We previously established that in principle, it is possible to quantum compute using passive linear optics with photo-detectors (quant-ph/0006088). Here we describe techniques based on error detection and correction that greatly improve the resource and device reliability requirements needed for scalability. The resource requirements are analyzed for ideal linear optics quantum computation (LOQC). The coding methods can be integrated both with loss detection and phase error-correction to deal with the primary relaxation processes in non-ideal optics, including detector inefficiencies. The main conclusion of our work is that the resource requirements for implementing quantum communication or computation with LOQC are reasonable. Furthermore, this work clearly demonstrates how special knowledge of the error behavior can be exploited for greatly improving the fault tolerance and overheads of a physical quantum computer.
Tremendous efforts have been paid for realization of fault-tolerant quantum computation so far. However, preexisting fault-tolerant schemes assume that a lot of qubits live together in a single quantum system, which is incompatible with actual situations of experiment. Here we propose a novel architecture for practically scalable quantum computation, where quantum computation is distributed over small-size (four-qubit) local systems, which are connected by quantum channels. We show that the proposed architecture works even with the error probability 0.1% of local operations, which breaks through the consensus by one order of magnitude. Furthermore, the fidelity of quantum channels can be very low
∼ 0.7, which substantially relaxes the difficulty of scaling-up the architecture. All key elements and their accuracy required for the present architecture are within reach of current technology. The present architecture allows us to achieve efficient scaling of quantum computer, as has been achieved in today's classical computer.