Squeezing and quantum state engineering with Josephson travelling wave amplifiers

We develop a quantum theory describing the input–output properties of Josephson traveling wave parametric amplifiers. This allows us to show how such a device can be used as a source of nonclassical radiation, and how dispersion engineering can be used to tailor gain profiles and squeezing spectra with attractive properties, ranging from genuinely broadband spectra to “squeezing combs” consisting of a number of discrete entangled quasimodes. The device’s output field can furthermore be used to generate a multi-mode squeezed bath—a powerful resource for dissipative quantum state preparation. In particular, we show how it can be used to generate continuous variable cluster states that are universal for measurement based quantum computing. The favorable scaling properties of the preparation scheme makes this a promising path towards continuous variable quantum computing in the microwave regime. Quantum information: Microwave quantum computer Microwaves, familiar from our everyday lives, can be used to perform quantum computation. We show how this is possible using a quantum electronic device called a Josephson Travelling Wave Parametric Amplifier, or TWPA for short. This device has already been made and extensively tested in the lab for the purpose of amplifying extremely weak signals. We here demonstrate that the TWPA is much more than just an amplifier: It is a remarkably powerful resource for generating just the type of entanglement that is needed for a quantum computer. Surprisingly, a full-fledged universal quantum computer can be made using just four such devices, if their output radiation is combined to interfere in just the right way. This proposal may lead to small-scale quantum computers using already existing technology.