Kevin S. Holabird

Coherent singlet-triplet oscillations in a silicon-based double quantum dot

Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.

Isotopically enhanced triple-quantum-dot qubit

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time. Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.

Pauli spin blockade in undoped Si/SiGe two-electron double quantum dots

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.

Measurement of valley splitting in high-symmetry Si/SiGe quantum dots

We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures.

Room-temperature electric-field controlled spin dynamics in (110)InAs quantum wells

We report the demonstration of room temperature gate control over the electron spin dynamics using the Rashba effect in a (110) InAs∕AlSb two-dimensional electron gas. Our calculations predict that the strong spin–orbit interaction in this system produces pseudomagnetic fields exceeding 1 T when only 140 mV is applied across a single quantum well. Using this large pseudomagnetic field, we demonstrate low-power spin manipulation on a picosecond time scale. Our findings are promising for the prospect of nonmagnetic low-power, high-speed spintronics.

Sb-Heterostructure Low Noise W-Band Detector Diode Sensitivity Measurements

Sb-heterostructure diodes have become the detector of choice for W-band millimeter wave imaging cameras. Here we demonstrate lower impedance versions that optimize noise equivalent power (NEP). The goal is to decrease the gain required of the RF pre-amplifier, ideally to zero. Measured W-band sensitivities for three diodes are 3500, 5500, and 6000 V/W. Their zero bias differential resistance values imply Johnson noise limited NEPs of 0.98, 0.83, and 0.79 pW/Hzfrac12, respectively, much less than obtained from conventional Schottky diodes. A wideband transition from a horn antenna to the 6000 V/W detector has produced an integrated bandwidth of 30 GHz with implied temperature sensitivity (NEDeltaT) close to 10degK

Undoped accumulation-mode Si/SiGe quantum dots.

We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function-control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the [Formula: see text] charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.

New tunnel diode for zero-bias direct detection for millimeter-wave imagers

High-resolution passive millimeter wave imaging cameras require per pixel detector circuitry that is simple, has high sensitivity, low noise, and low power. Detector diodes that do not require bias or local oscillator input, and have high cutoff frequencies are strongly preferred. In addition, they must be manufacturable in large quantities with reasonable uniformity and reproducibility. Such diodes have not been obtainable for W-band and above. We are developing zero-bias square-law detector diodes based on InAs/Alsb/GaAlSb heterostructures which for the first time offer a cost-effective solution for large array formats. The diodes have a high frequency response and are relatively insensitive to growth and process variables. The large zero- bias non-linearity in current floor necessary for detection arises from interband tunneling between the InAs and the GaAlSb layers. Video resistance can be controlled by varying an Alsb tunnel barrier layer thickness. Our analysis shows that capacitance can be further decreased and sensitivity increased by shrinking the diode area, as the diode can have very high current density. DC and RF characterization of these devices and an estimate of their ultimate frequency performance in comparison with commercially available diodes are presented.

Sb-heterostructure diode detector W-band NEP and NEDT optimization

Sb-heterostructure diodes have become the detector of choice for W-band millimeter wave imaging cameras now being commercialized or in prototype development. Here we optimize the diode impedance to yield a minimum noise equivalent power (NEP). The goal is to decrease the gain required of the front-end LNA. Measured W-band sensitivities for two diodes are 3500 and 5500V/W. Their zero bias differential resistance values imply Johnson noise limited NEPs of 0.98 and 0.83pW/Hz1/2, respectively, much less than obtained from conventional biased Schottky diodes. A MMIC version of the diode detector has been simulated with an integrated bandwidth of ~ 30 GHz at W-band. The simulated temperature sensitivity (NEΔT) with an HRL W-band LNA on the front end is <1°K.

High frequency performance of Sb-heterostructure millimeter-wave diodes

We have developed a new zero bias millimeter wave diode based on quantum tunneling in an InAs/AlSb/GaSb nanostructure. It is ideal for square law radiometry and passive millimeter wave imaging. Excellent sensitivity has been demonstrated at present up to 110 GHz, with higher bandwidth predicted for smaller area diodes.