Entangling superconducting qubits through an analogue wormhole

  1. Carlos Sabín
We propose an experimental setup to test the role of curved spacetime on entanglement extraction from the vacuum of a quantum field to a pair of artificial atoms. In particular, we
consider two superconducting qubits coupled to a dc-SQUID array embedded into an open microwave transmission line, where a suitable external bias is able to mimic a spacetime containing a traversable wormhole. We find that the amount of vacuum entanglement that can be extracted by the superconducting qubits depends on the parameters of the wormhole. At some distances qubits that would remain separable in flat spacetime become entangled due to the presence of the effective wormhole background.

Observation of Three-Photon Spontaneous Parametric Downconversion in a Superconducting Parametric Cavity

  1. C.W. Sandbo Chang,
  2. Carlos Sabín,
  3. P. Forn-Díaz,
  4. Fernando Quijandría,
  5. A. M. Vadiraj,
  6. I. Nsanzineza,
  7. G. Johansson,
  8. and C.M. Wilson
Spontaneous parametric downconversion (SPDC) has been a key enabling technology in exploring quantum phenomena and their applications for decades. For instance, traditional SPDC, which
splits a high energy pump photon into two lower energy photons, is a common way to produce entangled photon pairs. Since the early realizations of SPDC, researchers have thought to generalize it to higher order, e.g., to produce entangled photon triplets. However, directly generating photon triplets through a single SPDC process has remained elusive. Here, using a flux-pumped superconducting parametric cavity, we demonstrate direct three-photon SPDC, with photon triplets generated in a single cavity mode or split between multiple modes. With strong pumping, the states can be quite bright, with flux densities exceeding 60 photon/s/Hz. The observed states are strongly non-Gaussian, which has important implications for potential applications. In the single-mode case, we observe a triangular star-shaped distribution of quadrature voltages, indicative of the long-predicted „star state“. The observed star state shows strong third-order correlations, as expected for a state generated by a cubic Hamiltonian. By pumping at the sum frequency of multiple modes, we observe strong three-body correlations between multiple modes, strikingly, in the absence of second-order correlations. We further analyze the third-order correlations under mode transformations by the symplectic symmetry group, showing that the observed transformation properties serve to „fingerprint“ the specific cubic Hamiltonian that generates them. The observed non-Gaussian, third-order correlations represent an important step forward in quantum optics and may have a strong impact on quantum communication with microwave fields as well as continuous-variable quantum computation.

Generating Multimode Entangled Microwaves with a Superconducting Parametric Cavity

  1. C.W. Sandbo Chang,
  2. M. Simoen,
  3. José Aumentado,
  4. Carlos Sabín,
  5. P. Forn-Díaz,
  6. A. M. Vadiraj,
  7. Fernando Quijandría,
  8. G. Johansson,
  9. I. Fuentes,
  10. and C.M. Wilson
In this Letter, we demonstrate the generation of multimode entangled states of propagating microwaves. The entangled states are generated by parametrically pumping a multimode superconducting
cavity. By combining different pump frequencies, applied simultaneously to the device, we can produce different entanglement structures in a programable fashion. The Gaussian output states are fully characterized by measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated using an in situ microwave calibration source, a shot noise tunnel junction. Applying a variety of entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the states. Our method is easily extensible to more modes.

One-dimensional sections of exotic spacetimes with superconducting circuits

  1. Carlos Sabín
We introduce analog quantum simulations of 1+1 dimensional sections of exotic 3+1 dimensional spacetimes, such as Alcubierre warp-drive spacetime, G“{o}del rotating universe
and Kerr highly-rotating black hole metric. Suitable magnetic flux profiles along a SQUID array embedded in a superconducting transmission line allow to generate an effective spatiotemporal dependence in the speed of light, which is able to mimic the corresponding light propagation in a dimensionally-reduced exotic spacetime. In each case, we discuss the technical constraints and find the optimal region of parameters for the experimental implementation.

Superluminal Physics with Superconducting Circuit Technology

  1. Carlos Sabín,
  2. Borja Peropadre,
  3. Lucas Lamata,
  4. and Enrique Solano
We introduce a toolbox for the quantum simulation of superluminal motion with superconducting circuits. We show that it is possible to simulate the motion of a superconducting qubit
at constant velocities that exceed the speed of light in the electromagnetic medium and the subsequent emission of Ginzburg radiation. We consider as well possible setups for simulating the superluminal motion of a mirror, finding a link with the superradiant phase transition of the Dicke model.

Dynamical Casimir Effect for Gaussian Boson Sampling

  1. Borja Peropadre,
  2. Joonsuk Huh,
  3. and Carlos Sabín
We show that the Dynamical Casimir Effect (DCE), realized on two multimode coplanar waveguide resonators, implements a gaussian boson sampler (GBS). The appropriate choice of the mirror
acceleration that couples both resonators translates into the desired initial gaussian state and many-boson interference in a boson sampling network. In particular, we show that the proposed quantum simulator naturally performs a classically hard task, known as scattershot boson sampling. Our result unveils an unprecedented computational power of DCE, and paves the way for using DCE as a resource for quantum simulation.

Entanglement of superconducting qubits via acceleration radiation

  1. Laura García-Álvarez,
  2. Simone Felicetti,
  3. Enrique Rico,
  4. Enrique Solano,
  5. and Carlos Sabín
We show that simulated relativistic motion can generate entanglement between artificial atoms and protect them from spontaneous emission. We consider a pair of superconducting qubits
coupled to a resonator mode, where the modulation of the coupling strength can mimic the harmonic motion of the qubits at relativistic speeds, generating acceleration radiation. We find the optimal feasible conditions for generating a stationary entangled state between the qubits when they are initially prepared in their ground state. Furthermore, we analyze the effects of motion on the probability of spontaneous emission in the standard scenarios of single-atom and two-atom superradiance, where one or two excitations are initially present. Finally, we show that relativistic motion induces sub-radiance and can generate a Zeno-like effect, preserving the excitations from radiative decay.

Wormhole spacetimes, CTCs and chronology protection in a dc-SQUID array

  1. Carlos Sabín
We present an analog quantum simulator of spacetimes containing traversable wormholes. A suitable spatial dependence in the external bias of a dc-SQUID array mimics the propagation
of light in a 1D wormhole background. The impedance of the array places severe limitations on the type of spacetime that we are able to implement. However, we find that wormhole throat radius in the sub-mm range are achievable. We show how to modify this spacetime in order to allow the existence of closed timelike curves. The quantum fluctuations of the phase associated to the finite array impedance might be seen as an analogue of Hawking’s chronology protection mechanism.

Motion and gravity effects in the precision of quantum clocks

  1. Joel Lindkvist,
  2. Carlos Sabín,
  3. Göran Johansson,
  4. and Ivette Fuentes
We show that motion and gravity affect the precision of quantum clocks. We consider a localised quantum field as a fundamental model of a quantum clock moving in spacetime and show
that its state is modified due to changes in acceleration. By computing the quantum Fisher information we determine how relativistic motion modifies the ultimate bound in the precision of the measurement of time. While in the absence of motion the squeezed vacuum is the ideal state for time estimation, we find that it is highly sensitive to the motion-induced degradation of the quantum Fisher information. We show that coherent states are generally more resilient to this degradation and that in the case of very low initial number of photons, the optimal precision can be even increased by motion. These results can be tested with current technology by using superconducting resonators with tunable boundary conditions.

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.