Photons are unsurpassed as qubits in terms of their decoherence times, mobility, and the achievability of high-fidelity single-qubit operations. Entanglement experiments thus far in optics have been very clean, and an optical quantum processor would obviously have an advantage in connecting to a quantum "network" (no need to convert between stationary and flying qubits). We are exploring the limits of the original KLM linear optics quantum computing proposal, as well as the more recent cluster- and graph-state approaches, in addition to various nonlinear optical approaches. Several challenges must be overcome: the logic devices are often probabilistic, large numbers of ancilla photons must be generated in entangled states, and high-efficiency detectors are required. Other research programs are developing single-photon sources and detectors. The remaining issue is the practical scalability of quantum circuits - the ability to perform quantum logic gates with error rates below the fault-tolerant threshold and incorporate them into large-scale quantum circuits with realistic physical resources.
We are
investigating two main approaches: The first uses novel linear-optics
methods, such as "cluster state" techniques, to perform quantum
calculations: once the cluster of linked photon states has been created,
an arbitrary quantum computation can be performed deterministically,
thereby drastically reducing the resources required to perform
deterministic two-qubit operations over earlier linear optics schemes.
Our second approach incorporates weak nonlinearities to enable
deterministic capabilities. For instance, by employing the "quantum Zeno
effect", one can suppress the failure events that occur in linear
optics, and efficiently realize arbitrary quantum logic; alternatively,
quantum non-demolition measurements enable deterministic quantum logic
gates. Closely coordinated with experiment is a strong theoretical
effort, seeking to further minimize the required resources and optimize
the robustness of the logic circuits, by considering error correction,
decoherence, scaling limitations, and nonlinear schemes.
Quantum computation is possible if the error per operation is below a certain fault-tolerant threshold. We will determine:
(i) the relevant errors specific to optical quantum computing for determining a fault-tolerant threshold (decoherence and noisy gate operations).
(ii) if operations can be implemented with linear optics with errors below this threshold. The ability to reach such a threshold means that the architecture is scalable in principle.
However, scalable and practical are not the same thing. What is the resource overhead required to achieve scalability? How can the building blocks be combined into a large-scale circuit to perform useful quantum computations? We will realize high-fidelity fault-tolerant gates and demonstrate practical schemes for circuit scale-up by developing photonics technologies, including adaptive, micro-, and integrated-optics. These compact technologies leverage off the highly advanced photonics industry, offering the most promising route to large-scale Optical Quantum Computer.