Nanowire Superinductance Fluxonium Qubit

Abstract

We characterize a fluxonium qubit consisting of a Josephson junction inductively shunted with a NbTiN nanowire superinductance. We explain the measured energy spectrum by means of a multimode theory accounting for the distributed nature of the superinductance and the effect of the circuit nonlinearity to all orders in the Josephson potential. Using multiphoton Raman spectroscopy, we address multiple fluxonium transitions, observe multilevel Autler-Townes splitting and measure an excited state lifetime of T1 = 20 µs. By measuring T1 at different magnetic flux values, we find a crossover in the lifetime limiting mechanism from capacitive to inductive losses.The development of superinductors [1-5] has received significant interest due to their potential to provide noise protection in superconducting qubits [6][7][8]. Moreover, inductively shunted Josephson junction based superconducting circuits are known to be immune to charge noise [1], and to flux noise in the limit of large inductances [9][10][11][12]. Despite remarkable progress, the superinductances that have been so far reported in the literature are still small compared to those needed for qubit protection [7,8,11,12].A thin-film nanowire built from a disordered superconductor constitutes an alternative approach to reach the required superinductance regime. High-kinetic inductance superconducting materials, such as NbTiN and TiN, have been studied in the context of microwave detectors [13][14][15], parametric amplifiers [16][17][18], and rfSQUID qubits [19,20]. In a nanowire, the inertia of the Cooper pair condensate is manifested as the kinetic inductance of the superconducting wire, and can be expressed aswhere m is the free electron mass, e is the electron charge and n s is the density of Cooper pairs [14,21]. The second bracketed term in Eq. (1) is a geometric factor dependent on the length l, width w, and thickness d of the nanowire. By choosing a disordered superconductor with a low n s and fabricating a sufficiently long and thin wire, the kinetic inductance can be made large enough to reach the superinductance regime. In this regime, the presence of stray ground capacitance and the large kinetic inductance lower the frequencies of the self-resonant modes of the device. As is the case of long junction arrays [2], the multimode structure of the device needs to be taken into account to produce an accurate theoretical description [22,23].In this Letter, we demonstrate a fluxonium circuit integrating a NbTiN nanowire superinductance. We charac- * These authors contributed equally to this work.terize the effect of the nanowire modes on the qubit spectrum with a multimode circuit theory accounting for the distributed nature of the superinductance. Importantly, and in contrast to previous approaches tailored to weakly anharmonic qubits [24,25], our theory incorporates the circuit nonlinearity to all orders in the Josephson potential. Such difference allows us to treat the strong anharmonicity of the fluxonium qubit efficiently, and to retain the effect of ch...

Loss Mechanisms and Quasiparticle Dynamics in Superconducting Microwave Resonators Made of Thin-Film Granular Aluminum

“…Despite such remarkably long relaxation times, we show that grAl resonators with a kinetic inductance L kinetic as high as 2 nH=□ maintain internal quality factors Q i > 10 5 for n ≈ 1 average circulating photons. These properties place grAl in the same class of low-loss, high kinetic inductance environments as Josephson junction arrays [37][38][39] and disordered superconducting thin films, such as TiN [40][41][42], NbTiN [43][44][45], and NbN [46][47][48].…”

Fluxonium Steps up to the Plate

“…It is important to realize that the effective loss tangent of the inductance must be in the 10 −7 -10 −8 range in order to reach the coherence times reported in this experiment. Whether such a low loss can be reached with dirty superconductors is an interesting question [52][53][54][55][56]. Our specific Josephson tunnel junction chain design [ Fig.…”

Efficient modeling of superconducting quantum circuits with tensor networks

“…such as NbTiN, TiN, and granular aluminum. Experiments on nanowire superinductors have recently demonstrated inductance values comparable to some of the junction-array counterparts, with low levels of loss and residual nonlinearity [32][33][34][35] . Beyond fluxonium, superinductances are also exploited by other qubit designs, with the notable example of the noise-protected 0 − π qubit 25,36 .…”