3 research outputs found
Quantum computer error structure probed by quantum error correction syndrome measurements
With quantum devices rapidly approaching qualities and scales needed for
fault tolerance, the validity of simplified error models underpinning the study
of quantum error correction needs to be experimentally evaluated. In this work,
we have directly assessed the performance of superconducting devices
implementing heavy-hexagon code syndrome measurements with increasing circuit
sizes up to 23 qubits, against the error assumptions underpinning code
threshold calculations. Data from 16 repeated syndrome measurement cycles was
found to be inconsistent with a uniform depolarizing noise model, favouring
instead biased and inhomogeneous noise models. Spatial-temporal correlations
investigated via stabilizer measurements revealed significant temporal
correlation in detection events. These results highlight the non-trivial
structure which may be present in the noise of quantum error correction
circuits and support the development of noise-tailored codes and decoders to
adapt
A scalable and fast artificial neural network syndrome decoder for surface codes
Surface code error correction offers a highly promising pathway to achieve
scalable fault-tolerant quantum computing. When operated as stabiliser codes,
surface code computations consist of a syndrome decoding step where measured
stabiliser operators are used to determine appropriate corrections for errors
in physical qubits. Decoding algorithms have undergone substantial development,
with recent work incorporating machine learning (ML) techniques. Despite
promising initial results, the ML-based syndrome decoders are still limited to
small scale demonstrations with low latency and are incapable of handling
surface codes with boundary conditions and various shapes needed for lattice
surgery and braiding. Here, we report the development of an artificial neural
network (ANN) based scalable and fast syndrome decoder capable of decoding
surface codes of arbitrary shape and size with data qubits suffering from a
variety of noise models including depolarising errors, biased noise, and
spatially inhomogeneous noise. Based on rigorous training over 50 million
random quantum error instances, our ANN decoder is shown to work with code
distances exceeding 1000 (more than 4 million physical qubits), which is the
largest ML-based decoder demonstration to-date. The established ANN decoder
demonstrates an execution time in principle independent of code distance,
implying that its implementation on dedicated hardware could potentially offer
surface code decoding times of O(sec), commensurate with the
experimentally realisable qubit coherence times. With the anticipated scale-up
of quantum processors within the next decade, their augmentation with a fast
and scalable syndrome decoder such as developed in our work is expected to play
a decisive role towards experimental implementation of fault-tolerant quantum
information processing.Comment: 11 pages, 6 figure
Direct observation of narrow electronic energy band formation in 2D molecular self-assembly
Surface-supported molecular overlayers have demonstrated versatility as platforms for fundamental research and a broad range of applications, from atomic-scale quantum phenomena to potential for electronic, optoelectronic and catalytic technologies. Here, we report a structural and electronic characterisation of self-assembled magnesium phthalocyanine (MgPc) mono and bilayers on the Ag(100) surface, via low-temperature scanning tunneling microscopy and spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), density functional theory (DFT) and tight-binding (TB) modeling. These crystalline close-packed molecular overlayers consist of a square lattice with a basis composed of a single, flat-adsorbed MgPc molecule. Remarkably, ARPES measurements at room temperature on the monolayer reveal a momentum-resolved, two-dimensional (2D) electronic energy band, 1.27 eV below the Fermi level, with a width of ∼20 meV. This 2D band results from in-plane hybridization of highest occupied molecular orbitals of adjacent, weakly interacting MgPc's, consistent with our TB model and with DFT-derived nearest-neighbor hopping energies. This work opens the door to quantitative characterisation – as well as control and harnessing – of subtle electronic interactions between molecules in functional organic nanofilms