applicable to a system comprising
three nearest-neighbor-coupled superconducting artificial atoms. For each
three-qubit gate, the numerical simulation of the proposed scheme achieves
99.9% fidelity, which is an accepted threshold fidelity for fault-tolerant
quantum computing. We test our procedure in the presence of decoherence-induced
noise as well as show its robustness against random external noise generated by
the control electronics. The three-qubit gates are designed via the machine
learning algorithm called Subspace-Selective Self-Adaptive Differential
Evolution (SuSSADE).Comment: 18 pages, 13 figures. Accepted for publication in Phys. Rev. Applie
Quantum metrology with full and fast quantum control
We establish general limits on how precise a parameter, e.g. frequency or the
strength of a magnetic field, can be estimated with the aid of full and fast
quantum control. We consider uncorrelated noisy evolutions of N qubits and show
that fast control allows to fully restore the Heisenberg scaling (~1/N^2) for
all rank-one Pauli noise except dephasing. For all other types of noise the
asymptotic quantum enhancement is unavoidably limited to a constant-factor
improvement over the standard quantum limit (~1/N) even when allowing for the
full power of fast control. The latter holds both in the single-shot and
infinitely-many repetitions scenarios. However, even in this case allowing for
fast quantum control helps to increase the improvement factor. Furthermore, for
frequency estimation with finite resource we show how a parallel scheme
utilizing any fixed number of entangled qubits but no fast quantum control can
be outperformed by a simple, easily implementable, sequential scheme which only
requires entanglement between one sensing and one auxiliary qubit.Comment: 17 pages, 7 figures, 6 appendice
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