It is known that the quantum nature of the electromagnetic vacuum is responsible for the Lamb shift, which is a crucial phenomenon in quantum electrodynamics (QED). In circuit QED,the readout or bus resonators that are dispersively coupled can result in a significant Lamb shift, much larger than that in the original broadband cases. However, previous approaches or proposals for controlling the Lamb shift in circuit QED demand overheads in circuit designs or non-perturbative renormalization of the system’s eigenbases, which can impose formidable this http URL this work, we propose and demonstrate an efficient and cost-effective method for controlling the Lamb shift of fixed-frequency transmons. We employ the drive-induced longitudinal coupling between the transmon and resonator. By simply using an off-resonant monochromatic driving near the resonator frequency, we can regulate the Lamb shift from 32 to -30 MHz without facing the aforementioned challenges. Our work establishes an efficient way of engineering the fundamental effects of the electromagnetic vacuum and provides greater flexibility in non-parametric frequency controls of multilevel systems.
Harmonic oscillators belong to the most fundamental concepts in physics and are central to many current research fields such as circuit QED, cavity optomechanics and photon-pressuresystems. Here, we engineer an effective negative mass harmonic oscillator mode in a superconducting microwave LC circuit and couple it via photon-pressure to a second low-frequency circuit. We demonstrate that the effective negative mass leads to an inversion of dynamical backaction and to sideband-cooling of the low-frequency circuit by a blue-detuned pump field, naturally explained by the inverted energy ladder of the negative mass oscillator.
Microwave driving is a ubiquitous technique for superconducting qubits (SCQs), but the dressed states description based on the conventionally used perturbation theory and rotating waveapproximation cannot fully capture the dynamics in the strong driving limit. Comprehensive experimental works beyond these approximations applicable for transmons is unfortunately rare, which receive rising interests in quantum technologies. In this work, we investigate a microwave-dressed transmon over a wide range of driving parameters. We find significant renormalization of Rabi frequencies, energy relaxation times, and the coupling rates with a readout resonator, all of which are not quantified without breaking the conventional approximations. We also establish a concise non-Floquet theory beyond the two-state model while dramatically minimizing the approximations, which excellently quantifies the experiments. This work expands our fundamental understanding of time-periodically driven systems and will have an important role in accurately estimating the dynamics of weakly anharmonic qubits. Furthermore, our non-Floquet approach is beneficial for theoretical analysis since one can avoid additional efforts such as the choice of proper Floquet modes, which is more complicated for multi-level systems.
Nonlinear damping, a force of friction that depends on the amplitude of motion, plays an important role in many electrical, mechanical and even biological oscillators. In novel technologiessuch as carbon nanotubes, graphene membranes or superconducting resonators, the origin of nonlinear damping is sometimes unclear. This presents a problem, as the damping rate is a key figure of merit in the application of these systems to extremely precise sensors or quantum computers. Through measurements of a superconducting circuit, we show that nonlinear damping can emerge as a direct consequence of quantum fluctuations and the conservative nonlinearity of a Josephson junction. The phenomenon can be understood and visualized through the flow of quasi-probability in phase space, and accurately describes our experimental observations. Crucially, the effect is not restricted to superconducting circuits: we expect that quantum fluctuations or other sources of noise give rise to nonlinear damping in other systems with a similar conservative nonlinearity, such as nano-mechanical oscillators or even macroscopic systems.
Observing quantum phenomena in macroscopic objects, and the potential discovery of a fundamental limit in the applicability of quantum mechanics, has been a central topic of modernexperimental physics. Highly coherent and heavy micro-mechanical oscillators controlled by superconducting circuits are a promising system for this task. Here, we focus in particular on the electrostatic coupling of motion to a weakly anharmonic circuit, namely the transmon qubit. In the case of a megahertz mechanical oscillator coupled to a gigahertz transmon, we explain the difficulties in bridging the large electro-mechanical frequency gap. To remedy this issue, we explore the requirements to reach phonon-number resolution in the resonant coupling of a megahertz transmon and a mechanical oscillator.
Driving quantum systems periodically in time plays an essential role in the coherent control of quantum states. A good approximation for weak and nearly resonance driving fields, experimentsoften require large detuning and strong driving fields, for which the RWA may not hold. In this work, we experimentally, numerically, and analytically explore strongly driven two-mode Josephson circuits in the regime of strong driving and large detuning. Specifically, we investigate beam-splitter and two-mode squeezing interaction between the two modes induced by driving two-photon sideband transition. Using numerical simulations, we observe that the RWA is unable to correctly capture the amplitude of the sideband transition rates, which we verify using an analytical model based on perturbative corrections. Interestingly, we find that the breakdown of the RWA in the regime studied does not lead to qualitatively different dynamics, but gives the same results as the RWA theory at higher drive strengths, enhancing the coupling rates compared to what one would predict. Our work provides insight into the behavior of time-periodically driven systems beyond the RWA, and provides a robust theoretical framework for including these in the calculation and calibration of quantum protocols in circuit quantum electrodynamics.
We perform extensive analysis of graphene Josephson junctions embedded in microwave circuits. By comparing a diffusive junction at 15 mK with a ballistic one at 15 mK and 1 K, we areable to reconstruct the current-phase relation.
Investigation of intrinsic loss mechanism of superconducting resonator is a crucial task toward the study of the constituent material as well as application in quantum information process.Typical approach from transmission or reflection spectrum is however subjected to Fano-effect, which can induce systematic errors in discerning intrinsic and external losses. To avoid such requires under-coupled resonator and consequently sets a challenge when a large quality factor is expected and measurements at single-photon power levels is required. In this work, we propose and demonstrate a new approach with additional qubit coupled dispersively. Inducing electromagnetically induced transparency (EIT) in qubit spectrum, we can extract the resonator’s single-photon internal linewidth. Our work demonstrates a practical application of EIT for device spectroscopy.
We present the design, measurement and analysis of a current sensor based on a process of Josephson parametric upconversion in a superconducting microwave cavity. Terminating a coplanarwaveguide with a nanobridge constriction Josephson junction, we observe modulation sidebands from the cavity that enable highly sensitive, frequency-multiplexed output of small currents for applications such as transition-edge sensor array readout. We derive an analytical model to reproduce the measurements over a wide range of bias currents, detunings and input powers. Tuning the frequency of the cavity by more than \SI{100}{\mega\hertz} with DC current, our device achieves a minimum current sensitivity of \SI{8.9}{\pico\ampere\per\sqrt{\hertz}}. Extrapolating the results of our analytical model, we predict an improved device based on our platform, capable of achieving sensitivities down to \SI{50}{\femto\ampere\per\sqrt{\hertz}}}, or even lower if one could take advantage of parametric amplification in the Josephson cavity. Taking advantage of the Josephson architecture, our approach can provide higher sensitivity than kinetic inductance designs, and potentially enables detection of currents ultimately limited by quantum noise.
We propose a scheme for controlling a radio-frequency mechanical resonator at the quantum level using a superconducting qubit. The mechanical part of the circuit consists of a suspendedmicrometer-long beam that is embedded in the loop of a superconducting quantum interference device (SQUID) and is connected in parallel to a transmon qubit. Using realistic parameters from recent experiments with similar devices, we show that this configuration can enable a tuneable optomechanical interaction in the single-photon ultrastrong-coupling regime, where the radiation-pressure coupling strength is larger than both the transmon decay rate and the mechanical frequency. We investigate the dynamics of the driven system for a range of coupling strengths and find an optimum regime for ground-state cooling, consistent with previous theoretical investigations considering linear cavities. Furthermore, we numerically demonstrate a protocol for generating hybrid discrete- and continuous-variable entanglement as well as mechanical Schrödinger cat states, which can be realised within the current state of the art. Our results demonstrate the possibility of controlling the mechanical motion of massive objects using superconducting qubits at the single-photon level and could enable applications in hybrid quantum technologies as well as fundamental tests of quantum mechanics.