Independent high-purity photons created in domain-engineered crystals

Abstract

Advanced photonic quantum technology relies on multi-photon interference which requires bright sources of high-purity single photons. Here, we implement a novel domain-engineering technique for tailoring the nonlinearity of a parametric down-conversion crystal. We create pairs of independentlyheralded telecom-wavelength photons and achieve high heralding, brightness and spectral purities without filtering.The ability of generating and manipulating single quanta of light enables the possibility of exploring new quantumenhanced technologies. Thanks to their robust coherence and the possibility of travelling over long distances with low losses, single photons are the ideal carriers of quantum information in large scale quantum networks [1-3]. Recent works have shown how photonic platforms are suitable not only for communication purposes, but also play a role in other areas of quantum information, such as quantum computing [4][5][6] and simulation [7,8]. In particular, scalable/fault-tolerant linear optical quantum computing (LOQC) appears to be a promising platform for quantum computing [9,10]. However, LOQC requires photons with near-unity purities and heralding efficiencies [11,12], as each percentage point in unsuccessful gate operation or photon loss comes at a significant overhead cost in the required number of photon sources, detectors and circuit complexity [13,14].

Photonic quantum information processing: A concise review

“…Combined with optimized mode matching with the optical fiber 121 and high efficiency detection technology in telelcom wavelength range, GVM allowed realization of pulsed telecom photon-pair sources that are simultaneously pure, highly efficient and (if desired) entangled in a chosen degree of freedom 50,51,53,122 . Further tailoring of the crystal's nonlinearity profile [123][124][125] provides photons that are fully uncorrelated in their spectrum 126,127 , completely removing the need for lossy spectral filtering. Investigation of the performance and limitations of periodically poled SPDC sources continues [128][129][130][131] and even tools for complete SPDC optimization are now available 132 .…”

Direct Generation of Tailored Pulse-Mode Entanglement

“…A polarising beamsplitter separates the PDC photons before they are coupled into single-mode fibres. We measured a source brightness of ∼ 4 KHz/mW photon pairs with a symmetric heralding efficiency > 60%, a reasonable trade-off achieved optimising the pump, signal and idler focusing conditions [19].…”

“…However, generating entangled TFMs in a controlled way can be very challenging [13][14][15][16][17], limiting their usefulness in realistic scenarios. Here, we overcome this problem exploiting domain-engineered nonlinear crystals [18,19] for generating TFM entanglement from standard ultrafast laser pulses in a single-pass PDC experiment. We experimentally validate this technique by benchmarking a maximally antisymmetric state at telecom wavelength † These two authors contributed equally.with near unity fidelity, and implement a four-photon entanglement swapping scheme.…”

“…TFMs can therefore be engineered by shaping the JSA, either by modifying the pump-pulse amplitude function [10] or, as we demonstrate here, by shaping the PMF via nonlinearity engineering. Domain-engineered crystals have been employed successfully for the generation of spectrally-pure heralded photons [18,19], where undesired frequency correlations are eliminated by tailoring a Gaussian nonlinearity profile. Here we extend this technique to the direct, controlled generation of custom TFM entanglement.We use the Hermite-Gauss modes [10] basis to encode the TFM quantum state, with the goal of generating the maximally-entangled antisymmetric Bell-state:where "s" ("i") labels the signal (idler) photon.…”

“…We instead characterise the PDC state using an indirect approach that exploits joint spectral intensity (JSI) reconstruction via dispersive fibre spectroscopy [28] and two-photon interference [19] to infer information on the populations and the entanglement of the quantum state, respectively.…”