Configurable heralded two-photon Fock-states on a chip

TL;DR

This work addresses scalable on-chip generation and manipulation of heralded two-photon states using a monolithic lithium niobate platform. By integrating SPDC-based photon-pair sources, wavelength demultiplexing, and an electro-optically tunable coupler, the chip enables controllable two-photon interference and routing to heralded path-entangled states in the telecom band. Key results include a 0.94 Hong-Ou-Mandel visibility, a heralding rate around 200 Hz, and the ability to switch between product and N00N states on sub-millisecond timescales, illustrating the device's suitability for quantum networks and metrology. The approach promises high stability, low power consumption, and compatibility with telecom infrastructure, with potential extensions to continuous-variable and hybrid DV-CV quantum optics.

Abstract

Progress in integrated photonics enables combining several elementary functions on single substrates for realizing advanced functionnalized chips. We report a monolithic integrated quantum photonic realization on lithium niobate, where nonlinear optics and electro-optics properties have been harnessed simultaneously for generating heralded configurable, two-photon states. Taking advantage of a picosecond pump laser and telecom components, we demonstrate the production of various path-coded heralded two-photon states, showing 94\% raw visibility for Hong-Ou-Mandel interference. The versatility and performance of such a highly integrated photonic entanglement source enable exploring more complex quantum information processing protocols finding application in communication, metrology and processing tasks.

Figures (5)

• Figure 1: Schematic of the monolithic LN chip with its associated pump, filtering and detection environment. The chip is partitioned into 3 regions, I for photon pair generation, II for photon pair splitting, and III for heralded entanglement manipulation. HWP: half-wave plate; QWP: quater-wave plate; PBS: polarizing beam splitter; M: mirror; L: lens; V-Groove: standard fiber array 127 $\mu$m-spacing; C1/C2/C3: evanescently coupled waveguides; V: voltage; $H_1$/$H_2$: heralding modes; $S_1$/$S_2$: heralded modes; FBG: Fibre Bragg Grating filters; SNSPDs: superconducting nanowire single photon detectors; TDC: time-to-digital converter.
• Figure 2: Idler power spectra of the SPDC emission.
• Figure 3: Coupling ratio calibration of C3 as a function of the applied votage. Laser light at 1560 nm is injected through source PPLN/w1 and measured at output $S_1$ and $S_2$. The dashed lines figure out two extreme cases: $16$ V for 100:0, $34$ V for 50:50 operation.
• Figure 4: Raw 4-fold coincidence count rate as a function of the relative delay in picoseconds between photons $S_1$ and $S_2$ for an integration time of 2 hours. The multipair contribution is the principal noise factor and is reported on the graph in green area. The error bars are assumed as $\sqrt N$ due to the poissonian statistic of photon pair detections ($N$ represents the measured coincidence count).
• Figure 5: 4-fold coincidence count rate measurement for relative delay = 0 ps as a function of the pulse offset in time. (a) V = 34 V, N00N state case, and (b) V = 18 V, product state case. The first peak shows 4-fold coincidence counts for two pairs of photon simultaneously created, and the following peaks are for two pairs of photon created with a time interval $n\times\delta t$ (n = 1, 2, 3, ...), $\delta t$ being the repetition time of pump laser which is equal to 13 ns. The error bars are assumed to be $\sqrt N$ according to the poissonnian statistics of the 4-fold coincidences. The reduced number of 4-fold coincidences for $\Delta T\neq 0$ in the N00N case can be explained by the extra 3 dB loss of the 50:50 coupler.