226 research outputs found
Pseudo-digital quantum bits
Quantum computers are analog devices; thus they are highly susceptible to
accumulative errors arising from classical control electronics. Fast
operation--as necessitated by decoherence--makes gating errors very likely. In
most current designs for scalable quantum computers it is not possible to
satisfy both the requirements of low decoherence errors and low gating errors.
Here we introduce a hardware-based technique for pseudo-digital gate operation.
We perform self-consistent simulations of semiconductor quantum dots, finding
that pseudo-digital techniques reduce operational error rates by more than two
orders of magnitude, thus facilitating fast operation.Comment: 4 pages, 3 figure
Spin Readout and Initialization in a Semiconductor Quantum Dot
Electron spin qubits in semiconductors are attractive from the viewpoint of
long coherence times. However, single spin measurement is challenging. Several
promising schemes incorporate ancillary tunnel couplings that may provide
unwanted channels for decoherence. Here, we propose a novel spin-charge
transduction scheme, converting spin information to orbital information within
a single quantum dot by microwave excitation. The same quantum dot can be used
for rapid initialization, gating, and readout. We present detailed modeling of
such a device in silicon to confirm its feasibility.Comment: Published versio
Spin-orbit enhancement in Si/SiGe heterostructures with oscillating Ge concentration
We show that Ge concentration oscillations within the quantum well region of
a Si/SiGe heterostructure can significantly enhance the spin-orbit coupling of
the low-energy conduction-band valleys. Specifically, we find that for Ge
oscillation wavelengths near , a Dresselhaus
spin-orbit coupling is produced that is over an order of magnitude larger than
what is found in conventional Si/SiGe heterostructures without Ge concentration
oscillations. We also provide a detailed explanation for this resonance
phenomenon. This involves the Ge concentration oscillations producing
wavefunction satellite peaks a distance away in momentum space
from each valley, which then couple to the opposite valley through Dresselhaus
spin-orbit coupling. Our results indicate that the enhanced spin-orbit coupling
can enable fast spin manipulation within Si quantum dots using electric dipole
spin resonance in the absence of micromagnets. Indeed, our calculations yield a
Rabi frequency near the optimal Ge
oscillation wavelength Comment: 18 pages, 11 figure
Single-shot measurement of triplet-singlet relaxation in a Si/SiGe double quantum dot
We investigate the lifetime of two-electron spin states in a few-electron
Si/SiGe double dot. At the transition between the (1,1) and (0,2) charge
occupations, Pauli spin blockade provides a readout mechanism for the spin
state. We use the statistics of repeated single-shot measurements to extract
the lifetimes of multiple states simultaneously. At zero magnetic field, we
find that all three triplet states have equal lifetimes, as expected, and this
time is ~10 ms. At non-zero field, the T0 lifetime is unchanged, whereas the T-
lifetime increases monotonically with field, reaching 3 seconds at 1 T.Comment: 4 pages, 3 figures, supplemental information. Typos fixed; updated to
submitted versio
Pauli spin blockade and lifetime-enhanced transport in a Si/SiGe double quantum dot
We analyze electron transport data through a Si/SiGe double quantum dot in
terms of spin blockade and lifetime-enhanced transport (LET), which is
transport through excited states that is enabled by long spin relaxation times.
We present a series of low-bias voltage measurements showing the sudden
appearance of a strong tail of current that we argue is an unambiguous
signature of LET appearing when the bias voltage becomes greater than the
singlet-triplet splitting for the (2,0) electron state. We present eight
independent data sets, four in the forward bias (spin-blockade) regime and four
in the reverse bias (lifetime-enhanced transport) regime, and show that all
eight data sets can be fit to one consistent set of parameters. We also perform
a detailed analysis of the reverse bias (LET) regime, using transport rate
equations that include both singlet and triplet transport channels. The model
also includes the energy dependent tunneling of electrons across the quantum
barriers, and resonant and inelastic tunneling effects. In this way, we obtain
excellent fits to the experimental data, and we obtain quantitative estimates
for the tunneling rates and transport currents throughout the reverse bias
regime. We provide a physical understanding of the different blockade regimes
and present detailed predictions for the conditions under which LET may be
observed.Comment: published version, 18 page
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
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