2,929 research outputs found
Using deep learning to understand and mitigate the qubit noise environment
Understanding the spectrum of noise acting on a qubit can yield valuable
information about its environment, and crucially underpins the optimization of
dynamical decoupling protocols that can mitigate such noise. However,
extracting accurate noise spectra from typical time-dynamics measurements on
qubits is intractable using standard methods. Here, we propose to address this
challenge using deep learning algorithms, leveraging the remarkable progress
made in the field of image recognition, natural language processing, and more
recently, structured data. We demonstrate a neural network based methodology
that allows for extraction of the noise spectrum associated with any qubit
surrounded by an arbitrary bath, with significantly greater accuracy than the
current methods of choice. The technique requires only a two-pulse echo decay
curve as input data and can further be extended either for constructing
customized optimal dynamical decoupling protocols or for obtaining critical
qubit attributes such as its proximity to the sample surface. Our results can
be applied to a wide range of qubit platforms, and provide a framework for
improving qubit performance with applications not only in quantum computing and
nanoscale sensing but also in material characterization techniques such as
magnetic resonance.Comment: Accepted for publication, 15 pages, 10 figure
Spin Amplification for Magnetic Sensors Employing Crystal Defects
Recently there have been several theoretical and experimental studies of the
prospects for magnetic field sensors based on crystal defects, especially
nitrogen vacancy (NV) centres in diamond. Such systems could potentially be
incorporated into an AFM-like apparatus in order to map the magnetic properties
of a surface at the single spin level. In this Letter we propose an augmented
sensor consisting of an NV centre for readout and an `amplifier' spin system
that directly senses the local magnetic field. Our calculations show that this
hybrid structure has the potential to detect magnetic moments with a
sensitivity and spatial resolution far beyond that of a simple NV centre, and
indeed this may be the physical limit for sensors of this class
The classical nature of nuclear spin noise near clock transitions of Bi donors in silicon
Whether a quantum bath can be approximated as classical noise is a
fundamental issue in central spin decoherence and also of practical importance
in designing noise-resilient quantum control. Spin qubits based on bismuth
donors in silicon have tunable interactions with nuclear spin baths and are
first-order insensitive to magnetic noise at so-called clock-transitions (CTs).
This system is therefore ideal for studying the quantum/classical nature of
nuclear spin baths since the qubit-bath interaction strength determines the
back-action on the baths and hence the adequacy of a classical noise model. We
develop a Gaussian noise model with noise correlations determined by quantum
calculations and compare the classical noise approximation to the full quantum
bath theory. We experimentally test our model through dynamical decoupling
sequence of up to 128 pulses, finding good agreement with simulations and
measuring electron spin coherence times approaching one second - notably using
natural silicon. Our theoretical and experimental study demonstrates that the
noise from a nuclear spin bath is analogous to classical Gaussian noise if the
back-action of the qubit on the bath is small compared to the internal bath
dynamics, as is the case close to CTs. However, far from the CTs, the
back-action of the central spin on the bath is such that the quantum model is
required to accurately model spin decoherence.Comment: 5 pages, 3 figure
Remote capacitive sensing in two-dimension quantum-dot arrays
We investigate gate-defined quantum dots in silicon on insulator nanowire
field-effect transistors fabricated using a foundry-compatible fully-depleted
silicon-on-insulator (FD-SOI) process. A series of split gates wrapped over the
silicon nanowire naturally produces a bilinear array of quantum
dots along a single nanowire. We begin by studying the capacitive coupling of
quantum dots within such a 22 array, and then show how such couplings
can be extended across two parallel silicon nanowires coupled together by
shared, electrically isolated, 'floating' electrodes. With one quantum dot
operating as a single-electron-box sensor, the floating gate serves to enhance
the charge sensitivity range, enabling it to detect charge state transitions in
a separate silicon nanowire. By comparing measurements from multiple devices we
illustrate the impact of the floating gate by quantifying both the charge
sensitivity decay as a function of dot-sensor separation and configuration
within the dual-nanowire structure.Comment: 9 pages, 3 figures, 35 cites and supplementar
A Silicon Surface Code Architecture Resilient Against Leakage Errors
Spin qubits in silicon quantum dots are one of the most promising building
blocks for large scale quantum computers thanks to their high qubit density and
compatibility with the existing semiconductor technologies. High fidelity
single-qubit gates exceeding the threshold of error correction codes like the
surface code have been demonstrated, while two-qubit gates have reached 98\%
fidelity and are improving rapidly. However, there are other types of error ---
such as charge leakage and propagation --- that may occur in quantum dot arrays
and which cannot be corrected by quantum error correction codes, making them
potentially damaging even when their probability is small. We propose a surface
code architecture for silicon quantum dot spin qubits that is robust against
leakage errors by incorporating multi-electron mediator dots. Charge leakage in
the qubit dots is transferred to the mediator dots via charge relaxation
processes and then removed using charge reservoirs attached to the mediators. A
stabiliser-check cycle, optimised for our hardware, then removes the
correlations between the residual physical errors. Through simulations we
obtain the surface code threshold for the charge leakage errors and show that
in our architecture the damage due to charge leakage errors is reduced to a
similar level to that of the usual depolarising gate noise. Spin leakage errors
in our architecture are constrained to only ancilla qubits and can be removed
during quantum error correction via reinitialisations of ancillae, which ensure
the robustness of our architecture against spin leakage as well. Our use of an
elongated mediator dots creates spaces throughout the quantum dot array for
charge reservoirs, measuring devices and control gates, providing the
scalability in the design
Ensemble based quantum metrology
The field of quantum metrology promises measurement devices that are
fundamentally superior to conventional technologies. Specifically, when quantum
entanglement is harnessed the precision achieved is supposed to scale more
favourably with the resources employed, such as system size and the time
required. Here we consider measurement of magnetic field strength using an
ensemble of spins, and we identify a third essential resource: the initial
system polarisation, i.e. the low entropy of the original state. We find that
performance depends crucially on the form of decoherence present; for a
plausible dephasing model, we describe a quantum strategy which can indeed beat
the standard quantum limit
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