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We propose a novel architecture for scalable quantum computation based on quantum actuated decoherence-free (DF) qubits. Each qubit is encoded by the DF subspace of a nuclear spin pair and has long coherence time. A nitrogen-vacancy center in diamond is chosen as the quantum actuator to realize initialization, readout and universal control of DF qubits with fidelities higher than 99%. It reduces the challenge of classical interfaces from controlling and observing complex quantum systems down to a simple quantum actuator. Our scheme also provides a novel way to handle complex quantum systems. [12]. In these quantum computation proposals, qubits are usually classically actuated, where classical interfaces work on qubits directly. There are several challenges for quantum computation driven by classical interfaces (upper panel of Fig. 5(a)). Firstly, if qubits are coupled to the classical interface more strongly for faster quantum gates, qubits may suffer more serious dephasing effect due to their stronger couplings to the general environment. Secondly, for large-scale quantum computation, with the increase of qubits, the Hilbert space will grown exponentially, which dramatically increases the difficulty to control such a complex quantum system by classical interfaces. Due to the imperfections and limitations of classical interfaces, to satisfy all the Divincenzo criteria is still very challenging, especially for large-scale quantum systems.Here, we propose a novel solid-state architecture for a scalable quantum computation based on DF qubits and a quantum actuator. Pairs of nuclear spins are utilized to construct DF qubits which are immune to collective noise and have long coherence time. The quantum actuator as described by S. Lloyd [13], is a medium that connects the quantum system of interest to the classical interfaces (see lower panel of Fig. 5(a)). All the processing of qubits, such as initializing, detecting and controlling are accomplished by the coupled quantum actuator. This reduces the difficulty of classical interfaces from manipulating an exponential Hilbert space down to a simple quantum system. In addition, the quantum actuator avoids the problems induced by classical interfaces, such as the voltage fluctuations of Kane's A-gate, which is one of the decoherence sources [8]. Obviously, it is important to find a suitable physical candidate for the quantum actuator, which requires to be well handled by classical interfaces and has appropriate couplings with qubits. In our proposal, a nitrogen-vacancy (NV) center in diamond is used as the quantum actuator, because it has long coherence time, even approaching one second [14], and can be individually polarized, manipulated and detected with high fidelity [15,16]. The effect of imperfection from classical control can be well suppressed by the state-of-art technology. Some other candidates for the quantum actuator, such as SiC defects [17], may also be feasible in our proposal. We show that initializing, detecting and universal controlling the DF qubits can...
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