Amplifying quantum signals with a RF-driven Josephson oscillator:
Quantum activation over a meta-potential barrier
R. Vijay, I. Siddiqi, M. Metcalfe, P. Hyafil and Michel Devoret
Applied Physics Department, Yale University
We have constructed a new type of cryogenic amplifier whose primary purpose is the
readout of superconducting quantum bits. It is based on the dynamical bifurcation of an
RF-driven Josephson tunnel junction from a low-amplitude to a high-amplitude plasma
oscillation state [1-3]. Although this bifurcation manifests itself experimentally by a
sudden phase-shift in a pulsed microwave reflection measurement, and can appear quite
similar to the switching of a DC current-biased junction out of its zero-voltage state, it is
fundamentally a strong non-equilibrium phenomenon.
The main advantages of the Josephson bifurcation amplifier are speed, high-sensitivity,
low back-action, and the absence of on-chip dissipation. In the quantum regime
corresponding to qubit operation, switching occurs not through tunneling but through the
novel phenomenon of quantum activation, which bears some resemblance to the Unruh
effect explaining the evaporation of black holes. We will present recent results of the
measurement of this quantum activation, and show what the bifurcation amplifier is
bringing to decoherence measurement and quantum non-demolition in the particular case
of the readout circuit of the quantronium qubit.
[1]
Siddiqi I., Vijay R., Pierre F., Wilson C.M., Metcalfe M., Rigetti C., Frunzio L., Devoret M.H., An
RF-Driven Josephson Bifurcation Amplifier for Quantum Measurements, Phys. Rev. Lett. 93, 207002
(2004) [cond-mat/0312623]
[2]
Siddiqi I., Vijay R., Pierre F., Wilson C.M., Frunzio L., Metcalfe M., Rigetti C., Schoelkopf R.J.,
Devoret M.H., Vion D. and Esteve D., Direct Observation of Dynamical Switching between Two
Driven Oscillation States of a Josephson Junction, Phys. Rev. Lett 94, 027005 (2005) [condmat/0312553]
[3]
Siddiqi I., Vijay R., Metcalfe M., E. Boaknin, Frunzio L., Schoelkopf R. and Devoret M.H.
Dispersive measurement of superconducting qubit coherence with a fast, latching readout, Phys. Rev.
B 73, 054510 (2006) [cond-mat/0507548]