Josephson Ratchet (rectifier) with tunable asymmetry

What is it about?

Ratchets are the devices that are trying to convert (ac or noisy) input signal with zero time average into dc motion thus producing useful work. This is done by breaking some symmetries. Paradigmatic example is a particle driven in a periodic saw-tooth potential. It is very tempting to construct such a ratchet on the base of a Josephson junction, where the Josephson phase (particle) moves in a 2pi periodic Josephson potential. Advantages are: (a) Josephson junctions are very fast devices and can capture and rectify a lot of spectral energy from dc up to 1THz, (b) one can construct underdamped as well as overdamped ratchets and (c) dc motion, if any, simply results in a dc voltage across the device. The "only" problem up to now was that Josephson energy profile in most Josephson junctions is SYMMETRIC. In the novel, so-called φ Josephson junctions, the asymmetry of the Josephson energy profile is tunable by applied magnetic field starting from the symmetric one at zero field. We demonstrate the operation of a deterministic ratchet based on a φ junction and determine key figures of merit such as maximum counterforce and energy efficiency.

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Why is it important?

This demonstrates yet another application of the novel Josephson φ junctions, which closely follows paradigmatic example.

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