68 research outputs found
A shuttling-based two-qubit logic gate for linking distant silicon quantum processors
Control of entanglement between qubits at distant quantum processors using a
two-qubit gate is an essential function of a scalable, modular implementation
of quantum computation. Among the many qubit platforms, spin qubits in silicon
quantum dots are promising for large-scale integration along with their
nanofabrication capability. However, linking distant silicon quantum processors
is challenging as two-qubit gates in spin qubits typically utilize short-range
exchange coupling, which is only effective between nearest-neighbor quantum
dots. Here we demonstrate a two-qubit gate between spin qubits via coherent
spin shuttling, a key technology for linking distant silicon quantum
processors. Coherent shuttling of a spin qubit enables efficient switching of
the exchange coupling with an on/off ratio exceeding 1,000 , while preserving
the spin coherence by 99.6% for the single shuttling between neighboring dots.
With this shuttling-mode exchange control, we demonstrate a two-qubit
controlled-phase gate with a fidelity of 93%, assessed via randomized
benchmarking. Combination of our technique and a phase coherent shuttling of a
qubit across a large quantum dot array will provide feasible path toward a
quantum link between distant silicon quantum processors, a key requirement for
large-scale quantum computation
Gate modulation of the hole singlet-triplet qubit frequency in germanium
Spin qubits in germanium gate-defined quantum dots have made considerable
progress within the last few years, partially due to their strong spin-orbit
coupling and site-dependent -tensors. While this characteristic of the
-factors removes the need for micromagnets and allows for the possibility of
all-electric qubit control, relying on these -tensors necessitates the need
to understand their sensitivity to the confinement potential that defines the
quantum dots. Here, we demonstrate a qubit whose frequency is a strong
function of the voltage applied to the barrier gate shared by the quantum dots.
We find a -factor that can be approximately increased by an order of
magnitude adjusting the barrier gate voltage only by 12 mV. We attribute the
strong dependence to a variable strain profile in our device. This work not
only reinforces previous findings that site-dependent -tensors in germanium
can be utilized for qubit manipulation, but reveals the sensitivity and
tunability these -tensors have to the electrostatic confinement of the
quantum dot
Practical Strategies for Enhancing the Valley Splitting in Si/SiGe Quantum Wells
Silicon/silicon-germanium heterostructures have many important advantages for
hosting spin qubits. However, controlling the energy splitting between the two
low-energy conduction-band valleys remains a critical challenge for scaling up
to large numbers of reliable qubits. Broad distributions of valley splittings
are commonplace, even among quantum dots formed on the same chip. Such behavior
has previously been attributed to imperfections such as steps at the quantum
well interface. The most common approaches for addressing this problem have
sought to engineer design improvements into the quantum well. In this work, we
develop a simple, universal theory of valley splitting based on the
reciprocal-space profile of the quantum well confinement potential, which
simultaneously explains the effects of steps, wide interfaces, alloy disorder,
and custom heterostructure designs. We use this understanding to characterize
theoretically the valley splitting in a variety of heterostructures, finding
that alloy disorder can explain the observed variability of the valley
splitting, even in the absence of steps. Moreover we show that steps have a
significant effect on the valley splitting only when the top interface is very
sharp. We predict a universal crossover from a regime where low valley
splittings are rare to a regime dominated by alloy disorder, in which valley
splittings can approach zero. We show that many recent experiments fall into
the latter category, with important implications for large-scale qubit
implementations. We finally propose a strategy to suppress the incidence of low
valley splittings by (i) increasing the random alloy disorder (to increase the
valley splitting variance), and then (ii) allowing for electrostatic tuning of
the dot position (to access locations with higher valley splitting).Comment: 34 pages, 22 figure
Germanium wafers for strained quantum wells with low disorder
We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor
deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy
enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading
dislocation density of (61)10 cm, nearly an order of
magnitude improvement compared to control strain-relaxed buffers on Si wafers.
The associated reduction in short-range scattering allows for a drastic
improvement of the disorder properties of the two-dimensional hole gas,
measured in several Ge/SiGe heterostructure field-effect transistors. We
measure an average low percolation density of (1.220.03)10
cm, and an average maximum mobility of (3.40.1)10
cm/Vs and quantum mobility of (8.40.5)10 cm/Vs when
the hole density in the quantum well is saturated to
(1.650.02)10 cm. We anticipate immediate application
of these heterostructures for next-generation, higher-performance Ge
spin-qubits and their integration into larger quantum processors
Rapid single-shot parity spin readout in a silicon double quantum dot with fidelity exceeding 99 %
Silicon-based spin qubits offer a potential pathway toward realizing a
scalable quantum computer owing to their compatibility with semiconductor
manufacturing technologies. Recent experiments in this system have demonstrated
crucial technologies, including high-fidelity quantum gates and multiqubit
operation. However, the realization of a fault-tolerant quantum computer
requires a high-fidelity spin measurement faster than decoherence. To address
this challenge, we characterize and optimize the initialization and measurement
procedures using the parity-mode Pauli spin blockade technique. Here, we
demonstrate a rapid (with a duration of a few us) and accurate (with >99%
fidelity) parity spin measurement in a silicon double quantum dot. These
results represent a significant step forward toward implementing
measurement-based quantum error correction in silicon
Probing resonating valence bonds on a programmable germanium quantum simulator
Simulations using highly tunable quantum systems may enable investigations of
condensed matter systems beyond the capabilities of classical computers.
Quantum dots and donors in semiconductor technology define a natural approach
to implement quantum simulation. Several material platforms have been used to
study interacting charge states, while gallium arsenide has also been used to
investigate spin evolution. However, decoherence remains a key challenge in
simulating coherent quantum dynamics. Here, we introduce quantum simulation
using hole spins in germanium quantum dots. We demonstrate extensive and
coherent control enabling the tuning of multi-spin states in isolated, paired,
and fully coupled quantum dots. We then focus on the simulation of resonating
valence bonds and measure the evolution between singlet product states which
remains coherent over many periods. Finally, we realize four-spin states with
-wave and -wave symmetry. These results provide means to perform
non-trivial and coherent simulations of correlated electron systems.Comment: Article main text and Supplementary Information Main text: 9 pages, 5
figures Supplementary Information: 15 pages, 9 figure
Single-hole pump in germanium
Abstract: Single-charge pumps are the main candidates for quantum-based standards of the unit ampere because they can generate accurate and quantized electric currents. In order to approach the metrological requirements in terms of both accuracy and speed of operation, in the past decade there has been a focus on semiconductor-based devices. The use of a variety of semiconductor materials enables the universality of charge pump devices to be tested, a highly desirable demonstration for metrology, with GaAs and Si pumps at the forefront of these tests. Here, we show that pumping can be achieved in a yet unexplored semiconductor, i.e. germanium. We realise a single-hole pump with a tunable-barrier quantum dot electrostatically defined at a Ge/SiGe heterostructure interface. We observe quantized current plateaux by driving the system with a single sinusoidal drive up to a frequency of 100 MHz. The operation of the prototype was affected by accidental formation of multiple dots, probably due to disorder potential, and random charge fluctuations. We suggest straightforward refinements of the fabrication process to improve pump characteristics in future experiments
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