Characterizing Niobium Nitride Superconducting Microwave Coplanar Waveguide Resonator Array for Circuit Quantum Electrodynamics in Extreme Conditions

The high critical magnetic field and relatively high critical temperature of niobium nitride (NbN) make it a promising material candidate for applications in superconducting quantum technology. However, NbN-based devices and circuits are sensitive to decoherence sources such as two-level system (TLS) defects. Here, we numerically and experimentally investigate NbN superconducting microwave coplanar waveguide resonator arrays, with a 100 nm thickness, capacitively coupled to a common coplanar waveguide on a silicon chip. We observe that the resonators' internal quality factor (Qi) decreases from Qi ~ 1.07*10^6 in a high power regime ( = 27000) to Qi ~ 1.36 *10^5 in single photon regime at temperature T = 100 mK. Data from this study is consistent with the TLS theory, which describes the TLS interactions in resonator substrates and interfaces. Moreover, we study the temperature dependence internal quality factor and frequency tuning of the coplanar waveguide resonators to characterise the quasiparticle density of NbN. We observe that the increase in kinetic inductance at higher temperatures is the main reason for the frequency shift. Finally, we measure the resonators' resonance frequency and internal quality factor at single photon regime in response to in-plane magnetic fields B||. We verify that Qi stays well above 10^4 up to B|| = 240 mT in the photon number = 1.8 at T = 100 mK. Our results may pave the way for realising robust microwave superconducting circuits for circuit quantum electrodynamics (cQED) at high magnetic fields necessary for fault-tolerant quantum computing, and ultrasensitive quantum sensing

Quantum Key Distribution on microwave band for superconducting quantum computing

We propose a quantum key distribution (QKD) system operating at cryogenic temperature to build secure microwave wireless link between quantum computers. One of the state-of-the-art methods of building high quality quantum computers is based on superconducting technology. Limited by the microwave components working at cryogenic temperature, the frequencies of superconducting quantum computing are generally from f=4GHz to f=10GHz. So building quantum secure link at this frequency range at cryogenic temperature is important for the accomplishment of private quantum communication. Continuous Variable (CV) QKD is one way to build secure link depending on quantum mechanics. We consider a one-way light-of-sight (LOS) point to point microwave communication link between two users: Alice and Bob. The process of this scheme is as follow: Alice starts from a single thermal state and prepares coherent states from a two-dimensional Gaussian distribution. Then she sends her states to Bob through an insecure diffraction-only channel with transmissivity T. Bob uses a homodyne detection to measure the income states and get the outcome. To examine the security of this link, we assume the attacker Eve hiding in the channel and operating collective attack to steal information. To minimize risk of leakage, Alice and Bob carry out privacy amplification in the post-processing and use reverse reconciliation (RR) to prepare their secret key. In conclusion, this work focuses on the secure wireless distance between different superconducting quantum computers’ communication systems by using microwave CVQKD. The results may benefit the communication systems between both quantum computers and inter-satellite

Nitride-based superconducting coplanar waveguide resonator with an internal quality factor above one million

As a type II superconductor, niobium nitride (NbN) has been widely used in quantum circuits [1-2]. Insensitivity to magnetic fields and high kinetic inductance make it a suitable material candidate for applications in complex superconducting quantum circuits. However, NbN is more susceptible to decoherence sources such as two-level system (TLS) defects, and optimising the fabrication procedure is one of the main challenges which should be overcome. Here, we investigate microwave photonic losses in NbN-based superconducting quantum circuits. Our circuits consist of an array of microwave resonators made from a 100 nm thin NbN film, capacitively coupled to a common coplanar waveguide. We designed, modelled, and optimised the resonators on various substrates such as Sapphire and intrinsic Silicon (Si) through analytical calculations and numerical simulations. We microfabricated the NbN resonators on Si and performed the microwave measurements at cryogenic temperatures down to T = 80 mK. We study the internal quality factor (Qi) of fabricated microwave resonators in millikelvin temperature in the power range of-140 dBm to -95 dBm. We further show an agreement between numerical calculation and measured internal quality factor at T = 100 mK. We report results for one of our samples, which includes three resonators, each with 4 µm widths and 2 µm gap, coupled to a feed line, all formed on a 525 µm thick Si substrate. The Si substrate was cleaned with HF solution to remove any native oxide before sputtering NbN. The resonators are fabricated using standard e-beam lithography and CF4 anisotropic dry etch. The resonator length was chosen to have a fundamental frequency mode between 3-6 GHz (see Fig.1 a, for results for one resonator). We observed that at low temperature (T = 100 mK), the internal quality factor increases from Qi = 1.4×105 ±4.7×104in the single photon regime to Qi = 106±1.4×105at high powers (Fig.1.b). We further found an agreement between the estimated TLS losses and our experimental results [2]

Recent Material Developments for Superconducting Quantum Circuits

No abstract available

Tantalum on Sapphire and Silicon Substrates for Superconducting Quantum Circuits

Materials science of superconducting circuits is considered with increasing importance, particularly as it directly affects qubit coherence. Appropriate nanofabrication and film growth techniques need to be developed to incorporate quality-factor engineered components. One emerging structure for superconductor ground planes and feedlines is tantalum (Ta) on a sapphire substrate, for which high coherence times were achieved for transmon qubits. The oxide formation and stoichiometry of α-phase Ta films leads to fewer sources of noise for the qubit to incoherently exchange energy with. In this presentation, we demonstrate growth techniques for deposition of Ta on heated sapphire substrates, and deposition of Ta on Si substrates using a Nb seed layer. We will also present different recipes that were used to dry etch Ta films into resonator structures, and discuss the extracted internal quality factors from these film. We discuss our investigations into fabricating Ta resonators on Si at room temperature which opens up a way to fabricate highly coherent circuits on systems without heating capabilities, and avoids thermally induced diffusion of pre-deposited materials. Finally, we detail the different dry etch chemistries that can be used and which one we have found to be optimal