Induced quantum dot probe for material characterization.

We propose a non-destructive means of characterizing a semiconductor wafer via measuring parameters of an induced quantum dot on the material system of interest with a separate probe chip that can also house the measurement circuitry. We show that a single wire can create the dot, determine if an electron is present, and be used to measure critical device parameters. Adding more wires enables more complicated (potentially multi-dot) systems and measurements. As one application for this concept we consider silicon metal-oxide-semiconductor (MOS) and silicon/silicon-germanium quantum dot qubits relevant to quantum computing and show how to measure low-lying excited states (so-called "valley" states). This approach provides an alternative method for characterization of parameters that are critical for various semiconductor-based quantum dot devices without fabricating such devices.

Electron spin resonance and spin-valley physics in a silicon double quantum dot.

Silicon quantum dots are a leading approach for solid-state quantum bits. However, developing this technology is complicated by the multi-valley nature of silicon. Here we observe transport of individual electrons in a silicon CMOS-based double quantum dot under electron spin resonance. An anticrossing of the driven dot energy levels is observed when the Zeeman and valley splittings coincide. A detected anticrossing splitting of 60 MHz is interpreted as a direct measure of spin and valley mixing, facilitated by spin-orbit interaction in the presence of non-ideal interfaces. A lower bound of spin dephasing time of 63 ns is extracted. We also describe a possible experimental evidence of an unconventional spin-valley blockade, despite the assumption of non-ideal interfaces. This understanding of silicon spin-valley physics should enable better control and read-out techniques for the spin qubits in an all CMOS silicon approach.

Qubit purification speed-up for three complementary continuous measurements.

We consider qubit purification under simultaneous continuous measurement of the three non-commuting qubit operators σ(x), σ(y), σ(z). The purification dynamics is quantified by (i) the average purification rate and (ii) the mean time of reaching a given level of purity, 1-ε. Under ideal measurements (detector efficiency η=1), we show in the first case an asymptotic mean purification speed-up of 4 when compared with a standard (classical) single-detector measurement. However, by the second measure-the mean time of first passage T(ε) of the purity-the corresponding speed-up is only 2. We explain these speed-ups using the isotropy of the qubit evolution that provides an equivalence between the original measurement directions and three simultaneous measurements, one with an axis aligned along the Bloch vector and the other with axes in the two complementary directions. For inefficient detectors, η=1 - δ < 1, the mean time of first passage T(δ,ε)increases because qubit purification competes with an isotropic qubit dephasing. In the asymptotic high-purity limit (ε,δ≪1), we show that the increase possesses a scaling behaviour: ΔT(δ,ε) is a function only of the ratio δ/ε. The increase ΔT(δ,ε) is linear for small arguments, but becomes exponential ~exp(δ,2ε) for δ/ε large.

Sound-based analogue of cavity quantum electrodynamics in silicon.

A quantum mechanical superposition of a long-lived, localized phonon and a matter excitation is described. We identify a realization in strained silicon: a low-lying donor transition (P or Li) driven solely by acoustic phonons at wavelengths where high-Q phonon cavities can be built. This phonon-matter resonance is shown to enter the strongly coupled regime where the "vacuum" Rabi frequency exceeds the spontaneous phonon emission into noncavity modes, phonon leakage from the cavity, and phonon anharmonicity and scattering. We introduce a micropillar distributed Bragg reflector Si/Ge cavity, where Q≃10(5)-10(6) and mode volumes V≲25λ(3) are reachable. These results indicate that single or many-body devices based on these systems are experimentally realizable.

Qubit state monitoring by measurement of three complementary observables.

We consider the evolution of a qubit (spin 1/2) under the simultaneous continuous measurement of three noncommuting qubit operators σ(x), σ(y), and σ(z). For identical ideal detectors, the qubit state evolves by approaching a pure state with a random direction in the Bloch vector space and by undergoing locally isotropic diffusion in the perpendicular directions. The quantum state conditioned on the complete detector record is used to assess the fidelity of classically inspired estimates based on running time averages and discrete time bin detector outputs.

Signatures of quantum behavior in single-qubit weak measurements.

With the recent surge of interest in quantum computation, it has become very important to develop clear experimental tests for "quantum behavior" in a system. This issue has been addressed in the past in the form of the inequalities due to Bell and those due to Leggett and Garg. These inequalities concern the results of ideal projective measurements, however, which are experimentally difficult to perform in many proposed qubit designs, especially in many solid-state qubit systems. Here, we show that weak continuous measurements, which are often practical to implement experimentally, can yield particularly clear signatures of quantum coherence, both in the measured correlation functions and in the measured power spectrum.

Mesoscopic quadratic quantum measurements.

We develop a theory of quadratic quantum measurements by a mesoscopic detector. It is shown that the quadratic measurements should have nontrivial quantum information properties, providing, for instance, a simple way of entangling two noninteracting qubits. We also calculate the output spectrum of a detector with both linear and quadratic response, continuously monitoring two qubits.