Max Hofheinz, Inac

ERC junior (ERC starting grant)

Wideband Quantum Optics with Josephson junctions

Microwave frequencies are a very interesting domain for quantum optics because microwaves can still be guided in electrical circuits as known from classical electronics. This allows for very efficient coupling between different circuit elements because photons can be guided and focused on length scales much shorter than the wavelength, making circuit quantum optics interesting as an interface for studying quantum behavior in various systems and for quantum computation. But two major limitations currently stunt circuit quantum optics:

First, there are currently no efficient single-photon detectors for the microwave regime or wide-band quantumlimited amplifiers. Efficient detection of single microwave quanta is currently only possible for localized modes.

Second, the frequency range of circuit quantum optics is extremely limited to a small range around 5 GHz by cryogenics and microwave engineering constraints. This hinders interaction with interesting atomic systems which are often at higher frequencies up the THz range.

In this project we want to overcome these limitations, making circuit quantum optics a versatile test bed for studying quantum behavior in various systems. We propose devices based on the ac Josephson effect in the single Cooper pair regime, where Cooper pairs crossing a voltage biased Josephson junction are forced to emit or absorb photons. Engineering the electromagnetic environment and applying appropriate voltages to the junction allows to select specific single- or multi-photon processes that we want to use to build photon sources, detectors and amplifiers. These devices have good potential for solving the detection problem of circuit quantum optics.

With this approach microwave signals are generated and detected on chip, lifting the constraints on operation frequency set by external microwave equipment. We want to implement these devices using NbN, a superconductor with large gap, allowing for operating frequencies up to 1 THz.

This project, if successful, will make circuit quantum optics not only an universal testbed for quantum behaviour but also a very versatile tool for any domain where radiation in the GHz to THz range at the single photon level is studied, such as quantum information processing, mesoscopic physics and astronomy.