The femtosecond laser direct-writing (FLDW) of waveguide circuits in glasses has seen interest from a number of fields over the previous 20 years. It has evolved from a curiosity to a viable platform for the rapid prototyping of small scale circuits. The field of quantum information science has exploited this capability and in the process advanced the fabrication technique. In this review the technological aspects of the laser inscription method relevant to quantum information science will be discussed. A range of demonstrations which have been enabled by laser written circuits will be outlined; these include novel circuits, simulations, photon sources and detection. This places the FLDW technique among the few integrated optical platforms to have produced individually every component required for scalable quantum computation.

A key resource for quantum optics experiments is an on-demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a heralded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro-optic switches. Hybrid interfacing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated technology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies. Spontaneous parametric downconversion (SPDC) is one of the most commonly exploited methods to generate photons in quantum information science (QIS) applications [1]. However, the spontaneous nature of the process, bounding the probability that N sources each simultaneously create a pair of photons, prevents the demonstration of advanced QIS protocols [2]. Furthermore, the power of the pump laser cannot simply be scaled to increase the pair generation rate since multi-photon events result in noise. Although schemes have been developed which can tolerate multi-photon pairs [3], the majority of protocols have stringent limitations on additional noise [4]. Increasing the repetition rate of the pump laser increases the number of photon pairs generated by a source per second [5], while the probability of generating a photon pair per pulse remains constant. In contrast, active temporal multiplexing schemes can increase the probability of obtaining a photon pair per pulse [6], however due to implementation challenges there has been to date no experimental demonstration. To address this, Migdall et al. proposed a spatial multiplexing scheme to increase photon rates while maintaining the corresponding noise level [7]. This scheme relies on spatially separated heralded sources being actively routed to a single output. A bulk optic demonstration of a multiplexed source used a pair of SPDC crystals [8]. This method suffered from large space and stability requirements and was not scalable beyond a pair of sources. Integrated optics offers a route to miniaturisation, stability and scalability which enabled a number of QIS circuit demonstrations such as a controlled-NOT operation [9], heralded multiphoton entanglement [10] and on-chip quantum relay operation [11]. Here we propose a hybrid photon source which uses SPDC waveguides and laser written components to produce four heralded photons (Fig. 1a). We then use fast fibre-based switches to actively route photons to the desired output (Fig. 1b). Using this architecture we have control over both the photon wavelength and number through temperature tuning and electronically addressed switches.The first demonstration of integrated spatial multiplexing, using a pair of χ (3) silicon photonic crystal (...