Charge Qubit Coupled to an Intense Microwave Electromagnetic Field in a Superconducting Nb Device: Evidence for Photon-Assisted Quasiparticle Tunneling

Abstract

We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg (LZS) interference structure of a longitudinally driven two-level system. For even stronger drives we observe a significant change in the LZS pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing. In a two-level system (TLS) under strong periodic nonadiabatic driving, the relative phase accumulated between successive tunneling events can play a significant role. The constructive or destructive interference between consecutive tunneling events are generally referred to as Landau-Zener-Stückelberg (LZS) oscillations. LZS oscillations are a celebrated phenomenon observed in a variety of quantum systems, ranging from optical lattices [1], nanomechanical circuits [2], single spins in diamond NV-centers [3], and semiconductor quantum dots [4,5], to Josephson qubits [6][7][8][9][10][11][12].In the regime of strong driving the steady state of a TLS coupled to a cavity is conveniently described in terms of photon dressed states, capturing the hybridization of the intense electromagnetic field and the TLS [12]. Such dressed states have successfully been applied in several applications, including lasing and amplification [13,14], suppression of decoherence in two-level systems [15], and single photon emission [16]. It is especially attractive to study dressed states in superconducting microcircuits, not only due to realizations of previously hardly accessible regimes [8,12], but also since they offer straightforward integration with other systems.In this article we present experimental observations of the interplay between photon dressed states of a superconducting charge qubit (Cooper-pair box, CPB) and quasiparticle creation. Remarkably, we find that when the CPB is driven into a regime where quasiparticle generation provides a dominant path for excitation/relaxation, the CPB exhibits a more pronounced interferometric response, while typically quasiparticle "poisoning" is degrading the performance of superconducting devices [17][18][19][20]. This effect emerges for strong driving * degraaf@chalmers.se of the qubit, when the two-level approximation of the CPB breaks down, and it is observed as a significant distortion of the typical LZS interference pattern. We attribute the increased response to the recovery of the qubit parity emerging in the ultra-strong driving regime. The populations of dressed states changes due to new quasiparticle-induced transition channels. At sufficient drive strengths the observed non-linear process involves...

Tunable superconducting microstrip resonators

“…The resonators are instrumental for the whole field of circuit quantum electrodynamics (c-QED) for having dramatically reduced microwave mode volume, as compared to bulk cavities, and accordingly increased coupling to engineered q-bits and spins. 7,8 Among the two generic planar resonator types, microstrip and coplanar, both are ubiquitous in industrial applications, 9,10 while only the latter is dominant in superconducting designs. This is because, in pursuit to squeeze the mode volume, the central line-to-ground gap in a typical coplanar resonator for c-QED applications is only a few microns wide.…”

Suppressing relaxation in superconducting qubits by quasiparticle pumping

“…As a superconducting circuit is cooled well below its critical temperature, the quasiparticle density via BCS theory is expected to be exponentially suppressed, but a number of experimental groups have reported higher-than-expected values in a wide variety of systems [7][8][9][10]. Although the reasons for the enhanced quasiparticle population or the mechanism behind quasiparticle generation are not fully understood, their presence has a number of adverse effects on the qubit performance, causing relaxation [11][12][13][14][15][16], dephasing [17][18][19], excess excited-state population [20], temporal variations in qubit parameters [21][22][23][24], and are predicted to be a major obstacle for realizing Majorana qubits in semiconductor nanowires [25]. Our results provide an in situ technique for removing quasiparticles, especially in conjunction with recent experiments showing that vortices in superconducting electrodes can act as quasiparticles traps, thus keeping the quasiparticles away from the Josephson junctions where they may contribute to qubit relaxation [22,[26][27][28].…”

Creating photon-number squeezed strong microwave fields by a Cooper-pair injection laser

“…In particular, we demonstrate that in a specific experimentally feasible parameter regime, squeezing at a high photon number can be achieved even in the presence of maximum gate-charge disorder. It follows that this device is robust against strong thermal noise in the DC-voltage bias 34 and nonequilibrium quasiparticles in the CPTs [35][36][37][38] .…”

“…In the limit of maximum disorder, it is also a rough model for (non-equilibrium) quasiparticle poisoning in the CPTs, which can be triggered in the high-intensity limit by quasiparticle tunneling due to direct multi-photon absorption 37 . For conventional lasers it has been shown that the lasing state is robust against disorder in the atomic detuning, the coupling strength to the resonator, and the pumping strength 28 .…”