Person:

Rashba, Emmanuel

Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements and non-orthogonal control axes.

The central-spin problem is a widely studied model of quantum decoherence. Dynamic nuclear polarization occurs in central-spin systems when electronic angular momentum is transferred to nuclear spins and is exploited in quantum information processing for coherent spin manipulation. However, the mechanisms limiting this process remain only partially understood. Here we show that spin–orbit coupling can quench dynamic nuclear polarization in a GaAs quantum dot, because spin conservation is violated in the electron–nuclear system, despite weak spin–orbit coupling in GaAs. Using Landau–Zener sweeps to measure static and dynamic properties of the electron spin–flip probability, we observe that the size of the spin–orbit and hyperfine interactions depends on the magnitude and direction of applied magnetic field. We find that dynamic nuclear polarization is quenched when the spin–orbit contribution exceeds the hyperfine, in agreement with a theoretical model. Our results shed light on the surprisingly strong effect of spin–orbit coupling in central-spin systems.

A theory of a two-dimensional electron-hole system in a strong magnetic field is developed. Three different models are considered, whose common distinguishing feature is electron-hole asymmetry. It is shown that this asymmetry influences radically the system properties. It leads to an exciton interaction that does not vanish when the momentum of the exciton relative motion tends to zero. As a result, the scattering amplitude diverges at small momenta, and magnetic biexcitons appear at arbitrarily weak asymmetry. The thermodynamics of such a system in the Hartree-Fock approximation is similar in many respects to the thermodynamics of a monopolar system. The high-temperature phase is the homogeneous one, and the low-temperature phase has the charge density waves. The homogeneous phase with exciton pairing is absolutely unstable.

A theory of the self-trapping barrier in crystal with degenerate bands (spin S = 1) is developed for holes and Frenkel excitons. It is shown that band degeneracy leads to spontaneous symmetry breaking of the barrier, which acquires a prolate or oblate shape. The dependence of the barrier height on a number of parameters, such as the ratio of the effective masses, the deformation potentials, and others is investigated. The barrier-height scale is determined in all cases by the mass of the heavy hole (exciton). The mechanism of formation of quasimolecular self-trapped holes and excitons is discussed. Similar results were obtained for the strong-coupling polaron, for which lowering of the symmetry leads to the appearance of two rotational degrees of freedom. The results are applied to a number of other problems: fluctuation levels in semiconductors, the Urbach rule, and others. It is shown also that in systems with multicomponent order parameters the critical nuclei may be nonspherical (e.g., cigar- or disk-shaped).

A theory of the free induction signal from biexcitons and bound excitons is presented. The simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative or dephasing processes. Radiative decay is anomalously fast and can result in strong picosecond pulses. The expanding area of a coherent exciton polarization (inflating antenna), produced by the exciting pulse, is the underlying physical mechanism. The developed formalism can be applied to different biexciton transients.

A recently discovered mechanism of electric dipole spin resonance, mediated by the hyperfine interaction, is investigated experimentally and theoretically. The effect is studied using a spin-selective transition in a GaAs double quantum dot. The resonant frequency is sensitive to the instantaneous hyperfine effective field, revealing a nuclear polarization created by driving the resonance. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. An unexplained additional signal at half the resonant frequency is presented.

We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron (^{13}C) nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time (T_1) first decreases with a parallel magnetic field and then goes through a minimum in a field of (1.4 T). We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which at high field enhances relaxation due to bending-mode phonons. The inhomogeneous dephasing time (T_2^*) is consistent with previous data on hyperfine coupling strength in (^{13}C) nanotubes.