arXiv subjects

Stephen McCoy

Publications and source records attributed to Stephen McCoy.

5 recordsLinked to original sources

arXiv2026

Transmon Architecture for Emission and Detection of Single Microwave Photons

We develop a compact transmon emitter/detector (TED) superconducting circuit and demonstrate its dual functionality as a single-photon source and detector. In our setup, photons emitted by a source TED are transmitted via a meter-long coaxial cable, routed through a circulator, and captured by a measurement TED. Both TED modules operate with nominally identical parameters, highlighting the flexibility of this novel architecture. Furthermore, we introduce an efficient microwave photon detection scheme tailored to the TED. Using this setup, we detect 60% of the emitted Fock state photons and infer a 95% detection efficiency at the input of the measurement TED, which we calibrate against coherent state measurements. The reset and photon emission/detection processes each require approximately $2\,\mu s$, yielding a minimum protocol duration of $4\,\mu s$ as constrained by our chosen TED parameters. Ultimately, the TED demonstrates a new use case for the recently developed double transmon coupler (DTC): a compact, drop-in, tunable, and transition-selective link between a coherent data transmon and a waveguide. Circuits like the TED will play a vital role in quantum information processing by facilitating unconditional fast reset, enabling microwave photon metrology, and serving as nascent quantum communication interfaces (QCIs). QCIs mediate entanglement distribution between quantum processing units (QPUs) within a cryostat or interface with microwave-to-optical transducers for long-range quantum networking.

quant-ph↗

arXiv2025

Ta-based Josephson junctions using insulating ALD TaN tunnel barriers

Josephson junctions form the core circuit element in superconducting quantum computing circuits, single flux quantum digital logic circuits, and sensing devices such as SQUIDs. Aluminum oxide has typically been used as the tunnel barrier. Its formation by exposure to low oxygen pressures at room temperature for short periods of time makes it susceptible to aging and limits the thermal budget of downstream processes. In this paper, we report the first demonstration of {\alpha}-Ta/insulating TaN/a-Ta superconductor/insulator/superconductor Josephson junctions fabricated on 300 mm wafers using CMOS-compatible processes. The junctions were fabricated on high-resistivity silicon substrates using standard processes available at 300 mm scale, including 193 nm optical lithography, ALD of TaN in a cluster tool, and chemical mechanical planarization to enable highly planar interfaces. Junction areas ranging from 0.03 um2 to 9 um2 with ALD TaN thickness between 2 nm and 7 nm were characterized. A critical current density of 76 uA/um2 was observed in junctions using 4 nm ALD TaN in the tunnel barrier. The dependence of Jc on ALD TaN layer thickness is analyzed, and the influence of junction geometry, packaging, and temperature on I-V characteristics is discussed. Junctions were retested after a period of 4 months to quantify junction aging. The potential of this novel material system and a 300 mm superconducting junction process flow to fabricate thermally and environmentally stable junctions is discussed. The vision of a Superconducting Quantum Process Design Kit for a Multi-Project Wafer program to enable rapid development and proliferation of superconducting quantum and digital digital logic systems is presented. This work represents the first step towards establishing such a Quantum Foundry, providing access to high quality qubits and single-flux quantum logic circuits at 300 mm wafer scale.

cond-mat.supr-con↗

arXiv2025

Noise-Aware Entanglement Generation Protocols for Superconducting Qubits with Impedance-Matched FBAR Transducers

Connecting superconducting quantum processors to telecommunications-wavelength quantum networks is critically necessary to enable distributed quantum computing, secure communications, and other applications. Optically-mediated entanglement heralding protocols offer a near-term solution that can succeed with imperfect components, including sub-unity efficiency microwave-optical quantum transducers. The viability and performance of these protocols relies heavily on the properties of the transducers used: the conversion efficiency, resonator lifetimes, and added noise in the transducer directly influence the achievable entanglement generation rate and fidelity of an entanglement generation protocol. Here, we use an extended Butterworth-van Dyke (BVD) model to optimize the conversion efficiency and added noise of a Thin Film Bulk Acoustic Resonator (FBAR) piezo-optomechanical transducer. We use the outputs from this model to calculate the fidelity of one-photon and two-photon entanglement heralding protocols in a variety of operating regimes. For transducers with matching circuits designed to either minimize the added noise or maximize conversion efficiency, we theoretically estimate that entanglement generation rates of greater than $160\;\mathrm{kHz}$ can be achieved at moderate pump powers with fidelities of $>90\%$. This is the first time a BVD equivalent circuit model is used to both optimize the performance of an FBAR transducer and to directly inform the design and implementation of an entanglement generation protocol. These results can be applied in the near term to realize quantum networks of superconducting qubits with realistic experimental parameters.

quant-ph↗

arXiv2025

Quantitative assessment of biological dynamics with aggregate data

We develop and apply a learning framework for parameter estimation in initial value problems that are assessed only indirectly via aggregate data such as sample means and/or standard deviations. Our comprehensive framework follows Bayesian principles and consists of specialized Markov chain Monte Carlo computational schemes that rely on modified Hamiltonian Monte Carlo to align with constraints induced by summary statistics and a novel elliptical slice sampler adapted to the parameters of biological models. We benchmark our methods with synthetic data on microbial growth in batch culture and test them with real growth curve data from laboratory replication experiments on $\textit{Prochlorococcus}$ microbes. The results indicate that our learning framework can utilize experimental or historical data and lead to robust parameter estimation and data assimilation in ODE models that outperform least-squares fitting.

q-bio.QM↗

arXiv2022

Discovery of an electronic crystal in a cuprate Mott insulator

Copper oxide high temperature superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. The interplay between these orders and the superconducting ground state, as well as how they arise through doping a Mott insulator, is essential to decode the mechanisms of high-temperature superconductivity. Over the years, various forms of electronic liquid crystal phases including charge/spin stripes and incommensurate charge-density-waves (CDWs) were found to emerge out of a correlated metallic ground state in underdoped cuprates. Early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly-doped, insulating limit of the CuO2 planes. Here, we use resonant X-ray scattering, electron transport, and muon spin rotation measurements to fully resolve the electronic and magnetic ground state and search for signatures of charge order in very lightly hole-doped cuprates from the RBa2Cu3O7-d family (RBCO; R: Y or rare earth). X-ray scattering data from RBCO films reveal a breaking of translational symmetry more pervasive than was previously known, extending down to the Mott limit. The ordering vector of this charge crystal state is linearly connected to the charge-density-waves of underdoped RBCO, suggesting that the former phase is a precursor to the latter as hole doping is increased. Most importantly, the coexistence of charge and spin order in RBCO suggests that this electronic symmetry-breaking state is common to the CuO2 planes in the very lightly-doped regime. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role of Coulomb-frustrated electronic phase separation among all cuprates.

cond-mat.str-el↗