Bell-state tomography in a silicon many-electron artificial molecule.

An error-corrected quantum processor will require millions of qubits, accentuating the advantage of nanoscale devices with small footprints, such as silicon quantum dots. However, as for every device with nanoscale dimensions, disorder at the atomic level is detrimental to quantum dot uniformity. Here we investigate two spin qubits confined in a silicon double quantum dot artificial molecule.

Spin-orbit Interactions for Singlet-Triplet Qubits in Silicon.

Spin-orbit coupling is relatively weak for electrons in bulk silicon, but enhanced interactions are reported in nanostructures such as the quantum dots used for spin qubits. These interactions have been attributed to various dissimilar interface effects, including disorder or broken crystal symmetries. In this Letter, we use a double-quantum-dot qubit to probe these interactions by comparing the spins of separated singlet-triplet electron pairs.

Probing a Spin Transfer Controlled Magnetic Nanowire with a Single Nitrogen-Vacancy Spin in Bulk Diamond.

The point-like nature and exquisite magnetic field sensitivity of the nitrogen vacancy (NV) center in diamond can provide information about the inner workings of magnetic nanocircuits in complement with traditional transport techniques. Here, we use a single NV in bulk diamond to probe the stray field of a ferromagnetic nanowire controlled by spin transfer (ST) torques. We first report an unambiguous measurement of ST tuned, parametrically driven, large-amplitude magnetic oscillations.

Coherent coupling between a quantum dot and a donor in silicon.

Individual donors in silicon chips are used as quantum bits with extremely low error rates. However, physical realizations have been limited to one donor because their atomic size causes fabrication challenges. Quantum dot qubits, in contrast, are highly adjustable using electrical gate voltages.

Telegraphic noise in transport through colloidal quantum dots.

We report measurements of electrical transport through single CdSe/CdS core/shell colloidal quantum dots (cQDs) connected to source and drain contacts. We observe telegraphic switching noise showing few plateaus at room temperature. We model and interpret these results as charge trapping of individual trap states, and therefore we resolve individual charge defects in these high-quality low-strain cQDs.

Nanofabrication of gate-defined GaAs/AlGaAs lateral quantum dots.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space.

Single-spin readout in a double quantum dot including a micromagnet.

We use photon-assisted tunneling (PAT) and an inhomogeneous Zeeman field to demonstrate spin-selective PAT readout with a double quantum dot. The inhomogeneous Zeeman field is generated by a proximal micromagnet, which provides different stray fields between the two dots, resulting in an energy difference between the interdot PAT of the up-spin state and that of the down-spin state. We apply various external magnetic fields to modify the relative filling weight between the up-spin and down-spin states and detect it by using a charge detection technique to probe the PAT induced charge delocalization in the double dot.

Insitu reduction of charge noise in GaAs/AlxGa1-xAs Schottky-gated devices.

We show that an insulated electrostatic gate can be used to strongly suppress ubiquitous background charge noise in Schottky-gated GaAs/AlGaAs devices. Via a 2D self-consistent simulation of the conduction band profile we show that this observation can be explained by reduced leakage of electrons from the Schottky gates into the semiconductor through the Schottky barrier, consistent with the effect of "bias cooling." Upon noise reduction, the noise power spectrum generally changes from Lorentzian to 1/f type.

Microwave band on-chip coil technique for single electron spin resonance in a quantum dot.

Microwave band on-chip microcoils are developed for the application to single electron spin resonance measurement with a single quantum dot. Basic properties such as characteristic impedance and electromagnetic field distribution are examined for various coil designs by means of experiment and simulation. The combined setup operates relevantly in the experiment at dilution temperature.

Pairing of spin excitations in lateral quantum dots.

We demonstrate the existence of correlated electronic states as paired spin excitations of lateral quantum dots in the integer quantum Hall regime. Starting from the spin-singlet filling-factor nu=2 droplet, by increasing the magnetic field we force the electrons to flip spins and increase the spin polarization. We identify the second spin-flip process as one accompanied by correlated, spin depolarized phases, interpreted as pairs of spin excitons.