Ultimate quantum limit for amplification: a single atom in front of a mirror

  1. Emely Wiegand,
  2. Ping-Yi Wen,
  3. Per Delsing,
  4. Io-Chun Hoi,
  5. and Anton Frisk Kockum
We investigate three types of amplification processes for light fields coupling to an atom near the end of a one-dimensional semi-infinite waveguide. We consider two setups where a
drive creates population inversion in the bare or dressed basis of a three-level atom and one setup where the amplification is due to higher-order processes in a driven two-level atom. In all cases, the end of the waveguide acts as a mirror for the light. We find that this enhances the amplification in two ways compared to the same setups in an open waveguide. Firstly, the mirror forces all output from the atom to travel in one direction instead of being split up into two output channels. Secondly, interference due to the mirror enables tuning of the ratio of relaxation rates for different transitions in the atom to increase population inversion. We quantify the enhancement in amplification due to these factors and show that it can be demonstrated for standard parameters in experiments with superconducting quantum circuits.

Transmon in a semi-infinite high-impedance transmission line — appearance of cavity modes and Rabi oscillations

  1. Emely Wiegand,
  2. Benjamin Rousseaux,
  3. and Göran Johansson
In this letter, we investigate the dynamics of a single superconducting artificial atom capacitively coupled to a transmission line with a characteristic impedance comparable or larger
than the quantum resistance. In this regime, microwaves are reflected from the atom also at frequencies far from the atom’s transition frequency. Adding a single mirror in the transmission line then creates cavity modes between the atom and the mirror. Investigating the spontaneous emission from the atom, we then find Rabi oscillations, where the energy oscillates between the atom and one of the cavity modes.

Characterizing decoherence rates of a superconducting qubit by direct microwave scattering

  1. Yong Lu,
  2. Andreas Bengtsson,
  3. Jonathan J. Burnett,
  4. Emely Wiegand,
  5. Baladitya Suri,
  6. Philip Krantz,
  7. Anita Fadavi Roudsari,
  8. Anton Frisk Kockum,
  9. Simone Gasparinetti,
  10. Göran Johansson,
  11. and Per Delsing
We experimentally investigate a superconducting qubit coupled to the end of an open transmission line, in a regime where the qubit decay rates to the transmission line and to its own
environment are comparable. We perform measurements of coherent and incoherent scattering, on- and off-resonant fluorescence, and time-resolved dynamics to determine the decay and decoherence rates of the qubit. In particular, these measurements let us discriminate between non-radiative decay and pure dephasing. We combine and contrast results across all methods and find consistent values for the extracted rates. The results show that the pure dephasing rate is one order of magnitude smaller than the non-radiative decay rate for our qubit. Our results indicate a pathway to benchmark decoherence rates of superconducting qubits in a resonator-free setting.