We study spin dynamics for two electrons confined to a double quantum dot under the influence of an oscillating exchange interaction. This leads to driven Rabi oscillations between the |↑↓ -state and the |↓↑ -state of the two-electron system. The width of the Rabi resonance is proportional to the amplitude of the oscillating exchange. A measurement of the Rabi resonance allows one to narrow the distribution of nuclear spin states and thereby to prolong the spin decoherence time. Further, we study decoherence of the two-electron states due to the hyperfine interaction and give requirements on the parameters of the system in order to initialize in the |↑↓ -state and to perform a √ SWAP operation with unit fidelity.

We study, both theoretically and experimentally, driven Rabi oscillations of a single electron spin coupled to a nuclear-spin bath. Because of the long correlation time of the bath, two unusual features are observed in the oscillations. The decay follows a power law, and the oscillations are shifted in phase by a universal value of =4. These properties are well understood from a theoretical expression that we derive here in the static limit for the nuclear bath. This improved understanding of the coupled electronnuclear system is important for future experiments using the electron spin as a qubit. DOI: 10.1103/PhysRevLett.99.106803 PACS numbers: 73.21.La, 03.65.Yz, 71.70.Jp, 76.20.+q A quantum bit is engineered such that its coupling to the disturbing environment is minimized. Understanding and controlling this coupling is therefore a major subject in the field of quantum information processing. It is not solely the coupling strength but also the dynamics of the environment that governs the quantum coherence. In particular, the limit where these dynamics are slow compared to the evolution of the quantum system is interesting. The well-known Markovian Bloch equations that describe the dynamics of a driven system, including the exponential decay of the longitudinal and transverse magnetization [1], then lose their validity. Such deviations from the exponential behavior have been studied theoretically [2,3] and experimentally, for instance in superconducting qubit systems [4].An electron spin confined in the solid state is affected predominantly by phonons via the spin-orbit interaction [5][6][7][8][9], and by nuclear spins in the host material via the hyperfine interaction. At low temperature, coupling to the nuclear spins is the dominant decoherence source [10 -17]. Although this strong coupling leads to an apparent decoherence time T 2 of the order of 20 ns when time averaged over experimental runs, the decoherence time T 2 strongly depends on the dynamics in the nuclear-spin bath. This typical nuclear-spin dynamics is very slow, because the nuclear spins are only weakly coupled with each other and the bath itself is coupled very weakly to its dissipative environment (like phonons). This implies that here, the Markovian Bloch equations are not valid.Here we study the dynamics and decoherence of an electron spin in a quantum dot that is coherently driven via pulsed magnetic resonance, and is coupled to a nuclearspin bath with a long correlation time. We find experimentally that, remarkably, the electron spin oscillates coherently, even when the Rabi period is much longer than T 2 10-20 ns. In addition, the characteristics of the driven electron-spin dynamics are unusual. The decay of the Rabi oscillations is not exponential but follows a power law and a universal (parameter independent) phase shift emerges. We compare these experimental results with a theoretical expression, derived in the limit of a static nuclear-spin bath.We consider a double quantum dot with one electron in each dot and a static external ...

We analyze nuclear spin dynamics in quantum dots and defect centers with a bound electron under electron-mediated coupling between nuclear spins due to the hyperfine interaction ("J-coupling" in NMR). Our analysis shows that the Overhauser field generated by the nuclei at the position of the electron has short-time dynamics quadratic in time for an initial nuclear spin state without transverse coherence. The quadratic short-time behavior allows for an extension of the Overhauser field lifetime through a sequence of projective measurements (quantum Zeno effect). We analyze the requirements on the repetition rate of measurements and the measurement accuracy to achieve such an effect. Further, we calculate the long-time behavior of the Overhauser field for effective electron Zeeman splittings larger than the hyperfine coupling strength and find, both in a Dyson series expansion and a generalized master equation approach, that for a nuclear spin system with a sufficiently smooth polarization the electron-mediated interaction alone leads only to a partial decay of the Overhauser field by an amount on the order of the inverse number of nuclear spins interacting with the electron.Comment: 11 pages, 3 figure

Abstract. We review our investigation of the spin dynamics for two electrons confined to a double quantum dot under the influence of the hyperfine interaction between the electron spins and the surrounding nuclei. Further we propose a scheme to narrow the distribution of difference in polarization between the two dots in order to suppress hyperfine induced decoherence.

In this article we review our work on the dynamics and decoherence of electron and hole spins in single and double quantum dots. The first part, on electron spins, focuses on decoherence induced via the hyperfine interaction while the second part covers decoherence and relaxation of heavy-hole spins due to spin-orbit interaction as well as the manipulation of heavy-hole spin using electric dipole spin resonance.