AVS 71 Session QS-ThP: Quantum Science and Technology Mini-Symposium Poster Session

Niobium (Nb) is a widely used superconducting material in thin-film form for quantum and superconducting electronic applications. In this work, we develop a robust Nb sputtering process optimized for evaluating candidate substrates for superconducting qubits. Thin Nb films were deposited on Si (001) substrates using direct current (DC) magnetron sputtering under varying powers and deposition times. Initial process optimization was guided by DC transport measurements, including critical temperature, critical field, and residual resistance ratio. Promising recipes were further assessed by fabricating Nb/Si coplanar waveguide resonators and measuring their internal quality factors at microwave frequencies. To mitigate dielectric losses at the metal–substrate interface, we implemented surface cleaning methods such as ex-situ HF etching (solution and vapor) and in-situ argon ion milling. Film morphology and interface quality were characterized using atomic force microscopy and scanning electron microscopy. Building on the optimized deposition parameters, we outline a pathway for screening alternative qubit-compatible substrates, including sapphire and silicon carbide.

Quantum photonic processors offer major advantages over classical computing through massive parallelism. Conventional two-photon couplers, however, are limited in scalability and flexibility. We present a photonic quantum circuit architecture using a Spontaneous Six-Wave Mixing (SSWM) microring resonator, adapted from classical photonics, as an active logic element generating triphoton entanglement. These three-photon interactions reduce coupler requirements and enable more qubits (greater integration density), hence allowing for greater computational power.

Decoupling allows qubits to temporarily store quantum information without unwanted interactions. Using MEEP simulations, the decoupled state was achieved by sweeping the ring index nring (semiconductor-compatible refractive index tuning) from 2.05 to 2.20 with the bus waveguide fixed at 2.05, until no light propagated into the ring. Coupling coefficients decreased from 0.1094 to 0.0200 (showing detuning) while the Q-factor remained stable (335.9 to 433.38), confirming robust functionality. Coupling spectra retained their shapes with only amplitude reductions, indicating unchanged behavior.

Photon state integrity was evaluated using a custom QuTiP-based photon–photon interaction model, with three modes of four photons each evolving under a Hamiltonian derived from the kappa interaction matrix. For both states, photon purity remained near 1, fidelity averaged 0.9 ± 0.1, and entropy stayed close to zero. Wigner function analysis showed minimal but consistent state differences, with their time-dependent rotation revealing precise windows for targeted control.

To measure circuit-level performance, a custom quantum photonic architecture was created: three data qubits underwent five different protocol classes i.e. teleportation, simple squeezing, etc. They operated under realistic noise channels with ancilla-assisted measurement. Kappa matrices (k) determined entanglement strengths and were swept over time from [0, kmax] to simulate detuning. This was compared to the control without kappa to determine if detuning affected circuit performance.

Metrics showed that the tunable SSWM microring resonator demonstrates strong potential in quantum photonic circuits with 4/5 protocols receiving better or consistent performance with (bi-state) detuning. Its protocol-dependent performance highlights opportunities for targeted applications, and future work will focus on simulation denoising and broader frequency sweeps to ensure robust, energy-efficient operation.