Diamond photonic crystal cavities offer exceptional properties for interfacing color centers in diamond to integrated photonic circuits. Leveraging Purcell enhancement of the color centers’ emission into the zero-phonon line, the development of high quality, low mode volume single-crystal diamond resonators remains one of the key challenges for building diamond quantum networks. In this paper, we present both a robust, versatile photonic crystal design as well as a scalable fabrication process realizing suspended nanobeam cavities. Measurements of confocal microscopy with broadband excitation yield quality factors of more than 5000. We further demonstrate a fast and facile characterization of our structures based on confocal photoluminescence imaging. This method not only provides a more detailed look at the higher order modes within diamond nanobeam resonators but also serves as a non-destructive diagnostic of the modes’ susceptibility to fabrication imperfections, providing critical feedback for scalable, high-yield integrated quantum photonic device development.

Superconducting nanowire single-photon detectors (SNSPDs) have emerged as leading cryogenic photon detectors, thanks to their high detection efficiency and low jitter. However, their large-scale integration remains limited by the wiring bottleneck between the cryogenic detectors and their room-temperature readout electronics. In applications such as color-center-based quantum computers (QCs), thousands of detectors may need to operate in parallel within a limited cryogenic cooling budget, thus asking for a scalable, low-power cryogenic electronic readout. To address these needs, this work introduces a cryogenic readout circuit directly wire-bonded to the SNSPD and using a high-impedance input to maximize the quality of the detector signal, thus relaxing the requirement of the cascaded amplifier and reducing its power consumption. An active quenching circuit is then adopted to ensure a reliable reset after the latching of the detector induced by such high input impedance. Implemented in 40-nm CMOS with an active area of <0.14 mm², the system achieves competitive performance at 0.1 K, delivering low timing jitter (<40 ps), high speed (dead time of ≈5 ns), and dark count rates (DCRs) below 1 Hz, while achieving a 5× reduction in power consumption (down to 20 μW) with respect to the cryogenic-readout state-of-the-art. Its ultralow-power operation and compact footprint make the proposed solution well-suited for integration within large-scale quantum-computing architectures.

Micro-transfer printing (uTP) facilitates the integration of pre-characterized components from different substrates onto one common carrier die. Its highly parallel nature allows for exceptional throughput [1]. Up to now, uTP has been successfully applied to print III-V amplifiers, lasers [2] , and quantum dot emitters [3] , among others. Our research focuses on fabricating devices containing waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) [4], that are ready to be transferred. Thanks to their outstanding performance, SNSPDs integrated on low-loss Silicon Nitride waveguides enable optical quantum computing [5] and quantum key distribution [6]. Enabling the transfer of SNSPDs would allow for precise on-demand detector placement and circumvents fabrication constraints that could compromise the quality of the superconducting film.
The fabrication process involves the patterning nanowires, half-etched Si₃N₄ waveguides, and metal electrodes via electron beam lithography (EBL), followed by reactive ion etching (RIE), physical vapor deposition (PVD) or sputtering. In order for the Si₃N₄ device to be transferred, the final step employs hydrofluoric acid etching to dissolve the SiO₂ layer (see Fig. 1a and b) and make the device freestanding. Thereafter, a PDMS stamp is used to pick up the device, breaking its support structures and accurately place them on the dedicated carrier substrate.
This photonic architecture is optimized for detecting photons at a 930 nm wavelength. For sacrificial test devices, grating couplers at both ends of the suspended structures facilitate integrity and functionality testing during fabrication, while metal contacts to the SNSPDs allow for electrical pretesting. This approach allows for a precise placement of pre-characterized waveguide-integrated SNSPDs on any substrate and thus the creation of hybrid architectures necessary for fully on-chip quantum photonic applications.

This paper presents a scalable cryogenic readout solution for Superconducting Nanowire Single-Photon Detectors (SNSPDs) tailored for the readout of color-center-based qubits. The readout circuit, wire-bonded directly to the SNSPD, utilizes high input impedance to boost the signal amplitude, hence reducing the power consumption, and active quenching to prevent the latching induced by the high impedance. Fabricated in 40-nm CMOS in a 0.14-mm ² active area, the proposed system demonstrates competitive performance at 0.1 K, featuring low jitter [<60 ps Full Width at Half Maximum (FWHM)], high speed (dead time ≈ 5 ns) and low dark count rate (<1 Hz), while dissipating only 20 μ W. Such an ultra-low power and compact area enables the readout integration within a large-scale colorcenter quantum computer.

Micro-transfer printing (uTP) facilitates the integration of pre-characterized components from different substrates onto one common carrier die, enabling high-yield processes. Its highly parallel nature allows for exceptional throughput [1]. uTP has been successfully applied to III-V amplifiers, lasers [2], and quantum dot emitters [3], among others.