Coherent Frequency Conversion in a Superconducting Artificial Atom with Two Internal Degrees of Freedom

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The richness of the energy level structure and the diversity of the associated transitions are at the heart of atomic and quantum optics experiments. They are provided by the multiples degrees of freedom (DoF) existing in these systems. On the opposite, studies on superconducting artificial atoms were focused on devices with only one DoF. In order to reach new regimes and increase the range of applications of these systems, we report here the fabrication and measurement of a superconducting device with two DoF. This device can be described by two anharmonic oscillators (the symmetric (s) and the antisymmetric (a) plasma modes) coupled through strong non-linear coupling terms, similar to Kerr effects [1]. We show independent coherent control of the two DoF and we use here one of the coupling terms to demonstrate a time resolved up and down frequency conversion in the microwave domain between the s and a mode[2]. This device can be a new building block for superconducting electronics providing substantials benefits with regard to quantum computation, qubit readout and parametric effects.




Figure: Free coherent oscillations between a single excitation state |0s1a> of the a-mode and a double excitation state |2s0a>of the s-mode produced by a strong nonlinear coupling. (a) Schematic pulse sequence. The energy diagram, without coupling in blue and red and with coupling in black, is represented for both the preparation and interaction steps. (b) Escape probability Pesc versus interaction time shows coherent oscillation at 815 MHz in perfect agreement with theoretical predictions (see in Inset the power spectral density (PSD) of these oscillations and the theoretically expected frequency(red arrow)). The coherent frequency conversion are realized due to the strong nonlinear coupling. Starting from the state |0s1a>, an excitation pair |2s0a> is then deterministically produced in about a single nanosecond.


[1] Nonlinear Coupling between the Two Oscillation Modes of a dc SQUID, F. Lecocq, J. Claudon, O. Buisson, and P. Milman, Phys. Rev. Lett. 107, 197002 (2011).

[2] Coherent Frequency Conversion in a Superconducting Artificial Atom with Two Internal Degrees of Freedom, F. Lecocq, I. M. Pop, I. Matei, E. Dumur, A. Feofanov, C. Naud, W. Guichard, O. Buisson,

Phys. Rev. Lett. 108, 107001 (2012).