Controlled integration of selected detectors and emitters in photonic integrated circuits

Integration of superconducting nanowire single photon detectors and quantum sources with photonic waveguides is crucial for realizing advanced quantum integrated circuits. However, scalability is hindered by stringent requirements on high performance detectors. Here we overcome the yield limitation by controlled coupling of photonic channels to pre-selected detectors based on measuring critical current, timing resolution, and detection efficiency. As a proof of concept of our approach, we demonstrate a hybrid on-chip full-transceiver consisting of a deterministically integrated detector coupled to a selected nanowire quantum dot through a filtering circuit made of a silicon nitride waveguide and a ring resonator filter, delivering 100 dB suppression of the excitation laser. In addition, we perform extensive testing of the detectors before and after integration in the photonic circuit and show that the high performance of the superconducting nanowire detectors, including timing jitter down to 23 $\pm$ 3 ps, is maintained. Our approach is fully compatible with wafer level automated testing in a cleanroom environment.


Introduction
From visible to mid-infrared, superconducting nanowire single photon detectors (SNSPDs) have demonstrated excellent performances in terms of efficiency, 1-3 temporal resolution, 4,5 dark counts 6 and detection rates. 2 The fact that SNSPDs are compact and require only a single lithographic step for their realization allows for building large scale optical circuits.
All these features make SNSPDs ideal candidates for photon detection in advanced quantum 7 and neuromorphic integrated optical circuits. 8,9 Waveguide-integrated SNSPDs are particularly interesting, because the evanescent coupling allows for close to unity detection efficiencies, 10,11 while still maintaining short detection length due to the strong coupling between the SNSPD and the guided optical mode. Several superconducting materials such as NbN and NbTiN have been used to fabricate detectors in a wide variety of photonic platforms: silicon-on-insulator, 11 silicon nitride-on-insulator, 12 diamond, 13 and GaAs/AlGaAs. 14 The scalability of complex quantum photonic integrated circuits is limited by the fabrication yield of each component. 15 High performance single-photon detectors based on SNSPD technology are demanding due to challenges in sputtering high quality superconducting films and imperfections in the nanowire during lithography and etching steps. Constrictions along the nanowire affect the performance of the detector by limiting the device switching current well below its theoretical critical current. 16 Due to these subtle non-determinisms in fabrication, without careful characterization, it is difficult to predict if all detectors meet the required performances. In this work, we report on a deterministic approach for integrating high performance SNSPDs with photonic circuits. Additionally, we demonstrate an on-chip 2 full transceiver consisting of a source, an optical link, and a detector on the same circuit. In the following sections we discuss the fabrication process and present the results. The process for fabricating the devices is shown in Figure 1a. We started with a silicon wafer and thermally oxidized it to form 3.6 µm SiO 2 , which serves as the bottom cladding for the photonic waveguide [ Figure 1 Figure 1d, forms a "U"-shape [11][12][13]17 with 70 nm wide nanowires separated by a 200 nm gap. Additionally, to avoid latching due to the small kinetic inductance of the nanowire, we included a 2.5 mm long and 400 nm wide section serving as a series inductor. 18 To characterize the detectors, the samples were directly immersed in liquid helium and illuminated from the top using attenuated CW laser at 881 nm. Detectors were biased using a tunable source and the detection events were counted by a high-speed counter.

Deterministic integration of the detectors
Silicon nitride was selected for the waveguide core in our photonic circuit. It offers a wide optical transparency window from visible to mid-IR, 19 which makes it a good candidate for a range of photonic applications. [20][21][22] Additionally, the relatively large refractive index-contrast The experimental set-up used to perform the timing-jitter measurements is discussed in detail in. 2 Figure 2c presents the temporal resolution of a selected device biased at 90 % of its critical current. The fitted data gives a full width half maximum (FWHM) timing-jitter = 23 ± 3 ps. A better time resolution can be achieved by using a cryogenic amplifier and by operating the SNSPD at a lower temperature which in turn increases the critical current and hence the signal to noise ratio. 4 Figure 2d shows the normalized efficiency versus bias current for the same detector. The critical current is 11.6 µA and 11.5 µA before and after deposition, respectively, the two curves overlap indicating a minimal influence of the deposition of silicon nitride on the performance of the detector. Based on timing-jitter, internal efficiency, and high critical current, we selected this detector for integration in a photonic circuit. The detector is coupled through a filtering circuit to a selected nanowire quantum dot to measure the quantum dot lifetime. The waveguides were patterned using e-beam lithography, the pattern was then transferred to Si 3 N 4 by dry etching in a CHF 3 /Ar chemistry [ Figure 2a7]. To conclude this section, all extensive measurements reveal that testing the SNSPDs performance is needed to select the detectors before etching the waveguide layer, and the integrated circuit should be designed according to the selected SNSPDs. 6 Quantum dot nanowire integration with SNSPDs nm. The nanowire quantum dot was excited from the top using a femtosecond 515 nm pulsed laser with a repetition rate of 20 MHz. The wavelength of the laser was chosen to be within the absorption window of silicon nitride, thus the waveguide core acts as a natural high-pass filter to eliminate the pump photons which can blind the SNSPDs from detecting the QD signal. Additionally, the ring resonator is highly under-coupled for the pump laser photons, 8 which provides an additional stage of filtering of the high intensity pump. The total suppression of the laser is estimated to be 100 dB. After excitation of the QD, the emitted photons are coupled to the silicon nitride waveguide, they are filtered by the ring resonator, and finally detected by the superconducting detector. Figure 3f shows a time-resolved start-stop correlation measurement with the laser signal, the SNSPD provides high time-resolution of 23 ps. We extracted the QD signal decay time of 0.62 ± 0.02 ns, in agreement with previous measurements performed on similar quantum dot nanowires configuration. 24,29 The presented measurement sets a standard for the level of determinism needed to realize large scale quantum photonic circuits, where detectors, as well as sources, are selected in a controlled process based on their individual characteristics.

Conclusion
In summary, we have shown a deterministic method to integrate high performance SNSPDs with photonic circuits. We realized on-chip full-transceiver, completely deterministic from source to detector and validated it by measuring the lifetime of a selected quantum dot.
The integration process demonstrated in this article is CMOS compatible, detectors can be mass-produced and their characterization can be fully automated. Afterwards, only the best detectors are integrated with the photonic circuit. We believe that our method provides the needed accuracy and performance to realize future large scale quantum and neuromorphic photonic circuits.