Introduction

fig:system — Figure 1: **Layout of experiment** a) Pulses from a 1539.47 nm mode locked laser (Pritel UOC) are split into two by an 80-ps delay-line interferometer before up-conversion and amplification in a second harmonic generation + erbium doped fiber amplifier (SHG + EDFA) module (Pritel). A short PM fiber from the SHG module connects to a nonlinear crystal generating photon pairs by spontaneous parametric down-conversion (SPDC). The coarse wavelength division multiplexing (CWDM) module separates the photon pair spectrum into eight 13 nm-wide bands around 1530 and 1550 nm, for the signal and idler photon, respectively. The signal and idler are directed to the Bob and Alice stations, respectively. The readout interferometers introduce the same time delay as the source interferometer. Polarization controllers are used to maximize the coincidence rates, as the detection efficiencies of each SNSPD is polarization sensitive (\(\pm20\%\)). Entanglement visibility is unaffected by readout polarization. 100 GHz spacing Dense wavelength division multiplexer (DWDM) modules are used to direct each frequency channnel into a distinct fiber. Two superconducting nanowire single photon detectors (SNSPDs) are used to measure a specific frequency multiplexed channel pair. Measurements for different multiplexed channels are performed in succession to resolve full system performance. b) ITU channels used in the experiment. Pairs of channels highlighted with the same color obey the phase and pump-energy matching condition for SPDC. To asses the full 16 channels (27-42) of Alice’s DWDM multiplexer, Bob’s 8-channel DWDM is replaced with a narrowband filter with tunable resonance frequency (not shown in figure).

Entangled photons play a vital role in the development of quantum information processing and communication systems ¹²³⁴⁵⁶. The ability to generate entangled photon pairs at a high rate is essential for establishing reliable and scalable quantum networks with quantum repeaters, as well as for implementing entanglement-based quantum key distribution (QKD) systems ⁷⁸⁹¹⁰. Unlike QKD implementations that rely on attenuated lasers ¹¹¹² entanglement distribution systems may fulfill the objectives of QKD while also serving as the foundation for advanced quantum networks that rely on entanglement as a fundamental resource.

Entanglement distribution and entanglement-based QKD have been demonstrated with impressive performance across a number of metrics. These include 40 kbps data rates in a QKD system deployed over 50 km of fiber ¹⁷ as well as multiple polarization entangled sources that leverage spectral multiplexing. These polarization sources include a demonstration of 181 kebits/s across 150 ITU channel pairs and a high-throughput source potentially capable of gigabit rates with many added channels and detectors ¹⁸¹⁹. Multiple works have highlighted the need to leverage high total brightness, spectral brightness, collection efficiency, and visibility from pair-generating non-linear crystals to realize practical high-rate entanglement distribution ¹⁹²⁰²¹²²²³²⁴²⁵.

A time-bin entangled photon source has certain advantages over a polarization-based system ²⁶. Time-bin entanglement can be measured with no moving hardware and does not require precise polarization tracking to maximize visibility ²⁷²⁸. Also, with suitable equipment, robust time-bin modulation is possible over free space links with turbulence ²⁹. Therefore, the possibility of simplified fiber-to-free-space interconnects and larger quantum networks based on a shared time-bin protocol motivates development of improved time-bin sources. Furthermore, time-bin encoding is suited for single-polarization light-matter interfaces ³⁰³¹.

We direct 4.09 GHz mode locked laser light into a nonlinear crystal via 80-ps delay-line interferometers (12.5 GHz free-spectral range) to realize a high-rate entanglement source. The ability to resolve time-bin qubits into 80 ps wide bins is enabled by newly developed low-jitter differential superconducting nanowire single-photon detectors (SNSPDs) ³². Wavelength multiplexing is used to realize multiple high visibility channel pairings which together sum to a high coincidence rate. Each of the pairings can be considered an independent carrier of photonic entanglement ³³³⁴ and therefore the system as a whole is applicable to flex-grid architectures through the use of wavelength selective switching ³⁵³⁶. However, we focus on maximizing the rate between two receiving stations, Alice and Bob (Fig. \ref{fig:system}a). Each station is equipped with a DWDM that separates the frequency multiplexed channel into multiple fibers for detection. The SNSPDs are used with a real-time pulse pileup and time-walk correction technique ³⁷ to keep jitter low even at high count rates.

We quantify per-channel brightness and visibility as a function of pump power, as well as collection efficiencies, coincidence rates across 8 channel pairs, and expected performance of a partially realized 16-channel pair configuration. We show that the 8 channel system achieves visibilities that average to 99.3% at low mean photon number \(\mu_{L} = 5.6{\times} 10^{-5}\,\pm\,9{\times} 10^{-6}\). At a higher power (\(\mu_{H} = 5.0{\times} 10^{-3}\,\pm\,3{\times} 10^{-4}\)), we demonstrate a total coincidence rate of 3.55 MHz with visibilities that average to 96.6%. Through quantum state tomography we bound the distillable entanglement rate of the system to between 69% and 91% of the \(\mu_{H}\) coincidence rate (2.46 - 3.25 Mebits/s).

Quantifying a source’s spectral mode purity is important for gauging its utility in advanced quantum networks that rely on interferometric measurements like two-photon interference which enables Bell-state measurements (BSM) ³⁸. With Schmidt decomposition we quantify the modal purity of single DWDM channel pairs and derive the inverse Schmidt number which serves as an estimate for two-photon interference visibility between two such sources. Ultimately, we demonstrate that an entanglement generation source of this design makes for a robust and powerful building block for future high-rate quantum networks.

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