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Journal of Experimental and Theoretical Physics Letters

Scattering spectroscopy of a superconducting artificial atom coupled to two half spaces

A. Yu. Dmitriev, A. Korenkov, O. V. Astafiev

Published Jan 1, 2017• 1 citation
Publisher

Abstract

We propose a novel approach for spectroscopic characterization of quantum systems. A superconducting quantum system—an artificial atom—is coupled asymmetrically to two open-end transmission lines (1D half-spaces). The lines themselves are strongly decoupled from each other. This results in suppression of the direct microwave propagation from one side to another. The atom, excited from the weaker coupled side relaxes with photon emission preferably to the stronger coupled side. By measuring the emission spectrum, we reconstruct the energy levels of the artificial atom. Our method allows to reject the excitation tone and to detect only the elastically scattered emission corresponding to intra-atomic transitions. We also demonstrate visualization of the higher-level transitions by populating the excited levels. Such a system does not have an optical analog with natural atoms or quantum dots coupled to two half spaces.

DOI
10.1134/s0021364017020072

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We propose a novel approach for spectroscopic characterization of quantum systems. A superconducting quantum system—an artificial atom—is coupled asymmetrically to two open-end transmission lines (1D half-spaces). The lines themselves are strongly decoupled from each other. This results in suppression of the direct microwave propagation from one side to another. The atom, excited from the weaker coupled side relaxes with photon emission preferably to the stronger coupled side. By measuring the emission spectrum, we reconstruct the energy levels of the artificial atom. Our method allows to reject the excitation tone and to detect only the elastically scattered emission corresponding to intra-atomic transitions. We also demonstrate visualization of the higher-level transitions by populating the excited levels. Such a system does not have an optical analog with natural atoms or quantum dots coupled to two half spaces.
We study dynamics of a two-level superconducting quantum system, analogous to a natural atom in an open space, by measuring the evolution of its coherent and incoherent emission. The emitted waves containing full information about the states of the artificial atom are efficiently collected due to strong atom-transmission-line coupling. This allows us to do simultaneous measurements of all the quantum state projections and perform a full characterization of the system. We derive coherence times and extract the two-time correlation function from the dynamics of the coherent emission.
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We experimentally demonstrate the resonance fluorescence of a two-level artificial atom strongly coupled to a single-mode cavity field. The effect was theoretically predicted thirty years ago by Savage [Phys. Rev. Lett. 63, 1376 (1989)]. The system consists of a superconducting qubit circuit and a one-dimensional transmission line resonator. In addition, a one-dimensional transmission line strongly coupled to the atom serves as an open space. The effect takes place, when a microwave field is applied to the cavity, which in turn is resonantly coupled to the atom. The fluorescence spectrum is measured via the emission into the transmission line. We find that the central peak is determined by the atom spontaneous emission to the open space and the widths of side peaks are largely determined by the coherent interaction between the atom and the cavity, that is, the fluorescence spectrum here is very different from that of the Mollow triplet. We also derive analytical form for the spectrum. Our experimental results agree well with theoretical calculations.
We address recent advances in quantum optics with propagating microwaves\nin superconducting circuits. This research field exploits on the fact that the coupling between a superconducting artificial atom and propagating microwave photons in a one-dimensional (1D) open transmission line can be made strong enough to observe quantum effects, without using any cavity to confine the microwave photons.\nWe embed an artificial atom, a superconducting transmon qubit, in a 1D open\ntransmission line and investigate the scattering properties of coherent microwaves. When an input coherent state, with an average photon number much less than 1, is on resonance with the artificial atom, we observe extinction of up to 99% in the forward propagating field. We observe the strong nonlinearity of the artificial atom and under strong driving we observe the Mollow triplet. We also study the statistics of the reflected and transmitted beams, which are predicted to be nonclassical states. In particular, we demonstrate photon antibunching in the reflected beam by measuring the second-order correlation function. By applying a second control tone, we observe the Autler-Townes splitting and a giant cross-Kerr effect.\nFurthermore, we demonstrate fast operation of a single-photon router using\nthe Autler-Townes splitting. This device provides important steps towards the realization of a quantum network. This thesis describes the motivation, theoretical background, design, implementation and measurement results.
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Abstract A dynamical control of the coupling strengths between dressed states and probe photon states is demonstrated with a transmon-like artificial atom coupled to two closely spaced resonant modes. When the atom is driven with one mode, the atom state and driving photon states are superposed, forming the dressed states. Dressed states with 1st, 2nd and 3rd excited states of the atom were prepared and probed via the strong coupling to the other resonant mode. Spectroscopy reveals that the coupling strengths are ``dressed" and can be modulated by the power and sideband index of the driving. The transmission of the probe tone is modulated by the driving microwave amplitude with a Bessel behavior, displaying multi-photon process associated with the inter-atomic level transitions.