Probing a transmon qubit via the ultra-strong coupling to a Josephson waveguide

  1. Javier Puertas Martinez,
  2. Sebastien Leger,
  3. Nicolas Gheereart,
  4. Remy Dassonneville,
  5. Luca Planat,
  6. Farshad Foroughi,
  7. Yuriy Krupko,
  8. Olivier Buisson,
  9. Cecile Naud,
  10. Wiebke Guichard,
  11. Serge Florens,
  12. Izak Snyman,
  13. and Nicolas Roch
Exploring the quantum world often starts by drawing a sharp boundary between a microscopic subsystem and the bath to which it is invariably coupled. In most cases, knowledge of the
physical processes occuring in the bath is not required in great detail. However, recent developments in circuit quantum electrodynamics are presenting regimes where the actual dynamics of engineered baths, such as microwave photon resonators, becomes relevant. Here we take a major technological step forward, by tailoring a centimeter-scale on-chip bath from a very long metamaterial made of 4700 tunable Josephson junctions. By monitoring how each measurable bosonic resonance of the circuit acquires a phase-shift due to its interaction with a transmon qubit, one can indirectly measure qubit properties, such as transition frequency, linewidth and non-linearity. This new platform also demonstrates the ultra-strong coupling regime for the first time in the context of Josephson waveguides. Our device combines a large number of modes (up to 10 in the present setup) that are simultaneously hybridised with the two-level system, and a broadening dominated by the artificial environment that is a sizeable fraction of the qubit transition frequency. Finally, we provide a quantitative and parameter-free model of this large quantum system, and show that the finite environment seen by the qubit is equivalent to a truly macroscopic bath.

Quantum trajectories and their statistics for remotely entangled quantum bits

  1. Areeya Chantasri,
  2. Mollie E. Kimchi-Schwartz,
  3. Nicolas Roch,
  4. Irfan Siddiqi,
  5. and Andrew N. Jordan
We experimentally and theoretically investigate the quantum trajectories of jointly monitored transmon qubits embedded in spatially separated microwave cavities. Using nearly quantum-noise
limited superconducting amplifiers and an optimized setup to reduce signal loss between cavities, we can efficiently track measurement-induced entanglement generation as a continuous process for single realizations of the experiment. The quantum trajectories of transmon qubits naturally split into low and high entanglement classes corresponding to half-parity collapse. The distribution of concurrence is found at any given time and we explore the dynamics of entanglement creation in the state space. The distribution exhibits a sharp cut-off in the high concurrence limit, defining a maximal concurrence boundary. The most likely paths of the qubits‘ trajectories are also investigated, resulting in three probable paths, gradually projecting the system to two even subspaces and an odd subspace. We also investigate the most likely time for the individual trajectories to reach their most entangled state, and find that there are two solutions for the local maximum, corresponding to the low and high entanglement routes. The theoretical predictions show excellent agreement with the experimental entangled qubit trajectory data.

Unexpectedly allowed transition in two inductively coupled transmons

  1. Étienne Dumur,
  2. Bruno Küng,
  3. Alexey Feofanov,
  4. Thomas Weißl,
  5. Yuriy Krupko,
  6. Nicolas Roch,
  7. Cécile Naud,
  8. Wiebke Guichard,
  9. and Olivier Buisson
We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual
magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.

Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits

  1. Nicolas Roch,
  2. Mollie E. Schwartz,
  3. Felix Motzoi,
  4. Christopher Macklin,
  5. Rajamani Vijay,
  6. Andrew W. Eddins,
  7. Alexander N. Korotkov,
  8. K. Birgitta Whaley,
  9. Mohan Sarovar,
  10. and Irfan Siddiqi
The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated
by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

Superconducting quantum node for entanglement and storage of microwave radiation

  1. Emmanuel Flurin,
  2. Nicolas Roch,
  3. Jean-Damien Pillet,
  4. François Mallet,
  5. and Benjamin Huard
Superconducting circuits and microwave signals are good candidates to realize quantum networks, which are the backbone of quantum computers. We have realized a universal quantum node
based on a 3D microwave superconducting cavity parametrically coupled to a transmission line by a Josephson ring modulator. We first demonstrate the time-controlled capture, storage and retrieval of an optimally shaped propagating microwave field, with an efficiency as high as 80 %. We then demonstrate a second essential ability, which is the timed-controlled generation of an entangled state distributed between the node and a microwave channel.

Stabilizing the trajectory of a superconducting qubit by projective measurement feedback

  1. Philippe Campagne-Ibarcq,
  2. Emmanuel Flurin,
  3. Nicolas Roch,
  4. David Darson,
  5. Pascal Morfin,
  6. Mazyar Mirrahimi,
  7. Michel H. Devoret,
  8. Francois Mallet,
  9. and Benjamin Huard
Making a system state follow a prescribed trajectory despite fluctuations and errors commonly consists in monitoring an observable (temperature, blood-glucose level…) and reacting
on its controllers (heater power, insulin amount …). In the quantum domain, there is a change of paradigm in feedback since measurements modify the state of the system, most dramatically when the trajectory goes through superpositions of measurement eigenstates. Here, we demonstrate the stabilization of an arbitrary trajectory of a superconducting qubit by measurement based feedback. The protocol benefits from the long coherence time ($T_2>10 mu$s) of the 3D transmon qubit, the high efficiency (82%) of the phase preserving Josephson amplifier, and fast electronics ensuring less than 500 ns delay. At discrete time intervals, the state of the qubit is measured and corrected in case an error is detected. For Rabi oscillations, where the discrete measurements occur when the qubit is supposed to be in the measurement pointer states, we demonstrate an average fidelity of 85% to the targeted trajectory. For Ramsey oscillations, which does not go through pointer states, the average fidelity reaches 75%. Incidentally, we demonstrate a fast reset protocol allowing to cool a 3D transmon qubit down to 0.6% in the excited state.

Generating Entangled Microwave Radiation Over Two Transmission Lines

  1. Emmanuel Flurin,
  2. Nicolas Roch,
  3. Francois Mallet,
  4. Michel H. Devoret,
  5. and Benjamin Huard
Using a superconducting circuit, the Josephson mixer, we demonstrate the first experimental realization of spatially separated two-mode squeezed states of microwave light. Driven by
a pump tone, a first Josephson mixer generates, out of quantum vacuum, a pair of entangled fields at different frequencies on separate transmission lines. A second mixer, driven by a $pi$-phase shifted copy of the first pump tone, recombines and disentangles the two fields. The resulting output noise level is measured to be lower than for vacuum state at the input of the second mixer, an unambiguous proof of entanglement. Moreover, the output noise level provides a direct, quantitative measure of entanglement, leading here to the demonstration of 6 Mebit.s$^{-1}$ (Mega entangled bits per second) generated by the first mixer.