Linear optical quantum computing (LOQC) seems attractively simple: Information is borne entirely by light and processed by components such as beam splitters, phase shifters, and detectors. However, this very simplicity leads to limitations, such as the lack of deterministic entangling operations, which are compensated for by using substantial hardware overheads. Here, we quantify the resource costs for full-scale LOQC by proposing a specific protocol based on the surface code. With the caveat that our protocol can be further optimized, we report that the required number of physical components is at least 5 orders of magnitude greater than in comparable matter-based systems. Moreover, the resource requirements grow further if the per-component photon-loss rate is worse than 10 −3 or the per-component noise rate is worse than 10 −5 . We identify the performance of switches in the network as the single most influential factor influencing resource scaling.

An on-demand single-photon source is a key requirement for scaling many optical quantum technologies. A promising approach to realize an on-demand single-photon source is to multiplex an array of heralded single-photon sources using an active optical switching network. However, the performance of multiplexed sources is degraded by photon loss in the optical components and the non-unit detection efficiency of the heralding detectors. We provide a theoretical description of a general multiplexed single-photon source with lossy components and derive expressions for the output probabilities of single-photon emission and multi-photon contamination. We apply these expressions to three specific multiplexing source architectures and consider their tradeoffs in design and performance. To assess the effect of lossy components on near-and long-term experimental goals, we simulate the multiplexed sources when used for many-photon state generation under various amounts of component loss. We find that with a multiplexed source composed of switches with ∼ − 0.2 0.4 dB loss and high efficiency number-resolving detectors, a single-photon source capable of efficiently producing 20-40 photon states with low multi-photon contamination is possible, offering the possibility of unlocking new classes of experiments and technologies.An approach to overcome all of the scaling problems with HSPSs is the multiplexed (MUX) single-photon source [14], which uses an array of HSPSs, delay lines, electronics for classical logic operations, and an active optical switching network to approximate a true on-demand source (figure 1(b)). Using an array of HSPSs as a collective unit means that the probability that at least one of the HSPSs emits a single photon is high, while a switching network driven by the heralding signals is used to route the generated photon into a specific spatialtemporal output mode. This results in the near-deterministic generation of single photons, allowing for much larger quantum circuits than could be feasibly built with HSPSs without active multiplexing. MUX sources inherit the same benefits as parametric HSPSs, including their mature theoretical and experimental investigation, and are especially appealing due the prospects of a fully integrated device with existing fabrication processes.Several theoretical schemes have been previously investigated [15][16][17][18][19][20][21][22][23] and experimental work using bulk [24] and integrated [25,26] components has been demonstrated. Previous work has highlighted the fundamental constraints for creating pure states using parametric processes assuming ideal MUX components [13]. However, in real physical settings, non-ideal components will limit the efficiency and output fidelity of MUX sources. The most significant sources of error in multiplexed sources are likely to be photon loss in the optical components and the non-unit detection efficiency of the heralding detectors (which can also be viewed as photon loss). Assessing the suitability of a MUX sources using lossy components ...

The maturation of many photonic technologies from individual components to next-generation system-level circuits will require exceptional active control of complex states of light. A prime example is in quantum photonic technology: while single-photon processes are often probabilistic, it has been shown in theory that rapid and adaptive feedforward operations are sufficient to enable scalability. Here, we use simple "off-the-shelf" optical components to demonstrate active multiplexing-adaptive rerouting to single modes-of eight single-photon "bins" from a heralded source. Unlike other possible implementations, which can be costly in terms of resources or temporal delays, our new configuration exploits the benefits of both time and space degrees of freedom, enabling a significant increase in the singlephoton emission probability. This approach is likely to be employed in future near-deterministic photon multiplexers with expected improvements in integrated quantum photonic technology.

Many existing schemes for linear-optical quantum computing (LOQC) depend on multiplexing (MUX), which uses dynamic routing to enable near-deterministic gates and sources to be constructed using heralded, probabilistic primitives. MUXing accounts for the overwhelming majority of active switching demands in current LOQC architectures. In this manuscript we introduce relative multiplexing (RMUX), a general-purpose optimisation which can dramatically reduce the active switching requirements for MUX in LOQC, and thereby reduce hardware complexity and energy consumption, as well as relaxing demands on performance for various photonic components. We discuss the application of RMUX to the generation of entangled states from probabilistic singlephoton sources, and argue that an order of magnitude improvement in the rate of generation of Bell states can be achieved. In addition, we apply RMUX to the proposal for percolation of a 3D cluster state by Gimeno-Segovia et al (2015 Phys. Rev. Lett. 115 020502), and we find that RMUX allows an 2.4×increase in loss tolerance for this architecture.