Andrew T. Hunter

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.

Carbon in semi‐insulating, liquid encapsulated Czochralski GaAs

We have investigated undoped, semi‐insulating GaAs, grown by the liquid encapsulated Czochralski technique in pyrolytic BN crucibles. The concentration of carbon, the principal shallow acceptor, depends on the water content of the B2O3 encapsulant. Dry B2O3 (100–150 ppm H2O) results in carbon concentrations as high as 8×1015 cm−3, while wet B2O3 (500 ppm H2O) results in carbon concentrations below the detection limit of 3–5×1014 cm−3. In samples with carbon below 5×1014 cm−3, the compensation (acceptors minus donors shallower than EL2) is also low, and the room‐temperature carrier concentration can exceed 1011 cm−3. For samples with carbon above 5×1014 cm−3, the room‐temperature carrier concentration is below 5×107 cm−3 for all samples measured. The compensation is less than the carbon concentration in high carbon samples, indicating that donors shallower than EL2 are significant in determining the value of the compensation in these samples.

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.

Substrate effects on the threshold voltage of GaAs field‐effect transistors

An array of field‐effect transistors fabricated by direct ion implantation on a liquid‐encapsulated‐Czochralski (LEC) In0.003Ga0.997As wafer exhibits greater uniformity of threshold voltage than arrays on similarly processed conventional LEC GaAs wafers. However, there is no correlation between a transistor’s threshold voltage and its proximity to a dislocation for either In‐alloyed or conventional LEC GaAs substrates. An observed correlation of threshold voltage with local dislocation density for GaAs substrates can lead to the erroneous inference that threshold voltage is affected by dislocation proximity.

Reduced Sensitivity to Charge Noise in Semiconductor Spin Qubits via Symmetric Operation.

We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations.

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.

Characterization study of a HgTe‐CdTe superlattice by means of transmission electron microscopy and infrared photoluminescence

We report the first transmission electron microscopy (TEM) study of a HgTe‐CdTe superlattice. The superlattice consists of 250 layer pairs of HgTe‐CdTe on a (100) CdTe substrate and was grown at 175 °C by molecular beam epitaxy. Vertical cross‐section TEM images show a highly regular structure of the superlattice from the CdTe substrate to the free surface, indicating that interdiffusion effects at interfaces are minimal. Diffraction patterns taken from the first 30 pairs of layers of the superlattice from the CdTe buffer layer show a series of satellite spots up to the sixth order. This implies that the interfacial sharpness of this HgTe‐CdTe superlattice is comparable to those interfaces of high quality III‐V semiconductor superlattices. The HgTe‐CdTe superlattice exhibits an infrared photoluminescence peak at 357 meV, in reasonable agreement with theoretical predictions of its band gap.

Effects of interface stoichiometry on the structural and electronic properties of Ga1−xInxSb/InAs superlattices

We report an investigation of the effects of interface layer composition on the structural and electronic properties of Ga1−xInxSb/InAs superlattices, which are of interest for infrared detector applications. Shutter sequencing during growth of a series of Ga0.75In0.25Sb (8 ML)/InAs (13 ML) superlattices by molecular‐beam epitaxy has been employed to select structures with GaInAs‐like interfaces, InSb‐like interfaces, and intermediate choices of interfacial composition. Comparison of x‐ray diffraction scans from the superlattices confirms that a superlattice with GaInAs‐like interfaces has a 1.6% smaller average interatomic spacing in the growth direction than a sample with InSb‐like interfaces, in near agreement with expected interfacial bond length differences. Hall measurements of intrinsic carrier concentrations at high temperatures indicate an increase in superlattice energy gap when interfaces are switched from InSb‐like to GaInAs‐like, consistent with intuitive expectations. Low temperature Hall me...

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.

Recent advances in Ga1−xInxSb/InAs superlattice IR detector materials

Development of Ga 1-x In X Sb/InAs superlattices has been motivated by potential applications to infrared (IR) detectors in the 8-12 μm band and the far IR region 12-20 + μm. Theoretical and experimental results to date support the prospect that very high performance III-V materials-based IR focal plane array (FPA) sensors that can operate at temperatures higher than HgCdTe photovoltaic devices are feasible. The heterostructures under development are grown by molecular beam epitaxy (MBE) and are type-II, broken gap superlattices of alternating layers of Ga 1-x In x Sb/InAs with compositions chosen to allow the superlattice to be symmetrically strained to GaSb substrates. The details of the interface structure at the GaSb surface, the character of the interfaces between the Ga 1-x In X Sb and InAs layers, and the chemical nature of the Group V flux used in the MBE growth process are among the critical factors in enabling the growth of superlattices of extremely high optical quality suitable for IR detectors. We review recent advances in the materials and epitaxial processes for growing these superlattices. These processes have been applied to producing IR detector structures tailored to the particularly difficult spectral range of very long wavelength cut-off of 12 + μm.