arXiv subjects

Kevin D. Osborn

Publications and source records attributed to Kevin D. Osborn.

At least 19 recordsLinked to original sources

arXiv2025

Simulation of a rapid qubit readout dependent on the transmission of a single fluxon

The readout speed of qubits is a major limitation for error correction in quantum information science. We show simulations of a proposed device that gives readout of a fluxonium qubit using a ballistic fluxon with an estimated readout time of less than 1 nanosecond, without the need for an input microwave tone. This contrasts the prevalent readout based on circuit quantum electrodynamics, but is related to previous studies where a fluxon moving in a single long Josephson junction (LJJ) can exhibit a time delay depending on the state of a coupled qubit. Our readout circuit contains two LJJs and a qubit coupled at their interface. We find that the device can exhibit single-shot readout of a qubit -- one qubit state leads to a single dynamical bounce at the interface and fluxon reflection, and the other qubit state leads to a couple of bounces at the interface and fluxon transmission. Dynamics are initially computed with a separate degree of freedom for all Josephson junctions of the circuit. However, a collective coordinate model reduces the dynamics to three degrees of freedom: one for the fluxonium Josephson junction and one for each LJJ. The large mass imbalance in this model allows us to simulate the mixed quantum-classical dynamics, as an approximation for the full quantum dynamics. Calculations give backaction on the qubit at $\leq 0.1\%$.

quant-ph↗

arXiv2024

Detection of low-energy fluxons from engineered long Josephson junctions for efficient computing

Single-Flux Quantum (SFQ) digital logic is typically energy efficient and fast, and logic that uses ballistic and reversible principles provides a new platform to improve efficiency. We are studying long Josephson junctions (long JJs), SFQs within them, and an SFQ detector, all intended for future ballistic logic gate experiments. Specifically, we launch low-energy SFQ into engineered long JJs made from an array of 80 JJs and connecting inductors. The component JJs have critical currents of only 7.5 uA such that the Josephson penetration depth is approximately 2.4 unit cells, and the SFQ's stationary energy in the LJJ is ~47 zJ. The circuit measured consisted of three components: an SFQ launcher, the LJJ, and an SFQ detector that uses JJ critical currents of only 15-20 uA. The circuit was measured in two environments: at 4.2 K in a helium dunk probe and 3.5~K in a cryogen-free refrigerator. According to calculations, the SFQ may traverse the LJJ ballistically, i.e., with a small change in velocity. Data show that SFQ detection events are synchronous with SFQ launch events in both setups. The jitter extracted from the launch and arrival times is predominantly attributed to the noise in the detector. This study shows that we can create and detect low-energy SFQs made from engineered LJJs, and the importance of jitter studies for future ballistic gate measurements.

physics.app-ph↗

arXiv2023

Quantum defects from single surface exhibit strong mutual interactions

Two-level system (TLS) defects constitute a major decoherence source of quantum information science, but they are generally less understood at material interfaces than in deposited films. Here we study surface TLSs at the metal-air interface, by probing them using a quasi-uniform field within vacuum-gap (VG) capacitors of resonators. The VG capacitor has a nano-gap which creates an order-of-magnitude larger contribution from the metal-air interface than typical resonators used in circuit QED. We measure three phenomena and find qualitative agreement with an interacting TLS model, where near-resonant TLSs experience substantial frequency jitter from the state switching of far-detuned low-frequency TLSs. First, we find that the loss in all of our VG resonators is weakly or logarithmically power dependent, in contrast to data from deposited dielectric films. Second, we add a saturation tone with power $P_{in}$ to a transmission measurement and obtain the TLS Rabi frequency $Ω_{0}$. These data show a substantially weaker $P_{in}$ dependence of $Ω_{0}$ than the prediction from the standard non-interacting TLS model. Lastly, we increase the temperature and find an increased TLS jitter rate and dephasing rate from power-dependent loss and phase noise measurements, respectively. We also anneal samples, which lowers the low-frequency TLS density and jitter rate, but the single-photon loss is found to be unchanged. The results are qualitatively consistent with a fast-switching interacting-TLS model and they contrast the standard model of TLSs which describes TLSs independently.

physics.app-ph↗

arXiv2023

Asynchronous Reversible Computing Unveiled Using Ballistic Shift Registers

Reversible logic can provide lower switching energy costs relative to all irreversible logic, including those developed by industry in semiconductor circuits, however, more research is needed to understand what is possible. Superconducting logic, an exemplary platform for both irreversible and reversible logic, uses flux quanta to represent bits, and the reversible implementation may switch state with low energy dissipation relative to the energy of a flux quantum. Here we simulate reversible shift register gates that are ballistic: their operation is powered by the input bits alone. A storage loop is added relative to previous gates as a key innovation, which bestows an asynchronous property to the gate such that input bits can arrive at different times as long as their order is clearly preserved. The shift register represents bit states by flux polarity, both in the stored bit as well as the ballistic input and output bits. Its operation consists of the elastic swapping of flux between the stored and the moving bit. This is related to a famous irreversible shift register, developed prior to the advent of superconducting flux quanta logic (which used irreversible gates). In the base design of our ballistic shift register (BSR) there is one 1-input and 1-output port, but we find that we can make other asynchronous ballistic gates by extension. The gate constitutes the first asynchronous reversible 2-input gate. Finally, for a better insight into the dynamics, we introduce a collective coordinate model. We find that the gate can be described as motion in two coordinates subject to a potential determined by the input bit and initial stored flux quantum. Aside from the favorable asynchronous feature, the gate is considered practical in the context of energy efficiency, parameter margins, logical depth, and speed.

cond-mat.supr-con↗

arXiv2022

Experimentally revealing anomalously large dipoles in a quantum-circuit dielectric

Quantum two-level systems (TLSs) intrinsic to glasses induce decoherence in many modern quantum devices, such as superconducting qubits. Although the low-temperature physics of these TLSs is usually well-explained by a phenomenological standard tunneling model of independent TLSs, the nature of these TLSs, as well as their behavior out of equilibrium and at high energies above 1 K, remain inconclusive. Here we measure the non-equilibrium dielectric loss of TLSs in amorphous silicon using a superconducting resonator, where energies of TLSs are varied in time using a swept electric field. Our results show the existence of two distinct ensembles of TLSs, interacting weakly and strongly with phonons, where the latter also possesses anomalously large electric dipole moment. These results may shed new light on the low temperature characteristics of amorphous solids, and hold implications to experiments and applications in quantum devices using time-varying electric fields.

quant-ph↗

arXiv2022

Probing hundreds of individual quantum defects in polycrystalline and amorphous alumina

Quantum two-level systems (TLSs) are present in the materials of qubits and are considered defects because they limit qubit coherence. For superconducting qubits, the quintessential Josephson junction barrier is made of amorphous alumina, which hosts TLSs. However, TLSs are not understood generally -- either structurally or in atomic composition. In this study, we greatly extend the quantitative data available on TLSs by reporting on the physical dipole moment in two alumina types: polycrystalline $\mathrm{\mathrm{γ-Al}_{2}\mathrm{O}_{3}}$ and amorphous $\mathrm{a-Al}\mathrm{O_{x}}$. To obtain the dipole moments $p_z$, rather from the less-structural coupling parameter g, we tune individual TLSs with an external electric field to extract the $p_z$ of the TLSs in a cavity QED system. We find a clear difference in the dipole moment distribution from the film types, indicating a difference in TLS structures. A large sample of approximately 400 individual TLSs are analyzed from the polycrystalline film type. Their dipoles along the growth direction $p_z$ have a mean value of 2.6$\pm$0.3 Debye (D) and standard deviation $σ$ = 1.6$\pm$0.2 D . The material distribution fits well to a single Gaussian function. Approximately 200 individual TLSs are analyzed from amorphous films. Both the mean $p_z$ =4.6$\pm$0.5 D and $σ$ =2.5$\pm$0.3 D are larger. Amorphous alumina also has some very large $p_z$, > 8.6 D, in contrast to polycrystalline which has none of this moment. These large moments agree only with oxygen-based TLS models. Based on data and the candidate models (delocalized O and hydrogen-based TLSs), we find polycrystalline alumina has smaller ratio of O-based to H-based TLS than amorphous alumina.

quant-ph↗

arXiv2020

Reversible Fluxon Logic with optimized CNOT gate components

Reversible logic gates were previously implemented in superconducting circuits as adiabatic-reversible gates, which are powered with a sufficiently slow clock. In contrast, we are studying ballistic-reversible gates, where fluxons serve to both encode the information and power the gates. No power is applied to the gate apart from the energy of the input fluxons, and the two possible flux polarities represent the bit states. Undamped long Josephson junctions (LJJs), where fluxons move at practically constant speed from inertia, form the input and output channels of the gates. LJJs are connected in the gates by circuit interfaces, which are designed to allow the ballistic scattering from input to output fluxon states, using the temporary excitation of a localized mode. The duration of the resonant scattering determines the operation time of the gate, approximately a few Josephson plasma periods. Due to the coherent conversions between fluxon and localized modes the ballistic gates can be very efficient: in our simulations only a few percent of the fluxon's energy are dissipated in the gate operation. Ballistic-reversible gates can be combined with other, non-ballistic gate circuits to extend the range of gate functionalities. Here we describe how the CNOT can be built as a structure that includes the IDSN (Identity-else-Same-gives-NOT) and Store-and-Launch (SNL) gates. The IDSN is a 2-bit ballistic gate, which we describe and analyze in terms of equivalent 1-bit circuits. The SNL is a clocking gate, that allows the storage of a bit and the clocked launch of a fluxon on a bit-state dependent output path. In the CNOT the SNL gates provide the necessary routing and fluxon synchronization for the input to the IDSN gate.

quant-ph↗

arXiv2019

Reversible Fluxon Logic: Topological particles allow ballistic gates along 1D paths

Digital computing currently uses irreversible logic gates whose energy dissipation is fundamentally limited. Reversible logic gates can provide an energy-efficient alternative since they can operate with reversible processes that have no dissipation, such as with scattering processes involving elastic particles. The presented logic uses fluxons, topological solitons in Long Josephson Junctions (LJJs), as inputs into and outputs from logic gates. An advantage of using LJJs for connections is that they restrict scattering to 1D paths, in contrast to previous ballistic logic which is based on 2D scattering. Furthermore, we find through simulation that there is almost no energy loss in the scattering of fluxons between LJJs of designed unpowered circuit gates. To switch bit states, the fluxons are made to change polarity during operations -- fluxons in an input LJJ freely propagate into the gate circuit, excite a nonlinear oscillatory interface mode, and quickly scatter deterministically to an output LJJ as a fluxon or antifluxon. The numerically simulated soliton dynamics shows that over $97\%$ of the total energy is preserved as fluxon energy after gate operations. These phenomena are further analyzed with a collective coordinate ansatz, reducing the dynamics of many degrees of freedom to two coordinates which characterize fluxon and antifluxon type excitations in the input and the output LJJs. The solutions of the reduced model accurately describe the four possible energy-conserving scattering processes found in simulations of one-bit gate circuits (with two scattering directions and two output polarities). Calculated parameter tolerances indicate that the gates can be manufactured and tested. Results are shown for 1-bit gates as well as a fundamental 2-bit gate.

cond-mat.supr-con↗

arXiv2018

Ballistic reversible gates matched to bit storage: Plans for an efficient CNOT gate using fluxons

New computing technologies are being sought near the end of CMOS transistor scaling, meanwhile superconducting digital, i.e., single-flux quantum (SFQ), logic allows incredibly efficient gates which are relevant to the impending transition. In this work we present a proposed reversible logic, including gate simulations and schematics under the name of Reversible Fluxon Logic (RFL). In the widest sense it is related to SFQ-logic, however it relies on (some approximately) reversible gate dynamics and promises higher efficiency than conventional SFQ which is logically irreversible. Our gates use fluxons, a type of SFQ which has topological-particle characteristics in an undamped Long Josephson junction (LJJ). The collective dynamics of the component Josephson junctions (JJs) enable ballistic fluxon motion within LJJs as well as good energy preservation of the fluxon for JJ-circuit gates. For state changes, the gates induce switching of fluxon polarity during resonant scattering at an interface between different LJJs. Related to the ballistic nature of fluxons in LJJ, the gates are powered, almost ideally, only by data fluxon momentum in stark contrast to conventionally damped logic gates which are powered continuously with a bias. At first the fundamental Identity and NOT gates are introduced. Then 2-bit gates are discussed, including the IDSN gate which actually allows low fluxon-number inputs for more than 4 input states. A digital CNOT, an important milestone for 2-bit reversible superconducting gates, is planned as a central result. It uses a store and launch gate to stop and then later route a fluxon. This use of the store and launch gate allows a clocked CNOT gate and synchronization within. The digital CNOT gate could enable high efficiency relative to conventional irreversible gates and shows the utility of the IDSN as a reversible gate primitive.

cond-mat.mes-hall↗

arXiv2018

Coulomb blockade regions in sputter-deposited titanium nitride films

We present topographic and spectroscopic scanning tunneling microscopy measurements taken on a 21 nm thick TiN film at a temperature of 4.2 K -- above the superconducting transition temperature (T_c = 3.8 K) of the sample. The film was polycrystalline with crystallite diameters of d~19 nm, consistent with other films prepared under similar conditions. The spectroscopic maps show on average a shallow V-shape around V_b = 0 V consistent with a sample near the Mott insulation transition. In selected regions on several samples we additionally observed signs of Coulomb blockade. The corresponding peak structures are typically asymmetric with respect to bias voltage indicating coupling to two very different tunneling barriers. Furthermore, the peak structures appear with constant peak-peak spacing which indicates quantum dot states within the Coulomb blockade island. In this paper we discuss one such Coulomb blockade area and its implications in detail.

cond-mat.mes-hall↗

arXiv2016

Projected dipole moments of individual two-level defects extracted using circuit quantum electrodynamics

Material-based two-level systems (TLSs), appearing as defects in low-temperature devices including superconducting qubits and photon detectors, are difficult to characterize. In this study we apply a uniform dc-electric field across a film to tune the energies of TLSs within. The film is embedded in a superconducting resonator such that it forms a circuit quantum electrodynamical (cQED) system. The energy of individual TLSs is observed as a function of the known tuning field. By studying TLSs for which we can determine the tunneling energy, the actual $p_z$, dipole moments projected along the uniform field direction, are individually obtained. A distribution is created with 60 $p_z$. We describe the distribution using a model with two dipole moment magnitudes, and a fit yields the corresponding values $p=p_1= 2.8\pm 0.2$ Debye and $p=p_2=8.3\pm0.4$ Debye. For a strong-coupled TLS the vacuum-Rabi splitting can be obtained with $p_z$ and tunneling energy. This allows a measurement of the circuit's zero-point electric field fluctuations, in a method that does not need the electric-field volume.

cond-mat.supr-con↗

arXiv2015

Cavity quantum electrodynamics using a near-resonance two-level system: emergence of the Glauber state

Random tunneling two-level systems (TLSs) in dielectrics have been of interest recently because they adversely affect the performance of superconducting qubits. The coupling of TLSs to qubits has allowed individual TLS characterization, which has previously been limited to TLSs within (thin) Josephson tunneling barriers made from aluminum oxide. Here we report on the measurement of an individual TLS within the capacitor of a lumped-element LC microwave resonator, which forms a cavity quantum electrodynamics (CQED) system and allows for individual TLS characterization in a different structure and material than demonstrated with qubits. Due to the reduced volume of the dielectric (80 $μ$m$^{3}$), even with a moderate dielectric thickness (250 nm), we achieve the strong coupling regime as evidenced by the vacuum Rabi splitting observed in the cavity spectrum. A TLS with a coherence time of 3.2 $μ$s was observed in a film of silicon nitride as analyzed with a Jaynes-Cummings spectral model, which is larger than seen from superconducting qubits. As the drive power is increased, we observe an unusual but explicable set of continuous and discrete crossovers from the vacuum Rabi split transitions to the Glauber (coherent) state.

cond-mat.supr-con↗

arXiv2013

Bulk and Surface Tunneling Hydrogen Defects in Alumina

We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al- and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz-frequency TLSs in alumina.

cond-mat.mtrl-sci↗

arXiv2013

Universal dielectric loss in amorphous solids from simultaneous bias and microwave field

We derive the ac dielectric loss in glasses due to resonant processes created by two-level systems and a swept electric field bias. It is shown that at sufficiently large ac fields and bias sweep rates the nonequilibrium loss tangent created by the two fields approaches a universal maximum determined by the bare linear dielectric permittivity. In addition this nonequilibrium loss tangent is derived for a range of bias sweep rates and ac amplitudes and show that the loss tangent creates a predicted loss function that can be understood in a Landau-Zener theory and which can be used to extract the TLS density, dipole moment, and relaxation rate.

cond-mat.dis-nn↗

arXiv2011

Squeezed noise due to two-level system defects in superconducting resonator circuits

Motivated by recent surprising experimental results for the noise output of superconducting microfabricated resonators used in quantum computing applications and astronomy, we develop a fully quantum theoretical model to describe quantum dynamics of these circuits. Building on theoretical techniques from quantum optics, we calculate the noise in the output voltage due to two-level system (TLS) defects. The theory predicts squeezing for the noise in the amplitude quadrature with respect to the input noise, which qualitatively reproduces the noise ellipse observed in experiment. We show that noise enhancement along the phase direction persists for pump frequencies away from resonance. Our results also suggest that intrinsic TLS fluctuations must be incorporated in the model in order to describe the experimentally observed dependence of the phase noise on input power.

cond-mat.supr-con↗

arXiv2010

Loss Dependence on Geometry and Applied Power in Superconducting Coplanar Resonators

The loss in superconducting microwave resonators at low-photon number and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator geometries were studied with resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain both a coplanar strip and a quasi-lumped element. Transmitted power measurements were taken at 30 mK as a function of frequency and probe power. We find that the resonator loss, expressed as the inverse of the internal quality factor, decreases slowly over four decades of photon number in a manner not merely explained by loss from a conventional uniform spatial distribution of two-level systems in an oxide layer on the superconducting surfaces of the resonator.

cond-mat.supr-con↗

arXiv2010

Superposition of Inductive and Capacitive Coupling in Superconducting LC Resonators

We present an experimental investigation of lumped-element superconducting LC resonators designed to provide different types of coupling to a transmission line. We have designed four resonator geometries including dipole and quadrupole configured inductors connected in parallel with low loss SiNx dielectric parallel-plate capacitors. The design of the resonator allows a small change in the symmetry of the inductor or grounding of the capacitor to allow LC resonators with: 1) inductive coupling, 2) capacitive coupling, 3) both types of coupling, or 4) greatly reduced coupling. We measured all four designs at a temperature of 30mK at different values of power. We compare the extracted data from the four resonator types and find that both capacitive and inductive coupling can be included and that when left off, only a minor change in the circuit design is necessary. We also find a variation in the measured loss tangent of less than a few percent, which is a test of the systematic precision of the measurement technique.

cond-mat.supr-con↗

arXiv2010

Reducing quantum-regime dielectric loss of silicon nitride for superconducting quantum circuits

The loss of amorphous hydrogenated silicon nitride (a-SiN$_{x}$:H) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system (TLS) defect model. Each a-SiN$_{x}$:H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent $\tanδ_{0}$ in a-SiN$_{x}$:H is strongly correlated with N-H impurities, including NH$_{2}$. By slightly reducing $x$ we are able to reduce $\tanδ_0$ by approximately a factor of 50, where the best films show $\tanδ_0$ $\simeq$ 3 $\times$ 10$^{-5}$.

cond-mat.supr-con↗