Academic literature on the topic 'Radio-Frequency superconducting qubit'

We describe a digital microwave platform called Presto, designed for measurement and control of multiple quantum bits (qubits) and based on the third-generation radio-frequency system on a chip. Presto uses direct digital synthesis to create signals up to 9 GHz on 16 synchronous output ports, while synchronously analyzing responses on 16 input ports. Presto has 16 DC-bias outputs, four inputs and four outputs for digital triggers or markers, and two continuous-wave outputs for synthesizing frequencies up to 15 GHz. Scaling to a large number of qubits is enabled through deterministic synchronization of multiple Presto units. A Python application programming interface configures a firmware for synthesis and analysis of pulses, coordinated by an event sequencer. The analysis integrates template matching (matched filtering) and low-latency (184–254 ns) feedback to enable a wide range of multi-qubit experiments. We demonstrate Presto’s capabilities with experiments on a sample consisting of two superconducting qubits connected via a flux-tunable coupler. We show single-shot readout and active reset of a single qubit; randomized benchmarking of single-qubit gates showing 99.972% fidelity, limited by the coherence time of the qubit; and calibration of a two-qubit iSWAP gate.

Background. The need to implement controlled coupling between qubits, which are the logical elements of quantum devices such as quantum computers and quantum networks, requires, along with the use of traditional methods, the development of new, more effective ways to organize the interaction of qubits with the microwave fields of resonators used to generate and control the entanglement of qubits. As one of these methods, a method based on the influence of frequency-regulated radio frequency signals on a superconducting Josephson qubit connected by a large Josephson junction to a free qubit has been proposed. Aim. The influence of the Kerr medium of the resonator, in which one of the two qubits is placed, on their entanglement induced by the coherent or thermal frequency-regulated radio frequency field of the resonator is considered. Methods. To analyze the dynamics of the system under consideration, the solution of the quantum Liouville equation for the full density matrix is studied. An exact solution o this equation is found in the case of initial separable and entangled states of qubits. The exact solution of the evolution equation is used to calculate the criterion of qubit-qubit entanglement – cconcurrence. Numerical modeling of the concurrence was carried out for various states of qubits, coherent and thermal fields of the resonator, as well as various values of the intensity of the resonator field and the Kerr nonlinearity parameter. Results. It is shown that for separable initial states of qubits, the inclusion of Kerr nonlinearity reduces the maximum degree of entanglement of qubits. For an entangled initial state of qubits, the possibility of creating long-lived entangled states in the presence of Kerr nonlinearity is shown. Conclusion. The type of initial states of qubits and the range of values of the intensities of the resonator fields and the Kerr nonlinearity parameters have been established, for which the most effective control and operation of the evolution of qubits, as well as the degree of their entanglement, in the physical system under consideration, is possible.

We present a control and measurement setup for superconducting qubits based on the Xilinx 16-channel radio-frequency system-on-chip (RFSoC) device. The proposed setup consists of four parts: multiple RFSoC boards, a setup to synchronize every digital to analog converter (DAC) and analog to digital converter (ADC) channel across multiple boards, a low-noise direct current supply for tuning the qubit frequency, and cloud access for remotely performing experiments. We also designed the setup to be free of physical mixers. The RFSoC boards directly generate microwave pulses using sixteen DAC channels up to the third Nyquist zone, which are directly sampled by its eight ADC channels between the fifth and the ninth zones.

Detecting weak radio-frequency electromagnetic fields plays a crucial role in a wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum optics, the ultimate limit of a weak field is a single photon. Detecting and manipulating single photons at megahertz frequencies presents a challenge because, even at cryogenic temperatures, thermal fluctuations are appreciable. Using a gigahertz superconducting qubit, we observed the quantization of a megahertz radio-frequency resonator, cooled it to the ground state, and stabilized Fock states. Releasing the resonator from our control, we observed its rethermalization with nanosecond resolution. Extending circuit quantum electrodynamics to the megahertz regime, we have enabled the exploration of thermodynamics at the quantum scale and allowed interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.

Abstract To reach large-scale quantum computing, three-dimensional integration of scalable qubit arrays and their control electronics in multi-chip assemblies is promising. Within these assemblies, the use of superconducting interconnections, as routing layers, offers interesting perspective in terms of (1) thermal management to protect the qubits from control electronics self-heating, (2) passive device performance with significant increase of quality factors and (3) density rise of low and high frequency signals thanks to minimal dispersion. We report on the fabrication, using 200 mm silicon wafer technologies, of a multi-layer routing platform designed for the hybridation of spin qubit and control electronics chips. A routing level couples the qubits and the control circuits through one layer of Al0.995Cu0.005 and superconducting layers of TiN, Nb or NbN, connected between them by W-based vias. Wafer-level parametric tests at 300 K validate the yield of these technologies while low temperature electrical measurements in cryostat are used to extract the superconducting properties of the routing layers. Preliminary low temperature radio-frequency characterizations of superconducting passive elements, embedded in these routing levels, are presented.

AbstractMassive mechanical resonators operating at the quantum scale can enable a large variety of applications in quantum technologies as well as fundamental tests of quantum theory. Of crucial importance in that direction is both their integrability into state-of-the-art quantum platforms as well as the ability to prepare them in generic quantum states using well-controlled high-fidelity operations. Here, we propose a scheme for controlling a radio-frequency mechanical resonator at the quantum scale using two superconducting transmon qubits that can be integrated on the same chip. Specifically, we consider two qubits coupled via a capacitor in parallel to a superconducting quantum interference device (SQUID), which has a suspended mechanical beam embedded in one of its arms. Following a theoretical analysis of the quantum system, we find that this configuration, in combination with an in-plane magnetic field, can give rise to a tuneable three-body interaction in the single-photon strong-coupling regime, while enabling suppression of the stray qubit-qubit coupling. Using state-of-the-art parameters and qubit operations at single-excitation levels, we numerically demonstrate the possibility of ground-state cooling as well as high-fidelity preparation of mechanical quantum states and qubit-phonon entanglement, i.e. states having negative Wigner functions and obeying non-classical correlations. Our work significantly extends the quantum control toolbox of radio-frequency mechanical resonators and may serve as a promising architecture for integrating such mechanical elements with transmon-based quantum processors.

In gate-based dispersive sensing, the response of a resonator attached to a quantum dot gate is detected by a reflected radio frequency signal. This enables fast readout of spin qubits and tune up of arrays of quantum dots but comes at the expense of increased susceptibility to crosstalk, as the resonator can amplify spurious signals and induce fluctuations in the quantum dot potential. We attach tank circuits with superconducting NbN inductors and internal quality factors Qi>1000 to the interdot barrier gate of silicon double quantum dot devices. Measuring the interdot transition in transport, we quantify radio frequency crosstalk that results in a ring-up of the resonator when neighboring plunger gates are driven with frequency components matching the resonator frequency. This effect complicates qubit operation and scales with the loaded quality factor of the resonator, the mutual capacitance between device gate electrodes, and with the inverse of the parasitic capacitance to ground. Setting qubit frequencies below the resonator frequency is expected to substantially suppress this type of crosstalk.

We sympathetically cool highly charged ions (HCI) in Coulomb crystals of Doppler-cooled Be+ ions confined in a cryogenic linear Paul trap that is integrated into a fully enclosing radio-frequency resonator manufactured from superconducting niobium. By preparing a single Be+ cooling ion and a single HCI, quantum logic spectroscopy toward frequency metrology and qubit operations with a great variety of species are enabled. While cooling down the assembly through its transition temperature into the superconducting state, an applied quantization magnetic field becomes persistent, and the trap becomes shielded from subsequent external electromagnetic fluctuations. Using a magnetically sensitive hyperfine transition of Be+ as a qubit, we measure the fractional decay rate of the stored magnetic field to be at the 10−10 s−1 level. Ramsey interferometry and spin-echo measurements yield coherence times of >400 ms, demonstrating excellent passive magnetic shielding at frequencies down to DC.

Superconducting qubits have arisen as a leading technology platform for quantum computing, which is on the verge of revolutionizing the world's calculation capacities. Nonetheless, the fabrication of computationally reliable qubit circuits requires increasing the quantum coherence lifetimes, which are predominantly limited by the dissipations of two-level system defects present in the thin superconducting film and the adjacent dielectric regions. In this paper, we demonstrate the reduction of two-level system losses in three-dimensional superconducting radio frequency niobium resonators by atomic layer deposition of a 10 nm aluminum oxide Al2O3 thin films, followed by a high vacuum heat treatment at 650 °C for few hours. By probing the effect of several heat treatments on Al2O3-coated niobium samples by x-ray photoelectron spectroscopy plus scanning and conventional high resolution transmission electron microscopy coupled with electron energy loss spectroscopy and energy dispersive spectroscopy, we witness a dissolution of niobium native oxides and the modification of the Al2O3-Nb interface, which correlates with the enhancement of the quality factor at low fields of two 1.3 GHz niobium cavities coated with 10 nm of Al2O3.

Les circuits supraconducteurs sont des systèmes quantiques artificiels dont les propriétés peuvent être choisies pour répondre aux exigences de chaque application spécifique. Un circuit supraconducteur typique est conçu pour avoir une transition suffisamment anharmonique pour être utilisée comme un qubit, qui peut être facilement manipulé et lu grâce à son interaction forte (dipolaire) avec le champ électromagnétique. Un fort moment dipolaire fort est particulièrement intéressant en vue d'interfacer le circuit supraconducteur avec d'autres systèmes quantiques. Par exemple, la fluorescence de spins électroniques individuels a été détectée avec succès en utilisant un détecteur de photons micro-ondes basé sur un qubit supraconducteur, opérant dans la bande des 5-10 GHz. Dans le domaine de l'acousto-dynamique sur circuit (cQAD), le couplage entre un qubit et un résonateur piézoélectrique est utilisé pour détecter et manipuler l'état phononique, typiquement dans la plage 2-10 GHz. Cependant, adapter ces schémas de détection à des fréquences inférieures, en dessous de la fréquence de fonctionnement conventionnelle des qubits supraconducteurs, s'accompagne de nouveaux défis. D'abord, les qubits supraconducteurs sont lus grâce à l'intéraction dispersive avec un résonateur supraconducteur auxiliaire. Le couplage dispersif diminuant rapidement lorsque le désaccord dépasse l'anharmonicité du qubit, les qubits faiblement anharmoniques, tels que les transmons, nécessitent des résonateurs presque résonants dont les dimensions évoluent inversement avec la fréquence (par exemple, une cavité coplanaire λ/2 de 1 MHz nécessite un guide d'onde de 100 m de long). Deuxièmement, les systèmes à basse fréquence sont couplés à un bain thermique chaud avec lequel ils échangent des photons de manière aléatoire, transformant rapidement les états quantiques purs en mélanges statistiques. Le qubit fluxonium, composé d'une jonction Josephson court-circuitée simultanément par une grande inductance et une capacité, présente des opportunités uniques dans le domaine des qubits supraconducteurs à basse fréquence. Dans ce travail, nous démontrons un fluxonium lourd avec une fréquence de transition exceptionnellement basse de 1.8 MHz, tout en maintenant la capacité de manipuler et de lire le qubit en utilisant des techniques micro-ondes standard. Cela est rendu possible par le spectre d'énergie hautement non linéaire du fluxonium, où la première transition se produit dans la plage des MHz tandis que les transitions vers des états excités supérieurs sont dans la plage 3-10 GHz. Nous démontrons avec succès le refroidissement par bande latérale résolue du fluxonium, réduisant sa température effective à 23 μK et atteignant une population de l'état fondamental de 97,7%. Nos expériences révèlent en outre les capacités de manipulation cohérente du qubit, avec des temps de cohérence de T1 = 34 μs et T2* = 39 μs, accompagnés d'une lecture d'état projective.Nous démontrons en outre la sensibilité accrue du qubit aux champs de radiofréquence, obtenue par interaction directe avec un guide d'onde couplé capacitivement. En employant un protocole de préparation et de mesure cyclique, nous transformons le fluxonium en un capteur de charge résolu en fréquence précis, affichant une sensibilité de charge de 33 μe/√Hz. Cela se traduit par une sensibilité énergétique de 2.8ℏ par hertz, rivalisant avec les capteurs basés sur le transport, tout en restant intrinsèquement immunisé aux offsets de charge DC. La grande capacité de l'électrode ce notre capteur de charge basé sur le fluxonium (~50 fF) est très bénéfique dans les applications réelles de détection de charge, où la sensibilité est diluée lorsque la capacité propre du système sondé dépasse celle du capteur. Ce travail ouvre la voie à de nouvelles investigations expérimentales sur les phénomènes quantiques dans la plage de 1 à 10 MHz, y compris le régime de couplage fort avec des résonateurs mécaniques macroscopiques
Radio-frequency fluxonium superconducting circuit for AC-charge sensing applicationsSuperconducting-circuits are artificial quantum systems whose properties can be engineered to match the requirements of each specific application. A typical superconducting circuit is engineered to have a sufficiently an-harmonic transition to be used as a qubit, which can be easily manipulated and read-out thanks to its strong (dipolar) interaction with electromagnetic fields. The property of having a strong dipole moment is particularly interesting for interfacing a superconducting circuit with other quantum systems. For instance, fluorescence from individual electronic spins was successfully detected using a superconducting qubit-based microwave-photon detector operating in the 5-10 GHz band. In the realm of circuit quantum acousto-dynamics (cQAD), the coupling between a qubit and a piezoelectric resonator is used to detect and manipulate the phononic state, typically within the 2-10 GHz range. However, adapting these sensing schemes to lower frequencies, below the conventional operating frequency of superconducting qubits, introduces distinct challenges. First, superconducting qubits are read out thanks to the dispersive shift imparted to a nearby superconducting resonator. As the dispersive shift quickly drops for a cavity detuning exceeding the qubit anharmonicity, weakly anharmonic qubits, such as transmons, require nearly resonant resonators with dimensions scaling inversely with the frequency (as an illustration, a 1 MHz λ/2-coplanar cavity requires a 100-m-long waveguide). Second, low-frequency systems are coupled to a hot thermal bath with which they exchange photons randomly, quickly turning pure quantum states into statistical mixtures. The fluxonium qubit, composed of a Josephson junction shunted simultaneously by a large inductance and a capacitance, presents unique opportunities in the realm of low-frequency superconducting qubits.In this work, we demonstrate a heavy fluxonium with an unprecedentedly low transition frequency of 1.8 MHz, while maintaining the ability to manipulate and read out the qubit using standard microwave techniques. This is made possible by the highly non-linear energy spectrum of the fluxonium, where the first transition occurs in the MHz range while transitions to higher excited states are within the 3-10 GHz range. We successfully demonstrate resolved sideband cooling of the fluxonium, reducing its effective temperature to 23 μK and achieving a ground state population of 97.7%. Our experiments further reveal the qubit's coherent manipulation capabilities, with coherence times of T1=34 μs and T2*=39 μs, along with reliable single-shot state readout.We furthermore demonstrate the qubit's enhanced sensitivity to radio-frequency fields, achieved through direct interaction with a capacitively coupled waveguide. By employing cyclic preparation and measurement protocols, we transform the fluxonium into a precise frequency-resolved charge sensor, boasting a charge sensitivity of 33 μe/√Hz. This translates to an energy sensitivity of 2.8ℏ per hertz, rivaling state-of-the-art transport-based sensors while remaining inherently resistant to dc-charge noise. The large gate-capacitance of our fluxonium-based charge sensor (~50 fF) is highly beneficial in real-world charge sensing applications, where the sensitivity gets diluted when the self-capacitance of the probed system exceeds that of the sensor. This work paves the way for new experimental investigations into quantum phenomena within the 1-10 MHz range, including the strong-coupling regime with macroscopic mechanical resonators