179 research outputs found
Recovering Quantum Gates from Few Average Gate Fidelities
Characterizing quantum processes is a key task in the development of quantum technologies, especially at the noisy intermediate scale of today’s devices. One method for characterizing processes is randomized benchmarking, which is robust against state preparation and measurement errors and can be used to benchmark Clifford gates. Compressed sensing techniques achieve full tomography of quantum channels essentially at optimal resource efficiency. In this Letter, we show that the favorable features of both approaches can be combined. For characterizing multiqubit unitary gates, we provide a rigorously guaranteed and practical reconstruction method that works with an essentially optimal number of average gate fidelities measured with respect to random Clifford unitaries. Moreover, for general unital quantum channels, we provide an explicit expansion into a unitary 2-design, allowing for a practical and guaranteed reconstruction also in that case. As a side result, we obtain a new statistical interpretation of the unitarity—a figure of merit characterizing the coherence of a process
Implementation of Grover's Quantum Search Algorithm in a Scalable System
We report the implementation of Grover's quantum search algorithm in the
scalable system of trapped atomic ion quantum bits. Any one of four possible
states of a two-qubit memory is marked, and following a single query of the
search space, the marked element is successfully recovered with an average
probability of 60(2)%. This exceeds the performance of any possible classical
search algorithm, which can only succeed with a maximum average probability of
50%.Comment: 4 pages, 3 figures, updated error discussio
Consistency of high-fidelity two-qubit operations in silicon
The consistency of entangling operations between qubits is essential for the
performance of multi-qubit systems, and is a crucial factor in achieving
fault-tolerant quantum processors. Solid-state platforms are particularly
exposed to inconsistency due to the materials-induced variability of
performance between qubits and the instability of gate fidelities over time.
Here we quantify this consistency for spin qubits, tying it to its physical
origins, while demonstrating sustained and repeatable operation of two-qubit
gates with fidelities above 99% in the technologically important silicon
metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed
study of the stability of these operations by analysing errors and fidelities
in multiple devices through numerous trials and extended periods of operation.
Adopting three different characterisation methods, we measure entangling gate
fidelities ranging from 96.8% to 99.8%. Our analysis tools also identify
physical causes of qubit degradation and offer ways to maintain performance
within tolerance. Furthermore, we investigate the impact of qubit design,
feedback systems, and robust gates on implementing scalable, high-fidelity
control strategies. These results highlight both the capabilities and
challenges for the scaling up of spin-based qubits into full-scale quantum
processors
Shadow estimation of gate-set properties from random sequences
With quantum computing devices increasing in scale and complexity, there is a growing need for tools that obtain precise diagnostic information about quantum operations. However, current quantum devices are only capable of short unstructured gate sequences followed by native measurements. We accept this limitation and turn it into a new paradigm for characterizing quantum gate-sets. A single experiment—random sequence estimation—solves a wealth of estimation problems, with all complexity moved to classical post-processing. We derive robust channel variants of shadow estimation with close-to-optimal performance guarantees and use these as a primitive for partial, compressive and full process tomography as well as the learning of Pauli noise. We discuss applications to the quantum gate engineering cycle, and propose novel methods for the optimization of quantum gates and diagnosing cross-talk
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