17 research outputs found
Two-Qubit Gate Set Tomography with Fewer Circuits
Gate set tomography (GST) is a self-consistent and highly accurate method for
the tomographic reconstruction of a quantum information processor's quantum
logic operations, including gates, state preparations, and measurements.
However, GST's experimental cost grows exponentially with qubit number. For
characterizing even just two qubits, a standard GST experiment may have tens of
thousands of circuits, making it prohibitively expensive for platforms. We show
that, because GST experiments are massively overcomplete, many circuits can be
discarded. This dramatically reduces GST's experimental cost while still
maintaining GST's Heisenberg-like scaling in accuracy. We show how to exploit
the structure of GST circuits to determine which ones are superfluous. We
confirm the efficacy of the resulting experiment designs both through numerical
simulations and via the Fisher information for said designs. We also explore
the impact of these techniques on the prospects of three-qubit GST.Comment: 46 pages, 13 figures. V2: Minor edits to acknowledgment
Precision tomography of a three-qubit electron-nuclear quantum processor in silicon
Nuclear spins were among the first physical platforms to be considered for
quantum information processing, because of their exceptional quantum coherence
and atomic-scale footprint. However, their full potential for quantum computing
has not yet been realized, due to the lack of methods to link nuclear qubits
within a scalable device combined with multi-qubit operations with sufficient
fidelity to sustain fault-tolerant quantum computation. Here we demonstrate
universal quantum logic operations using a pair of ion-implanted P
nuclei in a silicon nanoelectronic device. A nuclear two-qubit controlled-Z
gate is obtained by imparting a geometric phase to a shared electron spin, and
used to prepare entangled Bell states with fidelities up to 94.2(2.7)%. The
quantum operations are precisely characterised using gate set tomography (GST),
yielding one-qubit gate fidelities up to 99.93(3)%, two-qubit gate fidelity of
99.21(14)% and two-qubit preparation/measurement fidelities of 98.95(4)%. These
three metrics indicate that nuclear spins in silicon are approaching the
performance demanded in fault-tolerant quantum processors. We then demonstrate
entanglement between the two nuclei and the shared electron by producing a
Greenberger-Horne-Zeilinger three-qubit state with 92.5(1.0)% fidelity. Since
electron spin qubits in semiconductors can be further coupled to other
electrons or physically shuttled across different locations, these results
establish a viable route for scalable quantum information processing using
nuclear spins.Comment: 27 pages, 14 figures, plus 20 pages supplementary information. v2
includes new and updated references, and minor text change
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
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