Single photons, manipulated using integrated linear optics, constitute a promising platform for universal quantum computation. A series of increasingly efficient proposals have shown linear-optical quantum computing to be formally scalable. However, existing schemes typically require extensive adaptive switching, which is experimentally challenging and noisy, thousands of photon sources per renormalized qubit, and/or large quantum memories for repeat-until-success strategies. Our work overcomes all these problems. We present a scheme to construct a cluster state universal for quantum computation, which uses no adaptive switching, no large memories, and which is at least an order of magnitude more resource-efficient than previous passive schemes. Unlike previous proposals, it is constructed entirely from loss-detecting gates and offers a robustness to photon loss. Even without the use of an active loss-tolerant encoding, our scheme naturally tolerates a total loss rate ∼ 1.6% in the photons detected in the gates. This scheme uses only 3-GHZ states as a resource, together with a passive linear-optical network. We fully describe and model the iterative process of cluster generation, including photon loss and gate failure. This demonstrates that building a linear optical quantum computer need be less challenging than previously thought.In 2001, Knill, Laflamme and Milburn [1] showed that scalable quantum computation was possible using only linear optical elements -without the need for deterministic two-photon interactions. However, their proposal was more a proof of principle than a feasible construction as the scheme required tens of thousands of optical elements to acquire gates with a high probability of success. Since then, several proposals have developed the idea of a linear optical quantum computer (LOQC), including Nielsen's proposal [2] of combining linear optics with cluster states, Browne and Rudolph's fusion mechanisms [5] to efficiently create optical cluster states and Kieling's et al proposal [4] of building an imperfect cluster that can be renormalized using ideas of percolation theory. While alternative schemes for LOQC [5] using parity state encoding [6] or small amplitude coherent states [7] have been proposed, we do not address these approaches in this manuscript.Recent demonstrations [8][9][10][11][12] have made significant progress towards experimental linear-optical quantum computing. In particular, the use of integrated photonics to implement large-scale, complex interferometers on a chip shows great promise. However, active feed-forward remains challenging, it requires fast switching which is a dominant source of photon loss and has not yet been experimentally demonstrated in an integrated device.Of previous approaches to linear optical quantum computing, only Kieling et al's proposal [4] is ballistic -meaning that active switching is not required for the process of cluster state generation. It is thus the most suitable previous approach to LOQC in an integrated setting. It has a number o...

We introduce fusion-based quantum computing (FBQC) -a model of universal quantum computation in which entangling measurements, called fusions, are performed on the qubits of small constant-sized entangled resource states. We introduce a stabilizer formalism for analyzing fault tolerance and computation in these schemes. This framework naturally captures the error structure that arises in certain physical systems for quantum computing, such as photonics. FBQC can offer significant architectural simplifications, enabling hardware made up of many identical modules, requiring an extremely low depth of operations on each physical qubit and reducing classical processing requirements. We present two pedagogical examples of fault-tolerant schemes constructed in this framework and numerically evaluate their threshold under a hardware agnostic fusion error model including both erasure and Pauli error. We also study an error model of linear optical quantum computing with probabilistic fusion and photon loss. In FBQC the non-determinism of fusion is directly dealt with by the quantum error correction protocol, along with other errors. We find that tailoring the fault-tolerance framework to the physical system allows the scheme to have a higher threshold than schemes reported in literature. We present a ballistic scheme which can tolerate a 10.4% probability of suffering photon loss in each fusion.

The ability to create large highly entangled 'cluster' states is crucial for measurement-based quantum computing. We show that deterministic multi-photon entanglement can be created from coupled solid state quantum emitters without the need for any two-qubit gates and regardless of whether the emitters are identical. In particular, we present a general method for controlled entanglement creation by making direct use of the always-on exchange interaction, in combination with single-qubit operations. This is used to provide a recipe for the generation of two-dimensional, cluster-state entangled photons that can be carried out with existing experimental capabilities in quantum dots. arXiv:1801.02599v2 [cond-mat.mes-hall]