Amplifying Microwaves at the quantum limit

Nowadays, it is possible to control and measure the quantum state of systems with a few degrees of freedom, whether they are microscopic objects like cold atoms and single photons, or macroscopic objects like superconducting circuits. Such delicate experiments require an interface which bridges the gap of orders of magnitudes in energy between the quantum object and the data acquisition system. This problem is solved by an active device: the amplifier.
Although the amplifier is necessary to overcome the noise at the stage of data acquisition, it eventually alters the signal. We have developed an amplifier for microwave signals which adds close to the minimum of noise allowed by Quantum laws to the outgoing signal: the equivalent of half a photon of noise added to the input signal.

Our amplifier is based on a superconducting circuit of Josephson junctions in superconducting resonators. We found a way to make it practical to use by tuning its center frequency over 500 MHz. We are now trying to increase its bandwidth for a given gain using more compact resonators.

More information can be found here: Widely tunable, non-degenerate three-wave mixing microwave device operating near the quantum limit

Two-mode squeezing on spatially separate arms

Using our quantum limited amplifiers, we can produce highly entangled propagating microwave fields on separate cables at different frequencies. For instance, amplifying the vacuum fluctuations leads to a 2-mode squeezed state of the field. Ignoring one cable, an observer would be lead to believe that the signal on the other cable is produced by a thermal bath at high temperature (given by the power gain). On the opposite, by carefully designing an interfering device between both arms, one could in principle reverse the amplification completely and recover the zero entropy vacuum fluctuations.

This experiment would be a perfect illustration of the only postulate distinguishing quantum information from classical information, namely: it is possible to possess the maximal knowledge on a system without possessing maximal knowledge on its parts.


Control of a qubit by measurement feedback

Using quantum limited amplifiers, one can now determine precisely the state evolution of a superconducting qubit in cavity from the outcome of its measurement in real time. It is therefore possible to correct the evolution of a quantum object towards a targeted evolution by sending control pulses conditioned on the measurement result. We are currently developing a test bed for this type of experiments where a macroscopical object (an FPGA board) measures the trajectory of a quantum bit and corrects it in real time.