Göran Johansson

Towards universal quantum computation through relativistic motion

  1. David Edward Bruschi,
  2. Carlos Sabín,
  3. Pieter Kok,
  4. Göran Johansson,
  5. Per Delsing,
  6. and Ivette Fuentes
We show how to use relativistic motion to generate continuous variable Gaussian cluster states within cavity modes. Our results can be demonstrated experimentally using superconducting
circuits where tunable boundary conditions correspond to mirrors moving with velocities close to the speed of light. In particular, we propose the generation of a quadripartite square cluster state as a first example that can be readily implemented in the laboratory. Since cluster states are universal resources for universal one-way quantum computation, our results pave the way for relativistic quantum computation schemes.
arxiv pdf

Input-output description of microwave radiation in the dynamical Coulomb blockade

  1. Juha Leppäkangas,
  2. Göran Johansson,
  3. Michael Marthaler,
  4. and Mikael Fogelström
We study microwave radiation emitted by a small voltage-biased Josephson junction connected to a superconducting transmission line. An input-output formalism for the radiation field
is established, using a perturbation expansion in the junction’s critical current. Using output field operators solved up to the second order, we estimate the spectral density and the second-order coherence of the emitted field. For typical transmission line impedances and at frequencies below the main emission peak at the Josephson frequency, radiation occurs predominantly due to two-photon emission. This emission is characterized by a high degree of photon bunching if detected symmetrically around half of the Josephson frequency. Strong phase fluctuations in the transmission line make related nonclassical phase-dependent amplitude correlations short lived, and there is no steady-state two-mode squeezing. However, the radiation is shown to violate the classical Cauchy-Schwarz inequality of intensity cross-correlations, demonstrating the nonclassicality of the photon pair production in this region.
arxiv pdf

Steady state entanglement of two superconducting qubits engineered by dissipation

  1. Florentin Reiter,
  2. L. Tornberg,
  3. Göran Johansson,
  4. and Anders S. Sørensen
We present a scheme for dissipative preparation of an entangled steady state of two superconducting qubits in a circuit QED setup. Combining resonator photon loss, a dissipative process
already present in the setup, with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art 3D, as well as with the more commonly used 2D transmons. The promising results of our study thus open a route for the demonstration of an entangled steady state in circuit QED.
arxiv pdf

Relativistic Quantum Teleportation with superconducting circuits

  1. Nicolai Friis,
  2. Antony R. Lee,
  3. Kevin Truong,
  4. Carlos Sabín,
  5. Enrique Solano,
  6. Göran Johansson,
  7. and Ivette Fuentes
We study the effects of relativistic motion on quantum teleportation and propose a realizable experiment where our results can be tested. We compute bounds on the optimal fidelity of
teleportation when one of the observers undergoes non-uniform motion for a finite time. The upper bound to the optimal fidelity is degraded due to the observer’s motion however, we discuss how this degradation can be corrected. These effects are observable for experimental parameters that are within reach of cutting-edge superconducting technology.
arxiv pdf

Microwave Quantum Optics with an Artificial Atom

  1. Io-Chun Hoi,
  2. C.M. Wilson,
  3. Göran Johansson,
  4. Joel Lindkvist,
  5. Borja Peropadre,
  6. Tauno Palomaki,
  7. and Per Delsing
We address the recent advances on microwave quantum optics with artificial atoms. This field relies on the fact that the coupling between a superconducting artificial atom and propagating
microwave photons in a 1D open transmission line can be made strong enough to observe quantum coherent effects, without using any cavity to confine the microwave photons. We investigate the scattering properties in such a system with resonant coherent microwaves. We observe the strong nonlinearity of the artificial atom and under strong driving we observe the Mollow triplet. By applying two resonant tones, we also observe the Autler-Townes splitting. By exploiting these effects, we demonstrate two quantum devices at the single-photon level in the microwave regime: the single-photon router and the photon-number filter. These devices provide essential steps towards the realization of an on-chip quantum network.
arxiv pdf

Breakdown of the cross-Kerr scheme for Photon Counting

  1. Bixuan Fan,
  2. Anton F. Kockum,
  3. Joshua Combes,
  4. Göran Johansson,
  5. Io-chun Hoi,
  6. Christopher Wilson,
  7. Per Delsing,
  8. G. J. Milburn,
  9. and Thomas M. Stace
We show, in the context of single photon detection, that an atomic three-level model for a transmon in a transmission line does not support the predictions of the nonlinear polarisability
model known as the cross-Kerr effect. We show that the induced displacement of a probe in the presence or absence of a single photon in the signal field, cannot be resolved above the quantum noise in the probe. This strongly suggests that cross-Kerr media are not suitable for photon counting or related single photon applications. Our results are presented in the context of a transmon in a one dimensional microwave waveguide, but the conclusions also apply to optical systems.
arxiv pdf