Stefan Zeppetzauer

Using coherent feedback for a periodic clock

  1. Stefan Zeppetzauer,
  2. Leonardo Assis Morais,
  3. Xin He,
  4. Gerard Milburn,
  5. and Arkady Fedorov
A driven linear oscillator and a feedback mechanism are two necessary elements of any classical periodic clock. Here, we introduce a novel, fully quantum clock using a driven oscillator
in the quantum regime and coherent quantum feedback. We show that if we treat the model semiclassically, this system supports limit cycles, or self-sustained oscillations, as needed for a periodic clock. We then analyse the noise of the system quantum mechanically and prove that the accuracy of this clock is higher compared to the clock implemented with the classical measurement feedback. We experimentally implement the model using two superconducting cavities with incorporated Josephson junctions and microwave circulators for the realisation of the quantum feedback. We confirm the appearance of the limit cycle and study the clock accuracy both in frequency and time domains. Under specific conditions of noisy driving, we observe that the clock oscillations are more coherent than the drive, pointing towards the implementation of a quantum autonomous clock.
arxiv pdf

Energetics of a Single Qubit Gate

  1. Jeremy Stevens,
  2. Daniel Szombati,
  3. Maria Maffei,
  4. Cyril Elouard,
  5. Réouven Assouly,
  6. Nathanaël Cottet,
  7. Rémy Dassonneville,
  8. Quentin Ficheux,
  9. Stefan Zeppetzauer,
  10. Audrey Bienfait,
  11. Andrew N. Jordan,
  12. Alexia Auffèves,
  13. and Benjamin Huard
Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit
and the controller that implements the gate? Here we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows. We measure the energy change in the drive field conditioned on the outcome of a projective qubit measurement. We demonstrate that the drive’s energy change associated with the measurement backaction can exceed by far the energy that can be extracted by the qubit. This can be understood by considering the qubit as a weak measurement apparatus of the driving field.
arxiv pdf