Coherence Times of Dressed States of a Superconducting Qubit under Extreme Driving

Abstract

In this work, we measure longitudinal dressed states of a superconducting qubit, the single Cooperpair box, and an intense microwave field. The dressed states represent the hybridization of the qubit and photon degrees of freedom, and appear as avoided level crossings in the combined energy diagram. By embedding the circuit in an rf oscillator, we directly probe the dressed states. We measure their dressed gap as a function of photon number and microwave amplitude, finding good agreement with theory. In addition, we extract the relaxation and dephasing rates of these states.When matter and light interact at the quantum level, in the form of atoms and photons, it is often no longer possible to clearly distinguish the individual contribution of each to the overall behavior of the system. This mixing of the aspects of light and matter can then be described in terms of dressed states of the atoms and photons [1]. These dressed states have become an essential concept in many fields of physics. Recently, they have also been invoked to explain the behavior of electrical circuits operating in the quantum regime [2,3]. In this context, known as circuit quantum electrodynamics (QED), we directly measure a class of states, longitudinal dressed states (LDS), that have received little experimental attention in the past. We create these states by illuminating an artificial atom made from a nanofabricated superconducting circuit with microwave photons. We then observe the interaction of the dressed states and a radiofrequency (rf) oscillator. This measurement scheme allows us to directly map the dressed energy diagram and extract the relaxation and dephasing times of the states. LDS are the natural description of a strongly driven superconducting quantum bit (qubit), and may have applications in the field of quantum information processing.Significant advances have been made in the development of engineered systems that exhibit coherent quantum properties. In the new field of circuit QED, these artificial atoms have recently been used to not only reproduce results of atomic physics and quantum optics, but to explore regimes previously inaccessible to traditional experiments [4]. Here, we use circuit QED techniques to directly study LDS over a wide range of drive strengths, including the extreme driving regime where the driving field is much stronger than the polarizing field. In atomic systems, the field strengths required for this are technically difficult to achieve and often exceed the ionization threshold of the atoms. The field geometry studied is also unusual. In a typical atomic experiment, a strong, static field is used to polarize the atomic spins under study. A relatively weak ac field, aligned perpendicular to the polarizing field, is then used to drive the spins. This transverse field geometry in fact implies that the atomic and photon spins are aligned, leading to a variety of selection rules based on the conservation of angular momentum. In our experiment, the driving field is aligned parallel to the polarizing fi...

Multiphoton spectroscopy of a hybrid quantum system

“…Several techniques exist for treating a driving field when the perturbation is no longer "weak." [23][24][25] We employ a numerical expansion in the Floquet basis 26,27 as this allows us to include arbitrary multiphoton processes together with decoherence. The inclusion of higher lying states in the qubit can be achieved via direct diagonalization of the exact qubit potential or by using an anharmonic approximation.…”

Effects of quasiparticle tunnelling in a circuit-QED realization of a strongly driven two-level system

“…This situation is considered also in Refs. (5,6) and the results discussed here are equivalent, but presented as the cavity frequency shift (that is the measured quantity here) rather than the quantum capacitance. (4), a bimodal behavior of the frequency shift occurs.…”

“…In Refs. (4,5) this was studied in the case of longitudinally coupled noise and, for example, a population inversion induced by the dissipative environment was experimentally verified. The system was studied also in Ref.…”

Zeno–anti-Zeno crossover via external fields in a one-dimensional coupled-cavity waveguide

“…Then the spins of atoms and photon are aligned. However, diagonal couplings between the driving field and the TLS means that the driving field is parallel to the polar- izing field [45]. Here, the spontaneous emission of a TLS is studied, the Zeno-anti-Zeno crossover is investigated via Floquet formulation.…”