Session Z1: Solid-State Spin Qubits: Coherence Control and Protection

Control of single-spin decoherence by dynamical decoupling and spin bath manipulation

Controlling the interaction of a single quantum system with its environment is a fundamental challenge in quantum science and technology. We dramatically suppress the coupling of a single spin in diamond with the surrounding spin bath by using high-fidelity double-axis dynamical decoupling [1]. The coherence is preserved for arbitrary quantum states, as verified by quantum process tomography. The resulting coherence time enhancement is found to follow a general scaling with the number of decoupling pulses. No limit is observed for the decoupling action up to 136 pulses, for which the coherence time is enhanced more than 25 times compared to spin echo. Furthermore, we have exploited multi-pulse sequences to enhance the sensitivity of single-spin magnetometry and to measure properties of the decoupling sequences themselves [2]. In this talk, I will present an overview of this work combined with our latest results on coherent manipulation of the spin bath environment. \\[4pt] [1] Universal dynamical decoupling of a single solid-state spin from a spin bath, G. de Lange, Z.H. Wang, D. Rist\`{e}, V.V. Dobrovitski, and R. Hanson, Science 330, 60 (2010). \\[0pt] [2] Single-spin magnetometry with multi-pulse sequences, G. de Lange, D. Rist\`{e}, V. V. Dobrovitski, R. Hanson, arXiv:1008.4395 (2010).   

Control of electron spin decoherence in nuclear spin baths

Nuclear spin baths are a main mechanism of decoherence of spin qubits in solid-state systems, such as quantum dots and nitrogen-vacancy (NV) centers of diamond. The decoherence results from entanglement between the electron and nuclear spins, established by quantum evolution of the bath conditioned on the electron spin state. When the electron spin is flipped, the conditional bath evolution is manipulated. Such manipulation of bath through control of the electron spin not only leads to preservation of the center spin coherence but also demonstrates quantum nature of the bath. In an NV center system, the electron spin effectively interacts with hundreds of $^{13}$C nuclear spins. Under repeated flip control (dynamical decoupling), the electron spin coherence can be preserved for a long time ($>$1~ms). Therefore some characteristic oscillations, due to coupling to a bonded $^{13}$C nuclear spin pair (a dimer), are imprinted on the electron spin coherence profile, which are very sensitive to the position and orientation of the dimer. With such finger-print oscillations, a dimer can be uniquely identified. Thus, we propose magnetometry with single-nucleus sensitivity and atomic resolution, using NV center spin coherence to identify single molecules. Through the center spin coherence, we could also explore the many-body physics in an interacting spin bath. The information of elementary excitations and many-body correlations can be extracted from the center spin coherence under many-pulse dynamical decoupling control. Another application of the preserved spin coherence is identifying quantumness of a spin bath through the back-action of the electron spin to the bath. We show that the multiple transition of an NV center in a nuclear spin bath can have longer coherence time than the single transition does, when the classical noises due to inhomogeneous broadening is removed by spin echo. This counter-intuitive result unambiguously demonstrates the quantumness of the nuclear spin bath.   

Single spin qubits in self-assembled quantum dots

The search for a highly coherent electronic spin in the solid state has led most spectacularly to the NV colour centre in diamond. Have self-assembled quantum dots, InGaAs in GaAs, been left behind? The advantages of self-assembled quantum dots are considerable - there is a strong optical transition, advanced heterostructure technology and post-growth processing techniques - but so far the spin coherence has been at best modest. This talk will present some possible ways to out fox the decoherence processes in a semiconductor with the goal of creating a highly coherent spin.   

Preserving electron spin coherence in solids by optimal dynamical decoupling

To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero, is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities, such as quantum gates. Errors are inevitably introduced in each spin flip, so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision. Such optimal dynamical decoupling sequences have recently been explored. The experimental realization of optimal dynamical decoupling in solid-state systems, however, remains elusive. Here we use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50K to room temperature [1]. Using a seven-pulse optimal dynamical decoupling sequence, we prolonged the spin coherence time to about 30 ms; it would otherwise be about 0.04 ms without control or 6.2 ms under one-pulse control. By comparing experiments with microscopic theories, we have identified the relevant electron spin decoherence mechanisms in the solid. Recently, we demonstrate experimentally that dynamical decoupling can preserve bipartite pseudo-entanglement in phosphorous donors in a silicon system [2]. In particular, the lifetime of pseudo entangled states is extended from 0.4 us in the absence of decoherence control to 30 us in the presence of a two-flip dynamical decoupling sequence. \\[4pt] [1]. Jiangfeng Du, Xing Rong, Nan Zhao, Ya Wang, Jiahui Yang~and R. B. Liu, Preserving electron spin coherence in solids by optimal dynamical decoupling, Nature~461, 1265-1268 (2009). \\[0pt] [2] Ya Wang, Xing Rong, Pengbo Feng, Wanjie Xu, Bo Chong, Ji-Hu Su, Jiangbin Gong, and Jiangfeng Du, Preservation of bipartite pseudo-entanglement in solids using dynamical decoupling, submitted to Phys. Rev. Lett.   

Theory of qubit dephasing in a large nuclear spin bath

This abstract not available.