G. Johansson

Partial-measurement back-action and non-classical weak values in a superconducting circuit

  1. J. P. Groen,
  2. D. Ristè,
  3. L. Tornberg,
  4. J. Cramer,
  5. P. C. de Groot,
  6. T. Picot,
  7. G. Johansson,
  8. and L. DiCarlo
We realize indirect partial measurement of a transmon qubit in circuit quantum electrodynamics by interaction with an ancilla qubit and projective ancilla measurement with a dedicated
readout resonator. Accurate control of the interaction and ancilla measurement basis allows tailoring the measurement strength and operator. The tradeoff between measurement strength and qubit back-action is characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. Combining partial and projective qubit measurements, we provide the solid-state demonstration of the correspondence between a non-classical weak value and the violation of a Leggett-Garg inequality.
arxiv pdf

Proposal for a coherent quantum memory for propagating microwave photons

  1. M. Afzelius,
  2. N. Sangouard,
  3. G. Johansson,
  4. M. U. Staudt,
  5. and C. M. Wilson
We describe a multi-mode quantum memory for propagating microwave photons that combines a solid-state spin ensemble resonantly coupled to a frequency tunable single-mode microwave cavity.
We first show that high efficiency mapping of the quantum state transported by a free photon to the spin ensemble is possible both for strong and weak coupling between the cavity mode and the spin ensemble. We also show that even in the weak coupling limit unit efficiency and faithful retrieval can be obtained through time reversal inhomogeneous dephasing based on spin echo techniques. This is possible provided that the cavity containing the spin ensemble and the transmission line are impedance matched. We finally discuss the prospects for an experimental implementation using a rare-earth doped crystal coupled to a superconducting resonator.
arxiv pdf

Scattering of coherent states on a single artificial atom

  1. B. Peropadre,
  2. J. Lindkvist,
  3. I.-C. Hoi,
  4. C.M. Wilson,
  5. J.J. Garcia-Ripoll,
  6. P. Delsing,
  7. and G. Johansson
In this work we theoretically analyze a circuit QED design where propagating quantum microwaves interact with a single artificial atom, a single Cooper pair box. In particular, we derive
a master equation in the so-called transmon regime, including coherent drives. Inspired by recent experiments, we then apply the master equation to describe the dynamics in both a two-level and a three-level approximation of the atom. In the two-level case, we also discuss how to measure photon antibunching in the reflected field and how it is affected by finite temperature and finite detection bandwidth.
arxiv pdf

Nonclassical microwave radiation from the dynamical Casimir effect

  1. J. R. Johansson,
  2. G. Johansson,
  3. C. M. Wilson,
  4. P. Delsing,
  5. and F. Nori
We investigate quantum correlations in microwave radiation produced by the dynamical Casimir effect in a superconducting waveguide terminated and modulated by a superconducting quantum
interference device. We apply nonclassicality tests and evaluate the entanglement for the predicted field states. For realistic circuit parameters, including thermal background noise, the results indicate that the produced radiation can be strictly nonclassical and can have a measurable amount of intermode entanglement. If measured experimentally, these nonclassicalilty indicators could give further evidence of the quantum nature of the dynamical Casimir radiation in these circuits.
arxiv pdf

Undoing measurement-induced dephasing in circuit QED

  1. A. Frisk Kockum,
  2. L. Tornberg,
  3. and G. Johansson
We analyze the backaction of homodyne detection and photodetection on superconducting qubits in circuit quantum electrodynamics. Although both measurement schemes give rise to backaction
in the form of stochastic phase rotations, which leads to dephasing, we show that this can be perfectly undone provided that the measurement signal is fully accounted for. This result improves upon that of Phys. Rev. A, 82, 012329 (2010), showing that the method suggested can be made to realize a perfect two-qubit parity measurement. We propose a benchmarking experiment on a single qubit to demonstrate the method using homodyne detection. By analyzing the limited measurement efficiency of the detector and bandwidth of the amplifier, we show that the parameter values necessary to see the effect are within the limits of existing technology.
arxiv pdf