Optimized Spectral Purity of Heralded Single Photons at the Telecom O-Band

TL;DR

This work addresses the challenge of generating spectrally pure heralded single photons in the telecom O-band by jointly engineering group-velocity matching and poling structures in a potassium titanyl phosphate (KTP) crystal. The authors develop a framework combining a Gaussian pump envelope with a tailored phase-matching function (PMF) and optimize poling using coherence-length (CL) and sub-coherence-length (SCL) schemes to shape the PMF toward a near-Gaussian form, achieving heralded-photon spectral purities exceeding 99% across multiple GVM configurations. They demonstrate that pump wavelengths from 603.8 nm to 887.3 nm can yield pure photons at 1310 nm, with some configurations compatible with off-the-shelf lasers and detectors, while analyzing noise spectra that underscore the need for practical spectral filtering to maintain high purity in real systems. The results advance practical quantum light sources for quantum information applications in the O-band, with implications for quantum communication and networking that leverage low-loss, low-dispersion fiber transmission and existing detector technologies.

Abstract

We report on optimizing the spectral purity of heralded single photons in the telecom O-band, where single photons can be propagated with low loss and low dispersion in a standard telecom optical fiber. We numerically searched for various group-velocity-matching conditions and corresponding optimal poling structures of a potassium titanyl phosphate crystal for spontaneous parametric downconversion. Our poling optimization results using phase-matching coherence-length and sub-coherence-length modulation schemes show > 99.4% spectral purity with pump wavelengths ranging from 603.8 nm to 887.3 nm. Some optimized configurations are feasible with off-the-shelf lasers and single-photon detectors. Moreover, by investigating noise photon spectra for different poling optimization methods, we show that, in practice, appropriate, gentle spectral filtering helps achieve high purity. This study will pave the way for developing practical quantum sources for quantum information applications at the telecom O-band.

Figures (9)

• Figure 1: Conceptual diagram of a spectrally pure heralded single-photon source at the telecom O-band optimized by an appropriate GVM condition and a modulated poling structure of a potassium titanyl phosphate (KTP) crystal. We consider a Type-II quasi-phase-matching condition where pump, signal, and idler photons are polarized along the crystallographic $Y$, $Z(Y)$, and $Y(Z)$ axes, respectively.
• Figure 2: Flowchart of the poling optimization algorithm. The poling array $\vb*A$ is constructed for given parameters $\alpha$ and $\beta$, and then evaluated by the cost function $C(z; \vb*{A})$, where $\alpha$ is the Gaussian spatial width factor and $\beta$ is the domain division factor. The procedure is continued until the poling array shows heralded single-photon purity higher than the threshold ($P = 99.5$%).
• Figure 3: (a,c) GVM angle $\theta$ and (b,d) phase-matching coherence length $l_c$ as a function of pump and signal wavelengths in a KTP crystal. The signal mode is (a,b) $Z$-polarization and (c,d) $Y$-polarization. The GVM angle distribution is mapped for $0^{\circ} \leq \theta \leq 90^{\circ}$, where high heralded single-photon purity can be achieved. Dashed black lines indicate the degenerate case $\lambda_s = 2\lambda_p$. The insets of (a,c) show our selected GVM cases to perform poling optimization at $\lambda_s =$ 1310 nm and $\lambda_s =$ 1550 nm (see Appendix C for $\lambda_s =$ 1550 nm).
• Figure 4: Optimized poling structure of a KTP crystal for the GVM case (i) ($\theta = 26^{\circ}$, $\lambda_i = 1550$ nm). The crystal poling signs $+1$ and $-1$ denote UP and DOWN poling directions, respectively.
• Figure 5: JSA for the GVM cases (i,ii,iv,vii). (a,c,e,g) PP scheme. (b,d,f,h) CL and SCL schemes. $\lambda_i$, the center wavelength of the idler mode; $\Delta\lambda_p$, optimal pump bandwidth achieved the maximum purity with the corresponding PMF.