arXiv subjects

Raymond W. Simmonds

Publications and source records attributed to Raymond W. Simmonds.

At least 19 recordsLinked to original sources

arXiv2026

WSi weak link element with a non-sinusoidal current-phase relation

Nonlinearity is an essential ingredient for encoding quantum states with non-uniform energy spacing, implementing coherent quantum gates, reading out qubits, amplifying, and mixing electromagnetic signals. In this work, we demonstrate the nonlinear behavior of a constriction fabricated from an amorphous, high-kinetic inductance material, tungsten silicide, embedded in a three-dimensional RF-SQUID. We find that the results are consistent with the weak link behaving as a Josephson junction with a sawtooth-like current-phase relation or a quantum phase slip element. Finally, we measure relaxation times of the metastable, persistent-current states trapped in the local minima of the potential.

quant-ph↗

arXiv2026

High-fidelity iSWAP gate with Double Transmon Coupler

Entangling operations are at the heart of all approaches to quantum information processing. Parametric gates, in particular, offer a versatile solution to strongly couple off-resonant superconducting qubits with suppressed parasitic crosstalk to spectator qubits due to frequency-selective activation. In this work, we demonstrate a parametric iSWAP gate between two transmon qubits using the recently developed double transmon coupler (DTC). The DTC supports robust internally-defined cancellation point (``off'' state) for static interactions, while simultaneously mediating a fast parametric coupling between data qubits that can be deployed for high-fidelity two-qubit operations. We use robust phase estimation to calibrate non-commuting error terms in the parametric iSWAP gate, and achieve a 99.827% gate fidelity in 40ns without any numerical optimization. The circuit architecture and calibration techniques developed here are extensible to other gate implementations and qubit modalities, paving the way towards resource-efficient quantum information processing.

quant-ph↗

arXiv2026

Magnetic flux distribution, quasiparticle spectroscopy, and quality factors in Nb films for superconducting qubits

Niobium is a practical material platform for superconducting microwave circuits; however, device-level performance can vary significantly depending on film growth and processing conditions. We compare three epitaxial Nb films grown on $c-$plane sapphire substrates under nominally identical conditions, except for the deposition temperature. To correlate internal quality factors, $Q_{\mathrm {i}}$, with material properties, we combine magneto-optical imaging of magnetic flux distribution with quasiparticle spectroscopy via measurements of the London penetration depth, $\lambda(T)$. In the low-$Q_{\mathrm i}$ film, there is a lesser ability to screen the magnetic field and an irregular temperature variation of $\lambda(T)$, implying the existence of localized in-gap states. High $Q_{\mathrm i}$ films show the opposite trend. We conclude that our measurements provide an efficient method for characterizing and optimizing superconducting films for quantum informatics applications.

cond-mat.supr-con↗

arXiv2026

Quasiparticle spectroscopy in tantalum films with different Ta/sapphire interfaces

One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoherence. Here, we present results from quasiparticle spectroscopy of Ta/sapphire films by measuring the Meissner-state magnetic susceptibility using a precision frequency-domain resonator specifically designed for thin films. We find direct evidence for additional low-energy excitations in samples with lower internal quality factors. These excitations are consistent with deep subgap states due to two-level systems, Yu-Shiba-Rusinov states near the gap edge, and perhaps other pair-breaking mechanisms. The developed non-destructive frequency-domain quasiparticle spectroscopy is a valuable addition to the quantum materials toolbox.

cond-mat.supr-con↗

arXiv2025

Cryogenic growth of aluminum: structural morphology, optical properties, superconductivity and microwave dielectric loss

We explore the molecular beam epitaxy synthesis of superconducting aluminum thin films grown on c-plane sapphire substrates at cryogenic temperatures of 6 K and compare their behavior with films synthesized at room temperature. We demonstrate that cryogenic growth increases structural disorder, producing crystalline grains that modify the optical, electrical, and superconducting properties of aluminum. We observe that cryogenic deposition changes the color of aluminum from fully reflective to yellow and correlate the pseudo-dielectric function and reflectance with structural changes in the film. We find that smaller grain sizes enhance the superconductivity of aluminum, increasing its critical temperature and critical field. We then estimate the superconducting gap and coherence length of Cooper pairs in aluminum in the presence of disorder. Finally, we fabricate superconducting microwave resonators on these films and find that, independently of the growth temperature, the system is dominated by two-level system loss with similar quality factors in the high and low power regimes. We further measure a higher kinetic inductance in the cryogenically grown films.

cond-mat.supr-con↗

arXiv2024

Cryogenic growth of tantalum thin films for low-loss superconducting circuits

Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have explored the effect of cryogenic temperatures on the growth of Ta and its integration in superconducting circuits. Cryogenic growth of Ta using a low temperature molecular beam epitaxy (MBE) system is found to stabilize single phase $\alpha$-Ta on several different substrates, which include Al$\mathrm{_2}$O$\mathrm{_3}$(0001), Si(001), Si(111), SiN${_x}$, and GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain < 20 K during the Ta deposition. X-ray $\theta$-2$\theta$ diffraction after warming to room temperature indicates the formation of polycrystalline $\alpha$-Ta. The 50 nm $\alpha$-Ta films grown on Al$\mathrm{_2}$O$\mathrm{_3}$(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity ($\mathrm{\rho _{300 K}}$) of 13.4 $\mathrm{\mu \Omega}$cm, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (T$_C$) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an RMS roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher $\mathrm{\rho _{300 K}}$, lower RRR and Tc, and increased $\beta$-Ta content. Coplanar waveguide resonators with a gap width of 3 $\mathrm{\mu}$m fabricated from cryogenically grown Ta on Si(111) and Al$\mathrm{_2}$O$\mathrm{_3}$(0001) show low power Q$_i$ of 1.9 million and 0.7 million, respectively, indicating polycrystalline $\alpha$-Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.

cond-mat.supr-con↗

arXiv2021

Direct observation of deterministic macroscopic entanglement

Quantum entanglement of mechanical systems emerges when distinct objects move with such a high degree of correlation that they can no longer be described separately. Although quantum mechanics presumably applies to objects of all sizes, directly observing entanglement becomes challenging as masses increase, requiring measurement and control with a vanishingly small error. Here, using pulsed electromechanics, we deterministically entangle two mechanical drumheads with masses of 70 pg. Through nearly quantum-limited measurements of the position and momentum quadratures of both drums, we perform quantum state tomography and thereby directly observe entanglement. Such entangled macroscopic systems are uniquely poised to serve in fundamental tests of quantum mechanics, enable sensing beyond the standard quantum limit, and function as long-lived nodes of future quantum networks.

quant-ph↗

arXiv2021

Towards merged-element transmons using silicon fins: the FinMET

A merged-element transmon (MET) device, based on silicon (Si) fins, is proposed and the first steps to form such a "FinMET" are demonstrated. This new application of fin technology capitalizes on the anisotropic etch of Si(111) relative to Si(110) to define atomically flat, high aspect ratio Si tunnel barriers with epitaxial superconductor contacts on the parallel side-wall surfaces. This process circumvents the challenges associated with the growth of low-loss insulating barriers on lattice matched superconductors. By implementing low-loss, intrinsic float-zone Si as the barrier material rather than commonly used, potentially lossy AlOx, the FinMET is expected to overcome problems with standard transmons by (1) reducing dielectric losses, (2) minimizing the formation of two-level system spectral features, (3) exhibiting greater control over barrier thickness and qubit frequency spread, especially when combined with commercial fin fabrication and atomic-layer digital etching; (4) potentially reducing the footprint by several orders of magnitude; and (5) allowing scalable fabrication. Here, as a first step to making such a device, the fabrication of Si fin capacitors on Si(110) substrates with shadow-deposited Al electrodes is demonstrated. These fin capacitors are then fabricated into lumped element resonator circuits and probed using low-temperature microwave measurements. Further thinning of silicon junctions towards the tunneling regime will enable the scalable fabrication of FinMET devices based on existing silicon technology, while simultaneously avoiding lossy amorphous dielectrics for the tunnel barriers.

quant-ph↗

arXiv2019

Fast, High-Fidelity, Quantum Non-demolition Readout of a Superconducting Qubit Using a Transverse Coupling

While relatively easy to engineer, static transverse coupling between a qubit and a cavity mode satisfies the criteria for a quantum non-demolition (QND) measurement only if the coupling between the qubit and cavity is much less than their mutual detuning. This can put significant limits on the speed of the measurement, requiring trade-offs in the circuit design between coupling, detuning, and decoherence introduced by the cavity mode. Here, we study a circuit in which the qubit-cavity and the cavity-feedline coupling can be turned on and off, which helps to isolate the qubit. We do not rely on the rotating-wave or dispersive approximations, but solve the full transverse interaction between the qubit and the cavity mode. We show that by carefully choosing the detuning and interaction time, we can exploit a recurrence in the qubit-cavity dynamics in a way that makes it possible to perform very fast, high fidelity, QND measurements. Here, the qubit measurement is performed more like a gate operation between the qubit and the cavity, where the cavity state can be amplified, squeezed, and released in a time-sequenced fashion. In addition, we also show that the non-demolition property of the off-resonant approximation breaks down much faster than its dispersive property, suggesting that many of the dispersive measurements to date have been implemented outside the QND regime.

quant-ph↗

arXiv2017

Reconfigurable re-entrant cavity for wireless coupling to an electro-optomechanical device

An electro-optomechanical device capable of microwave-to-optics conversion has recently been demonstrated, with the vision of enabling optical networks of superconducting qubits. Here we present an improved converter design that uses a three-dimensional (3D) microwave cavity for coupling between the microwave transmission line and an integrated LC resonator on the converter chip. The new design simplifies the optical assembly and decouples it from the microwave part of the setup. Experimental demonstrations show that the modular device assembly allows us to flexibly tune the microwave coupling to the converter chip while maintaining small loss. We also find that electromechanical experiments are not impacted by the additional microwave cavity. Our design is compatible with a high-finesse optical cavity and will improve optical performance.

quant-ph↗

arXiv2016

Hybrid quantum systems with trapped charged particles

We study theoretically the possibilities of coupling the quantum mechanical motion of a trapped charged particle (e.g. ion or electron) to quantum degrees of freedom of superconducting devices, nano-mechanical resonators and quartz bulk acoustic wave resonators. For each case, we estimate the coupling rate between the charged particle and its macroscopic counterpart and compare it to the decoherence rate, i.e. the rate at which quantum superposition decays. A hybrid system can only be considered quantum if the coupling rate significantly exceeds all decoherence rates. Our approach is to examine specific examples, using parameters that are experimentally attainable in the foreseeable future. We conclude that those hybrid quantum system considered involving an atomic ion are unfavorable, compared to using an electron, since the coupling rates between the charged particle and its counterpart are slower than the expected decoherence rates. A system based on trapped electrons, on the other hand, might have coupling rates which significantly exceed decoherence rates. Moreover it might have appealing properties such as fast entangling gates, long coherence and flexible electron interconnectivity topology. Realizing such a system, however, is technologically challenging, since it requires accommodating both trapping technology and superconducting circuitry in a compatible manner. We review some of the challenges involved, such as the required trap parameters, electron sources, electrical circuitry and cooling schemes in order to promote further investigations towards the realization of such a hybrid system.

quant-ph↗

arXiv2016

Sideband Cooling Beyond the Quantum Limit with Squeezed Light

Quantum fluctuations of the electromagnetic vacuum impose an observable quantum limit to the lowest temperatures that can be reached with conventional laser cooling techniques. As laser cooling experiments continue to bring massive mechanical systems to unprecedented temperatures, this quantum limit takes on increasingly greater practical importance in the laboratory. Fortunately, vacuum fluctuations are not immutable, and can be "squeezed" through the generation of entangled photon pairs. Here we propose and experimentally demonstrate that squeezed light can be used to sideband cool the motion of a macroscopic mechanical object below the quantum limit. To do so, we first cool a microwave cavity optomechanical system with a coherent state of light to within 15% of this limit. We then cool by more than 2 dB below the quantum limit using a squeezed microwave field generated by a Josephson Parametric Amplifier (JPA). From heterodyne spectroscopy of the mechanical sidebands, we measure a minimum thermal occupancy of 0.19 phonons. With this novel technique, even low frequency mechanical oscillators can in principle be cooled arbitrarily close to the motional ground state, enabling the exploration of quantum physics in larger, more massive systems.

quant-ph↗

arXiv2016

Observation of Strong Radiation Pressure Forces from Squeezed Light on a Mechanical Oscillator

Quantum enhanced sensing is a powerful technique in which nonclassical states are used to improve the sensitivity of a measurement. For enhanced mechanical displacement sensing, squeezed states of light have been shown to reduce the photon counting noise that limits the measurement noise floor. It has long been predicted, however, that suppressing the noise floor with squeezed light should produce an unavoidable increase in radiation pressure noise that drives the mechanical system. Such nonclassical radiation pressure forces have thus far been hidden by insufficient measurement strengths and residual thermal mechanical motion. Since the ultimate measurement sensitivity relies on the delicate balance between these two noise sources, the limits of the quantum enhancement have not been observed. Using a microwave cavity optomechanical system, we observe the nonclassical radiation pressure noise that necessarily accompanies any quantum enhancement of the measurement precision. By varying both the magnitude and phase of the squeezing, we optimize the fundamental trade-off between mechanical imprecision and backaction noise in accordance with the Heisenberg uncertainty principle. As the strength of the measurement is further increased, radiation pressure forces eventually dominate the mechanical motion. In this regime, the optomechanical interaction can be exploited as an efficient quantum nondemolition (QND) measurement of the amplitude fluctuations of the light field. By overwhelming mechanical thermal noise with radiation pressure by two orders of magnitude, we demonstrate a mechanically-mediated measurement of the squeezing with an effective homodyne efficiency of 94%. Thus, with strong radiation pressures forces, mechanical motion enhances the measurement of nonclassical light, just as nonclassical light enhances the measurement of the motion.

quant-ph↗

arXiv2013

Tunable coupling to a mechanical oscillator circuit using a coherent feedback network

We demonstrate a fully cryogenic microwave feedback network composed of modular superconducting devices connected by transmission lines and designed to control a mechanical oscillator coupled to one of the devices. The network features an electromechanical device and a tunable controller that coherently receives, processes and feeds back continuous microwave signals that modify the dynamics and readout of the mechanical state. While previous electromechanical systems represent some compromise between efficient control and efficient readout of the mechanical state, as set by the electromagnetic decay rate, the tunable controller produces a closed-loop network that can be dynamically and continuously tuned between both extremes much faster than the mechanical response time. We demonstrate that the microwave decay rate may be modulated by at least a factor of 10 at a rate greater than $10^4$ times the mechanical response rate. The system is easy to build and suggests that some useful functions may arise most naturally at the network-level of modular, quantum electromagnetic devices.

cond-mat.supr-con↗

arXiv2013

Modulated Electromechanics: Large Enhancements of Nonlinearities

It is well-known that the nonlinear coupling between a mechanical oscillator and a superconducting oscillator or optical cavity can be used to generate a Kerr-nonlinearity for the cavity mode. We show that the strength of this Kerr-nonlinearity, as well as the effect of the photon-pressure force can be enormously increased by modulating the strength of the nonlinear coupling. We present an electromechanical circuit in which this enhancement can be readily realized.

quant-ph↗

arXiv2012

Dynamical Autler-Townes control of a phase qubit

Routers, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmission.

cond-mat.mes-hall↗

arXiv2011

Quantum interference between two single photons of different microwave frequencies

We have measured quantum interference between two single microwave photons trapped in a superconducting resonator, whose frequencies are initially about 6 GHz apart. We accomplish this by use of a parametric frequency conversion process that mixes the mode currents of two cavity harmonics through a superconducting quantum interference device, and demonstrate that a two-photon entanglement operation can be performed with high fidelity.

cond-mat.mes-hall↗

arXiv2011

Entangled State Synthesis for Superconducting Resonators

We present a theoretical analysis of methods to synthesize entangled states of two superconducting resonators. These methods use experimentally demonstrated interactions of resonators with artificial atoms, and offer efficient routes to generate nonclassical states. We analyze physical implementations, energy level structure, and the effects of decoherence through detailed dynamical simulations.

quant-ph↗