Research
Josephson Photonics
The Josephson junction is the key ingredient in all superconducting quantum circuits, because it is the only nonlinear dissipationless circuit element we know. In most superconducting quantum circuits it is used in the superconducting state, where it acts as a nonlinear inductor. However, the Josephson junction can also be used by biasing the junction at a voltage V below the gap 2Δ/e. In this regime, the junction becomes an active circuit element that transforms the power supplied by the voltage source into microwave photons, while remaining non-dissipative. Our team is focusing on bringing to use this voltage state of the Josephson junction for quantum electronics, a domain we call Josephson Photonics.
![[Translate to English:] Illustration du tunneling inélastique de paires de Cooper](/fileadmin/_processed_/8/d/csm_inelastic-tunneling_335ad1d19d.png)
In this regime a tunnelling Cooper pair has to give away its energy 2eV in order to reach the condensate on the other side of the junction. This can happen by emitting photons into the circuit in which it is integrated. In the simplest case, this energy is emitted as a single photon at the Josephson frequency 2eV/h, which is nothing other than the AC Josephson effect. However, quantum fluctuations in the circuit also give rise to more complicated processes in which the energy of a Cooper pair is shared between several photons. By engineering the linear microwave circuit in which the junction is integrated, we can select the dominant single-photon or multi-photon process. The control parameter is here the real part of the impedance of the circuit Re Z(ω) as seen by the junction, which sets the strength of photon emission at frequency ω.
Nanofabrication of superconducting quantum circuits
![[Translate to English:] Image of a quantum circuit](/fileadmin/_processed_/1/1/csm_chip_0b3d4e20fa.jpg)
We continuously improve our nanofabriction process at 3iT. It features two Nb superconducting routing layers, separated by SiN dielectrics, Al/AlOx/Al Josephson junctions with sizes from 80 nm × 80 nm to 300 nm × 300 nm, as well as Cr resistors.
We are actively working with CMC Microsystems to provide this process for other research groups and companies outside of 3iT as a multi-project foundry service for quantum superconducting circuits.