Researchers at the National Institute of Standards (NIST) and the University of Colorado have built the first noiseless and tunable (4 to 8-GHz) amplifier. It could take some of the uncertainty out of measuring microwaves and improve the speed and precision of future quantum computers and communications.

Conventional amps add electrical noise or random fluctuations when measuring or boosting electromagnetic signals. This noise forces scientists to make repeated measurements for accurate information on the quantum states of microwave fields, i.e., the amplitude (or power) and phase of the waves. The rules of quantum mechanics say noise in phase and amplitude cannot be zero. But the new amp exploits a loophole that says if you measure and amplify only one of these parameters (amplitude in this case) the amp will theoretically add no noise. In reality, the amp adds about half the noise expected when measuring both amplitude and phase.

The amp is a 5-mm-long niobium cavity lined with 480 magnetic sensors called Squids (superconducting quantum interference devices). Microwaves bounce back and forth inside the cavity. Researchers can tune the wave velocity by manipulating the magnetic fields inside the Squids and intensity of the microwaves. Injecting an intense pump tone at the right frequency makes the microwave amplitude oscillate at twice the pump frequency. And only the part of the signal synchronous with the pump gets amplified.