-
[show abstract]
[hide abstract]
ABSTRACT: Measurement of multiple quantum devices on a single chip increases
characterization throughput and enables testing of device repeatability,
process yield, and systematic variations in device design. We present a method
that uses on-chip field-effect transistor switches to enable multiplexed
cryogenic measurements of double quantum dot Si/SiGe devices. Multiplexing
makes it feasible to characterize a number of devices that scales exponentially
with the number of external wires, a key capability given the significant
constraints on cryostat wiring currently in common use. We use this approach to
characterize three nominally identical quantum-point contact channels, enabling
comparison of their threshold voltages for accumulation and their pinch-off
voltages during a single cool-down of a dilution refrigerator.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: We report transport measurements on a Si/SiGe quantum dot subject to
microwave excitation via an on-chip antenna. The response shows signatures of
photon-assisted tunneling and only a small effect on charge stability. We also
explore the use of a d.c. current applied to the antenna for generating
tunable, local magnetic field gradients and put bounds on the achievable field
gradients, limited by heating of the reservoirs.
01/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: We report measurement of the valley Kondo effect in a Si/SiGe quantum dot.
The Kondo peaks in two consecutive Coulomb diamonds show unusual behavior in a
magnetic field that we interpret as arising from the valley degree of freedom.
In one diamond two Kondo peaks due to screening of the valley index exist at
zero field, revealing a zero-field valley splitting of 0.28 meV. In a finite
field the peaks broaden and coalesce due to Zeeman splitting. In the other
diamond, a single resonance persists at zero bias for non-zero field, a
phenomenon characteristic of valley non-conservation in tunneling.
12/2012;
-
Zhan Shi,
C. B. Simmons,
D. R. Ward,
J. R. Prance,
R. T. Mohr,
Teck Seng Koh,
John King Gamble,
X. Wu, D. E. Savage,
M. G. Lagally,
Mark Friesen,
S. N. Coppersmith,
M. A. Eriksson
[show abstract]
[hide abstract]
ABSTRACT: Fast quantum oscillations of a charge qubit in a double quantum dot
fabricated in a Si/SiGe heterostructure are demonstrated and characterized
experimentally. The measured inhomogeneous dephasing time T2* ranges from 127
ps to 3.7 ns; it depends substantially on how the energy difference of the the
two qubit states varies with external voltages, consistent with a decoherence
process that is dominated by charge noise that changes the asymmetry of the
qubit's double-well potential (detuning noise). Applying a charge-echo pulse
sequence increases the measured inhomogeneous decoherence time from 127 ps to
760 ps.
08/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: Si quantum wells on plastically relaxed SiGe substrates have nanometer variations in crystallographic parameters crucial to quantum-information devices. Synchrotron X-ray nanodiffraction shows that the lattice of the Si quantum well varies in orientation and thickness over lateral distances of 100 nm to 1 μm. The result is that the energy levels of the confined states are shifted by energies similar to the electron temperature.
Advanced Materials 07/2012; 24(38):5217-21. · 13.88 Impact Factor
-
Zhan Shi,
C B Simmons,
J R Prance,
John King Gamble,
Teck Seng Koh,
Yun-Pil Shim,
Xuedong Hu, D E Savage,
M G Lagally,
M A Eriksson,
Mark Friesen,
S N Coppersmith
[show abstract]
[hide abstract]
ABSTRACT: We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S(2)=3/4 (S=1/2) and S(z)=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantum dot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
Physical Review Letters 04/2012; 108(14):140503. · 7.37 Impact Factor
-
J R Prance,
Zhan Shi,
C B Simmons, D E Savage,
M G Lagally,
L R Schreiber,
L M K Vandersypen,
Mark Friesen,
Robert Joynt,
S N Coppersmith,
M A Eriksson
[show abstract]
[hide abstract]
ABSTRACT: 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. When the magnetic field is zero, we find that all three triplet states have equal lifetimes, as expected, and this time is ~10 ms. When the field is nonzero, the T(0) lifetime is unchanged, whereas the T- lifetime increases monotonically with the field, reaching 3 sec at 1 T.
Physical Review Letters 01/2012; 108(4):046808. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We measure the excited-state spectrum of a Si/SiGe quantum dot as a function
of in-plane magnetic field, and we identify the spin of the lowest three
eigenstates in an effective two-electron regime. The singlet-triplet splitting
is an essential parameter describing spin qubits, and we extract this splitting
from the data. We find it to be tunable by lateral displacement of the dot,
which is realized by changing two gate voltages on opposite sides of the
device. We present calculations showing the data are consistent with a spectrum
in which the first excited state of the dot is a valley-orbit state.
09/2011;
-
C B Simmons,
J R Prance,
B J Van Bael,
Teck Seng Koh,
Zhan Shi, D E Savage,
M G Lagally,
R Joynt,
Mark Friesen,
S N Coppersmith,
M A Eriksson
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate single-shot readout of a silicon quantum dot spin qubit, and we measure the spin relaxation time T1. We show that the rate of spin loading can be tuned by an order of magnitude by changing the amplitude of a pulsed-gate voltage, and the fraction of spin-up electrons loaded can also be controlled. This tunability arises because electron spins can be loaded through an orbital excited state. Using a theory that includes excited states of the dot and energy-dependent tunneling, we find that a global fit to the loading rate and spin-up fraction is in good agreement with the data.
Physical Review Letters 04/2011; 106(15):156804. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report a robust process for fabrication of surface-gated Si/SiGe quantum
dots (QDs) with an integrated superconducting single-electron transistor
(S-SET) charge sensor. A combination of a deep mesa etch and AlOx backfill is
used to reduce gate leakage. After the leakage current is suppressed, Coulomb
oscillations of the QD and the current-voltage characteristics of the S-SET are
observed at a temperature of 0.3 K. Coupling of the S-SET to the QD is
confirmed by using the S-SET to perform sensing of the QD charge state.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We fabricate curled 3-D objects from semiconductor nanomembranes consisting of single-crystal silicon, which is epitaxially grown on silicon-germanium-on-insulator substrates. The curling is caused by relaxing the strain induced by lattice mismatch between silicon (Si) and germanium (Ge). Depending on the lithographically patterned geometries and their orientation with respect to the crystallographic direction, different shapes of tubes can be realized. Particularly interesting are tubes that are not completely closed, or partially open, whose mechanical response is ultraelastic. We demonstrate that applying acetone on such tubes generates a surface stress imbalance between the Si and Si-Ge layers, resulting in detectable shape changes. This mechanism has potential applications in chemical sensing, where the deformable curled structures act as dynamic-aperture reflector antennas. Our simulation suggests the curvature changes induced in the presence of certain chemical, such as acetone, will lead to distinctive far-field radiation patterns in the terahertz (THz) range.
IEEE Transactions on Nanotechnology 02/2011; · 2.29 Impact Factor
-
C. B. Simmons,
J. R. Prance,
B. J. Van Bael,
Teck Seng Koh,
Zhan Shi, D. E. Savage,
M. G. Lagally,
R. Joynt,
Mark Friesen,
S. N. Coppersmith,
M. A. Eriksson
[show abstract]
[hide abstract]
ABSTRACT: The remarkable properties of silicon have made it the central material for
the fabrication of current microelectronic devices. Silicon's fundamental
properties also make it an attractive option for the development of devices for
spintronics and quantum information processing. The ability to manipulate and
measure spins of single electrons is crucial for these applications. Here we
report the manipulation and measurement of a single spin in a quantum dot
fabricated in a silicon/silicon-germanium heterostructure. We demonstrate that
the rate of loading of electrons into the device can be tuned over an order of
magnitude using a gate voltage, that the spin state of the loaded electron
depends systematically on the loading voltage level, and that this tunability
arises because electron spins can be loaded through excited orbital states of
the quantum dot. The longitudinal spin relaxation time T1 is measured using
single-shot pulsed techniques and found to be ~3 seconds at a field of 1.85
Tesla. The demonstration of single spin measurement as well as a long spin
relaxation time and tunability of the loading are all favorable properties for
spintronics and quantum information processing applications.
10/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the tunnel rates and energies of excited states of small
numbers of electrons in a quantum dot fabricated in a Si/SiGe heterostructure.
Tunnel rates for loading and unloading electrons are found to be strongly
energy dependent, and they vary significantly between different excited states.
We show that this phenomenon enables charge sensing measurements of the average
electron occupation that are analogous to Coulomb diamonds. Excited-state
energies can be read directly from the plot, and we develop a rate model that
enables a quantitative understanding of the relative sizes of different
electron tunnel rates.
10/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Epitaxial growth of self-assembled quantum dots (QDs) on single-crystal nanomembranes yields organized arrays of QDs via a growth mode mediated by QD-induced strains in the membrane. A crucial aspect of this effect arises because epitaxial growth on thin Si sheets and nanostructures derived from them can occur simultaneously on two surfaces separated only by the 10-nm-scale thickness of the membrane. A QD on one surface of a free-standing membrane causes the nucleation of QDs in specific positions on the opposite surface. Control experiments using molecular beam epitaxy to deposit QDs on a single surface do not yield long-range order. Through-membrane elastic interactions consistent with predictions from finite-element-based mechanics models are observed using synchrotron x-ray microdiffraction. The role of crystallographic anisotropy is evident in finite-element predictions of the strains that bias the nucleation events. The scaling of the dot spacing with membrane thickness is consistent with the spacing of nucleation sites predicted using the mechanical model.
New Journal of Physics 10/2010; 12(10):103011. · 4.18 Impact Factor
-
C. B. Simmons,
Teck Seng Koh,
Nakul Shaji,
Madhu Thalakulam,
L. J. Klein,
Hua Qin,
H. Luo, D. E. Savage,
M. G. Lagally,
A. J. Rimberg,
Robert Joynt,
Robert Blick,
Mark Friesen,
S. N. Coppersmith,
M. A. Eriksson
[show abstract]
[hide abstract]
ABSTRACT: 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.
Phys. Rev. B. 08/2010; 82(24).
-
[show abstract]
[hide abstract]
ABSTRACT: We report the fabrication and measurement of one-electron single and double
quantum dots with fast tunnel rates in a Si/SiGe heterostructure. Achieving
fast tunnel rates in few-electron dots can be challenging, in part due to the
large electron effective mass in Si. Using charge sensing, we identify
signatures of tunnel rates in and out of the dot that are fast or slow compared
to the measurement rate. Such signatures provide a means to calibrate the
absolute electron number and verify single electron occupation. Pulsed gate
voltage measurements are used to validate the approach.
03/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Silicon-on-insulator (SOI) presents a unique model system for exploring the stability of crystalline nanomaterials in metastable configurations. We show that the initial destabilization of ultrathin SOI is related to mechanical stress, in contrast to phenomena at later times driven by the energy of the SiO2/Si interface. Stepped rectangular truncated pyramids, with lateral dimensions of tens of nanometers, are formed on the outer Si layer of ultrathin (001)-oriented SOI during heating in ultrahigh vacuum. Pyramid edges are bounded by doubled atomic steps, with corners consisting of a complex series of single-layer steps. The shape of these nanopyramids represents a balance between stress-induced roughening and the elastic interaction between steps. SOI allows the precisely known energetics of silicon surfaces to be readily adapted to materials with nanoscale dimensions.
Physical Review B. 01/2010; 81(4).
-
J. L. Truitt,
K. A. Slinker,
K. L. M. Lewis, D. E. Savage,
Charles Tahan,
L. J. Klein,
J. O. Chu,
P. M. Mooney,
A. M. Tyryshkin,
D. W. van der Weide,
Robert Joynt,
S. N. Coppersmith,
Mark Friesen,
M. A. Eriksson
[show abstract]
[hide abstract]
ABSTRACT: Silicon quantum devices have progressed rapidly over the past decade, driven by recent interest in spintronics and quantum
computing. Spin coherence has emerged as a leading indicator of suitable devices for quantum applications. In particular,
the technique of electron-spin resonance (ESR) has proven powerful and flexible for probing both the magnitude and the nature
of spin scattering, when compared to theoretical predictions. Here, we provide a short review of silicon quantum devices,
focusing on silicon/silicon-germanium quantum wells. Our review touches on the fabrication and lithography of devices including
quantum dots, and the development of Schottky top gates, which have recently enabled the formation of few-electron quantum
dots with integrated charge sensors. We discuss recent proposals for quantum-dot quantum computing, as well as spin- and valley-scattering
effects, which may limit device performance. Recent ESR studies suggest that spin scattering in high-mobility Si/SiGe two-dimensional
electron gases may be dominated by the D’yakonov and Perel’ mechanism arising from Bychkov–Rashba spin-orbit coupling. These
results rely on theoretical predictions for the dependence of the coherence time T
2* on the orientation of an external applied magnetic field. Here, we perform ESR experiments on a series of samples fabricated
by different methods, including samples recently used to obtain few-electron quantum dots. While we observe some similarities
with recent experiments, we find that for five out of six samples, the angular dependence of T
2* was far larger than the theoretical predictions. We discuss possible causes for this discrepancy, but conclude that the theoretical
understanding of these samples is not yet complete.
08/2009: pages 101-127;
-
C B Simmons,
Madhu Thalakulam,
B M Rosemeyer,
B J Van Bael,
E K Sackmann, D E Savage,
M G Lagally,
R Joynt,
Mark Friesen,
S N Coppersmith,
M A Eriksson
[show abstract]
[hide abstract]
ABSTRACT: We report integrated charge sensing measurements on a Si/SiGe double quantum dot. The quantum dot is shown to be tunable from a single, large dot to a well-isolated double dot. Charge sensing measurements enable the extraction of the tunnel coupling t between the quantum dots as a function of the voltage on the top gates defining the device. Control of the voltage on a single such gate tunes the barrier separating the two dots. The measured tunnel coupling is an exponential function of the gate voltage. The ability to control t is an important step toward controlling spin qubits in silicon quantum dots.
Nano Letters 08/2009; 9(9):3234-8. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Freestanding, ultracompliant crystalline-sheet substrates provide a new opportunity to control the growth of strained epitaxial films. Three-dimensional SiGe islands grown on thin silicon nanomembranes self-order as the strain field induced by initial island growth guides nucleation of subsequent islands on the opposite surface. A mechanics analysis explains this unique growth mode, possible only on ultracompliant substrates. The ordering can be tailored by manipulating the thickness and elastic properties of the membrane.
Physical Review Letters 07/2009; 102(22):226103. · 7.37 Impact Factor
