Session H27: Focus Session: Semiconductor Qubits- Silicon Spin Qubits

Considerations for spin-based quantum computing in the solid-state

We give an update on recent work towards practical quantum computing using gated spins in semiconductors, especially in silicon.   

Fast initialization of a silicon spin qubit via an excited orbital state

We present data showing the initialization and measurement of individual electron spins in a silicon quantum dot. Spectroscopy of the electronic excited states of the dot reveals a relatively low-lying excited orbital state that is much more strongly coupled to the reservoir than the ground orbital state. As a function of an applied magnetic field, Zeeman splitting is observed for both the ground and the excited orbital states. By tuning a gate voltage, electron spins can be preferentially loaded into the quantum dot via any of these spin-split orbital states. Loading at either of the excited orbital states is measured to be over an order of magnitude faster than loading at directly into the orbital ground state. We use single-shot readout to measure the spin state of the loaded electrons. We observe two clear peaks in the fraction of spin-up electrons that are loaded, and these peaks correlate with loading through the spin-up ground or excited orbitals.   

Measurement of the electron spin relaxation time in a silicon quantum dot using single-shot readout

Electron spins in Si/SiGe quantum dots are promising candidates as qubits for quantum information processing, because spins in silicon couple weakly to the host material. We present a measurement of the spin lifetime for electrons in a silicon quantum dot. The spin state of individual electrons is measured using single-shot charge readout and spin-to-charge conversion: only spin-up electrons will tunnel off the quantum dot. Charge sensing is performed with an integrated quantum point contact that detects single electron tunnel events as steps in current. We determine the relaxation time by measuring the fraction of measurements that contain spin-up tunneling events as a function of the time that the electron spins are held on the quantum dot. We observe a clear decay in this spin-up fraction versus time, and an exponential fit yields $\mathrm{T_1}\sim 2.8~\mathrm{seconds}$ at a magnetic field of $1.85~\mathrm{T}$.   

Measurement of the Spin Relaxation Lifetime (T$_{1}$) in a One-Electron Strained-Si Accumulation-Mode Quantum Dot

We report measurements of the spin-relaxation lifetime (T$_{1}$) as a function of magnetic field in a strained-Si, accumulation-mode quantum dot. An integrated quantum-point contact (QPC) charge sensor was used to detect changes in dot occupancy as a function of bias applied to a single gate electrode. The addition spectra we obtained are consistent with theoretical predictions starting at N=0. The conductance of the charge sensor was measured by applying an AC voltage across the QPC and a 3 k$\Omega$ resistor. Lifetime measurements were conducted using a three-pulse technique consisting of a load, read, and flush sequence. T$_{1}$ was measured by observing the decay of the spin bump amplitude as a function of the load pulse length. We measured decay times ranging from approximately 75 msec at 2T to 12 msec at 3T, consistent with previous reports and theoretical predictions. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Undoped Si/SiGe Depletion-Mode Few-Electron Double Quantum Dots

We have successfully formed a double quantum dot in the sSi/SiGe material system without need for intentional dopants. In our design, a two-dimensional electron gas is formed in a strained silicon well by forward biasing a global gate. Lateral definition of quantum dots is established with reverse-biased gates with $\sim $40 nm critical dimensions. Low-temperature capacitance and Hall measurements confirm electrons are confined in the Si-well with mobilities $>$10$^{4}$ cm$^{2}$/V-s. Further characterization identifies practical gate bias limits for this design and will be compared to simulation. Several double dot devices have been brought into the few-electron Coulomb blockade regime as measured by through-dot transport. Honeycomb diagrams and nonlinear through-dot transport measurements are used to quantify dot capacitances and addition energies of several meV. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Transport, Charge Sensing, and Quantum Control in Si/SiGe Double Quantum Dots

Si/SiGe quantum dots hold great promise as ultra-coherent qubits [1]. In comparison with the GaAs system, Si has a weaker hyperfine interaction due to the zero nuclear spin of $^{28}$Si and smaller spin-orbit coupling due to its lighter atomic weight [2]. However, the fabrication of highly controllable Si/SiGe quantum dots is complicated by valley degeneracy, the larger effective electron mass, and the difficulty of obtaining high quality samples [3]. Here we develop a robust fabrication process for depletion mode Si/SiGe quantum dots, demonstrating high quality ohmic contacts and low-leakage Pd top gates. We report DC transport measurements as well as charge sensing in single and double quantum dots. The quantum dot gate electrode pattern allows a relatively high level of control over the confinement potential, tunneling rates, and electron occupation. \\[4pt] [1] C. B. Simmons \textit{et al.}, arXiv:1010.5828v1 (2010). \\[0pt] [2] R. Hanson \textit{et al.}, Rev. Mod. Phys. \textbf{79}, 1217 (2007). \\[0pt] [3] F. Sch\"affler, Semicond. Sci. Tech. \textbf{12}, 1515 (1997).   

Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry

We present transport measurements of a tunable silicon metal- oxide-semiconductor double quantum dot device with lateral geometry. Experimentally extracted gate-to-dot capacitances show that the device is largely symmetric under the gate voltages applied. Intriguingly, these gate voltages themselves are not symmetric. Comparison with numerical simulations indicates that the applied gate voltages serve to offset an intrinsic asymmetry in the physical device. We also show a transition from a large single dot to two well isolated coupled dots, where the central gate of the device is used to controllably tune the interdot coupling. \\ This work was supported by the LDRD program at Sandia National Laboratories, a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary Lockheed-Martin Company, for the U. S. DOE NNSA under Contract No. DE-AC04-94AL85000   

The effect of donors on lateral gated quantum-devices in Si/SiGe heterostructures

Much activity has focused on the development of quantum dots in Si/SiGe because of its potentially very long decoherence times (T2). However, to fabricate well-controlled quantum dots in Si/SiGe heterostructures, one must overcome complications that do not arise in GaAs/AlGaAs heterostructures. We demonstrate that switching charge noise and donor-layer conduction can lead to instability and cross-coupling among the tunnel barriers, thus making it difficult to achieve highly stable and tunable quantum devices in a Si/SiGe heterostructure. In particular, we have used an integrated charge-sensing quantum point contact to investigate the charge motion that originates from the excess donors, and present a systematic capacitance measurement to show how the donor layer affects device function in devices with large ($\sim $100 $\mu$m$^2$) gates as well as nanometer-size ones.   

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, in both forward and reverse bias regimes, and show that excellent fits to all data sets were obtained using one consistent set of parameters. We also obtain quantitative estimates for the tunneling rates and currents in the reverse bias regime using the Lindblad formalism. [Ref: arXiv:1008.5398v1]   

Fabrication of Few-Electron Carbon Nanotube Single and Double Quantum Dots

We discuss fabrication methods for carbon nanotube quantum dot devices designed to satisfy the requirements of spin qubit applications. These requirements include low disorder for reliable access to the few-electron regime, detection of charge states, and rapid manipulation with multiple gates. Nanotube growth occurs at or near the end of the fabrication process, a scheme that has been shown previously to produce clean devices for transport studies. In our devices the nanotubes are grown over pre-patterned gates or the nanotubes are located and gates are placed on top. A new atomic layer deposition process was developed to coat the nanotubes in a high-k dielectric for effective gating and suppression of electron interactions. We find in these devices that disorder on the length scale of the quantum dot is made small enough for routine occupancy with few charges, but disorder with sufficiently short range to couple valleys remains an uncontrolled parameter that is important for qubit applications of nanotubes. We acknowledge support from NSF-MWN, IBM, and Harvard University.   

Undoped Heterostructure Materials for SiGe Quantum Devices

Quantum well heterostructures, widely used for the fabrication of quantum dots and related devices, typically make use of modulation doping. Removal of the dopants, by use of globally ``field-gated'' and/or back-gated heterostructure designs, eliminates the dominant sources of scattering, charge noise and instability in devices intended for low-temperature operation. In this talk we present recent progress in designing and fabricating undoped quantum well heterostructures in sSi/SiGe. A combination of simulation based modeling and experimental work has enabled us to successfully engineer materials for stable and quiet quantum dot operation. Specific topics to be presented include the important role of substrate and buffer layer background doping, concurrent MOS accumulation, leakage to front and back gates via barrier tunneling, and the expected range of electric fields that determine valley mixing in quantum dots. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.   

Heterostructure surface effects on Si/SiGe 2DEGs

We present the results of Hall and Shubnikov-de Haas measurements of the two-dimensional electron gas (2DEG) in Si/SiGe heterostructures at 2 K. We demonstrate that the condition of the surface has significant effects on the carrier density and mobility of electrons in the quantum well. Results from multiple samples show that the carrier density and mobility decrease with the amount of time that the samples are exposed to air. Surface treatment via a forming gas anneal or by dipping the samples in HF restores the carrier density and mobility of the degraded samples, and storing the samples in vacuum slows the rate of degradation. We believe that the reduction in carrier density of the 2DEG is a result of interface traps that form in the surface native oxide. Forming gas anneal passivates the interface traps, and HF strips the oxide. Illuminating the degraded samples at 2 K also improves the carrier density and mobility, possibly by activating electrons out of trap states. Deposition of AL2O3 on the surface using ALD caused a severe reduction in carrier density, which we believe is the result of a high trap density.   

Density and Depth of Natural Quantum Dots in Silicon MOS Structures

Electron spins in MOS structures have shown promise as qubits for quantum information processing. Typically, characteristics such as mobility, mid-gap interface states and oxide fixed charge are considered figures of merit for the Si/SiO$_{2}$ interface, however, other properties may be important. Recently, we have shown that, by biasing the gate above threshold and then reducing V$_{G}$ to 0V, we freeze electrons into natural quantum dots, where 2D electrons are confined by interface disorder. The depth of these dots is determined by the temperature and can be extracted using a Schottky-Hall-Read model. Additionally, we measure the density of confined electron states from the magnitude of the ESR signal. These measurements offer us a means to characterize the interface disorder in these MOS structures. Experiments have been performed on devices from different labs. Preliminary results from industrial quality devices fabricated at Sandia National Laboratories indicate a shallower dot depth, though a similar mobility. The shallower confinement suggests a higher quality for single-electron quantum devices.   

Real time electron counting through wavelet edge detection

We have recently demonstrated single-shot measurements of individual electron spins in a Si/SiGe quantum dot. These experiments were analyzed using a wavelet-based technique that allows detection of charging events in real time. An alternative method, based on level thresholding, is not well suited for real time detection, due to drifting background currents in the charge sensor. In contrast, the wavelet technique relies on edge detection and is hence robust against drifting currents levels. In this talk, we describe our wavelet algorithm and its applications for charge sensing. We benchmark the performance of the algorithm under realistic signal noise conditions.   

Triangulating the source of tunneling resonances in a point contact with nanometer scale sensitivity

We observe resonant tunneling in split gate point contacts defined in a double gate enhancement mode Si-MOS device structure. We determine the capacitances from the resonant feature to each of the conducting gates and the source/drain two dimensional electron gas regions. In our device, these capacitances provide information about the resonance location in three dimensions. Semi-classical electrostatic simulations of capacitance, already used to map quantum dot size and position [Stalford et al., IEEE Nanotechnology], identify a combination of location and confinement potential size that satisfy our experimental observations. The sensitivity of simulation to position and size allow us to triangulate possible locations of the resonant level with nanometer resolution. We discuss our results and how they may apply to resonant tunneling through a single donor. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.