Abstract
Spins have been extensively explored as carriers of qubits for quantum information processing (QIP). Quantum entanglement between spin qubits is a crucial resource and its efficient generation has been a central problem in QIP. On the other hand, uncontrollable spin degrees of freedom in the environment of spin qubits can be the major causes of errors. An outstanding problem for scalable QIP is to suppress the collective noises from such spin baths. In this talk, we address the problems of suppressing collective fluctuations and generating entanglement in spin ensembles.
In the first part of the talk, we introduce approaches for suppressing the collective fluctuations in the nuclear spin bath of electron spin qubits in semiconductor quantum dots. In a coupled double dot system, we show the nuclear-electron flip-flop process at a singlet-triplet resonance can lead to a feedback mechanism, which can be used to suppress the nuclear field fluctuations. We also show that strong pumping of the nuclear spins in dynamic nuclear polarization processes can saturate the nuclear spin bath towards the collective ``dark states", where the transverse nuclear field fluctuation can be substantially suppressed compared to the value at thermal equilibrium.
In the second part of the talk, we present schemes for generating large scale quantum entanglement in spin qubit systems. For atomic spin qubits in optical lattices, we show how to prepare pure spin coherent state with low collective spin value by incoherent pumping with collective raising and lowering operations. For donor nuclear spin qubits in silicon, we show how to deterministically prepare symmetric and asymmetric Dicke states which constitute an important class of multipartite entangled states.