25 research outputs found
Triple visual hemifield maps in a case of optic chiasm hypoplasia
In humans, each hemisphere comprises an overlay of two visuotopic maps of the contralateral visual field, one from each eye. Is the capacity of the visual cortex limited to these two maps or are plastic mechanisms available to host more maps? We determined the cortical organization of the visual field maps in a rare individual with chiasma hypoplasia, where visual cortex plasticity is challenged to accommodate three hemifield maps. Using high-resolution fMRI at 7T and diffusion-weighted MRI at 3T, we found three hemiretinal inputs, instead of the normal two, to converge onto the left hemisphere. fMRI-based population receptive field mapping of the left V1-V3 at 3T revealed three superimposed hemifield representations in the left visual cortex, i.e. two representations of opposing visual hemifields from the left eye and one right hemifield representation from the right eye. We conclude that developmental plasticity including the re-wiring of local intra- and cortico-cortical connections is pivotal to support the coexistence and functioning of three hemifield maps within one hemisphere
Real-time feedback protocols for optimizing fault-tolerant two-qubit gate fidelities in a silicon spin system
Recently, several groups have demonstrated two-qubit gate fidelities in
semiconductor spin qubit systems above 99%. Achieving this regime of
fault-tolerant compatible high fidelities is nontrivial and requires exquisite
stability and precise control over the different qubit parameters over an
extended period of time. This can be done by efficiently calibrating qubit
control parameters against different sources of micro- and macroscopic noise.
Here, we present several single- and two-qubit parameter feedback protocols,
optimised for and implemented in state-of-the-art fast FPGA hardware.
Furthermore, we use wavelet-based analysis on the collected feedback data to
gain insight into the different sources of noise in the system. Scalable
feedback is an outstanding challenge and the presented implementation and
analysis gives insight into the benefits and drawbacks of qubit parameter
feedback, as feedback related overhead increases. This work demonstrates a
pathway towards robust qubit parameter feedback and systematic noise analysis,
crucial for mitigation strategies towards systematic high-fidelity qubit
operation compatible with quantum error correction protocols
Spatio-temporal correlations of noise in MOS spin qubits
In quantum computing, characterising the full noise profile of qubits can aid
the efforts towards increasing coherence times and fidelities by creating error
mitigating techniques specific to the type of noise in the system, or by
completely removing the sources of noise. Spin qubits in MOS quantum dots are
exposed to noise originated from the complex glassy behaviour of two-level
fluctuators, leading to non-trivial correlations between qubit properties both
in space and time. With recent engineering progress, large amounts of data are
being collected in typical spin qubit device experiments, and it is beneficiary
to explore data analysis options inspired from fields of research that are
experienced in managing large data sets, examples include astrophysics, finance
and climate science. Here, we propose and demonstrate wavelet-based analysis
techniques to decompose signals into both frequency and time components to gain
a deeper insight into the sources of noise in our systems. We apply the
analysis to a long feedback experiment performed on a state-of-the-art
two-qubit system in a pair of SiMOS quantum dots. The observed correlations
serve to identify common microscopic causes of noise, as well as to elucidate
pathways for multi-qubit operation with a more scalable feedback system.Comment: updated referenc
Entangling gates on degenerate spin qubits dressed by a global field
Coherently dressed spins have shown promising results as building blocks for
future quantum computers owing to their resilience to environmental noise and
their compatibility with global control fields. This mode of operation allows
for more amenable qubit architecture requirements and simplifies signal routing
on the chip. However, multi-qubit operations, such as qubit addressability and
two-qubit gates, are yet to be demonstrated to establish global control in
combination with dressed qubits as a viable path to universal quantum
computing. Here we demonstrate simultaneous on-resonance driving of degenerate
qubits using a global field while retaining addressability for qubits with
equal Larmor frequencies. Furthermore, we implement SWAP oscillations during
on-resonance driving, constituting the demonstration of driven two-qubit gates.
Significantly, our findings highlight the fragility of entangling gates between
superposition states and how dressing can increase the noise robustness. These
results represent a crucial milestone towards global control operation with
dressed qubits. It also opens a door to interesting spin physics on degenerate
spins
Impact of electrostatic crosstalk on spin qubits in dense CMOS quantum dot arrays
Quantum processors based on integrated nanoscale silicon spin qubits are a
promising platform for highly scalable quantum computation. Current CMOS spin
qubit processors consist of dense gate arrays to define the quantum dots,
making them susceptible to crosstalk from capacitive coupling between a dot and
its neighbouring gates. Small but sizeable spin-orbit interactions can transfer
this electrostatic crosstalk to the spin g-factors, creating a dependence of
the Larmor frequency on the electric field created by gate electrodes
positioned even tens of nanometers apart. By studying the Stark shift from tens
of spin qubits measured in nine different CMOS devices, we developed a
theoretical frawework that explains how electric fields couple to the spin of
the electrons in increasingly complex arrays, including those electric
fluctuations that limit qubit dephasing times . The results will aid in
the design of robust strategies to scale CMOS quantum technology.Comment: 9 pages, 4 figure
High-fidelity operation and algorithmic initialisation of spin qubits above one kelvin
The encoding of qubits in semiconductor spin carriers has been recognised as
a promising approach to a commercial quantum computer that can be
lithographically produced and integrated at scale. However, the operation of
the large number of qubits required for advantageous quantum applications will
produce a thermal load exceeding the available cooling power of cryostats at
millikelvin temperatures. As the scale-up accelerates, it becomes imperative to
establish fault-tolerant operation above 1 kelvin, where the cooling power is
orders of magnitude higher. Here, we tune up and operate spin qubits in silicon
above 1 kelvin, with fidelities in the range required for fault-tolerant
operation at such temperatures. We design an algorithmic initialisation
protocol to prepare a pure two-qubit state even when the thermal energy is
substantially above the qubit energies, and incorporate high-fidelity
radio-frequency readout to achieve an initialisation fidelity of 99.34 per
cent. Importantly, we demonstrate a single-qubit Clifford gate fidelity of
99.85 per cent, and a two-qubit gate fidelity of 98.92 per cent. These advances
overcome the fundamental limitation that the thermal energy must be well below
the qubit energies for high-fidelity operation to be possible, surmounting a
major obstacle in the pathway to scalable and fault-tolerant quantum
computation
High-fidelity spin qubit operation and algorithmic initialization above 1 K
The encoding of qubits in semiconductor spin carriers has been recognized as a promising approach to a commercial quantum computer that can be lithographically produced and integrated at scale. However, the operation of the large number of qubits required for advantageous quantum applications will produce a thermal load exceeding the available cooling power of cryostats at millikelvin temperatures. As the scale-up accelerates, it becomes imperative to establish fault-tolerant operation above 1 K, at which the cooling power is orders of magnitude higher. Here we tune up and operate spin qubits in silicon above 1 K, with fidelities in the range required for fault-tolerant operations at these temperatures. We design an algorithmic initialization protocol to prepare a pure two-qubit state even when the thermal energy is substantially above the qubit energies and incorporate radiofrequency readout to achieve fidelities up to 99.34% for both readout and initialization. We also demonstrate single-qubit Clifford gate fidelities up to 99.85% and a two-qubit gate fidelity of 98.92%. These advances overcome the fundamental limitation that the thermal energy must be well below the qubit energies for the high-fidelity operation to be possible, surmounting a main obstacle in the pathway to scalable and fault-tolerant quantum computation
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