Electromagnetic signals can be detected and amplified almost noiselessly using a guitar-string like mechanical vibrating wire. In the ideal case the method adds only the minimum amount of noise required by quantum mechanics. Credit: Juha Juvonen
A team of Finnish physicists has developed a novel way to amplify a microwave signal that unlike other amplifiers, produces noise that is just barely above that which is necessary due to the laws of quantum mechanics. The team, as they describe in their paper published in Nature, use a microwave cavity and a mechanical resonator to amplify a signal by 25 decibels while introducing a noise that is just 20 times the quantum limit.
Figuring out how to amplify signals at the quantum level is one of the keys to figuring out how to build quantum computers, otherwise the signals used would be too faint to be of any use. The traditional approach up to now has been to use superconducting materials to create so-called Josephson junctions, which is obviously difficult and the resultant product isn’t able to amplify signals over a wide range. Now however, it appears having to go that route may become moot, as a Finnish team takes an entirely different approach.
To make the new amplifier, the team has figured out a way to boost amicrowave signal by swiping photons from a pump wave in such a way as to avoid matching problems due to photons varying in pitch over time. To achieve this they put together a device that has first a microwave cavity that is sort of like a maze with mirrored walls that causes the microwaves to bounce around. Next to the cavity is an orifice that leads to a very thin mechanical beam that begins to resonate under pressure from the bouncing microwaves. The resonating causes a loss of energy from the pump that goes on until it drops down to the level of the signal that is meant to be amplified. Once that happens, the two are merged, resulting in a single amplified signal.
Aluminium nanowire, whose vibrations are coupled to the superconducting cavity (on the right), enables almost noiseless amplification of microwaves. Credit: Tero Heikkilä
What would be perfect is if an amplifier could be created that would introduce zero noise of course, but that’s impossible according to the laws of quantum mechanics which says that any space in existence always has something called quantum jitters in it. Thus the next best thing is to create an amplifier that produces only the amount created by these quantum jitters, which would be a vast improvement on amplifiers currently in use in modern electronics which introduce noise in various ways mainly due to the heat that is present when atoms bang together as they move around. This new process clearly gets around that problem but the team also acknowledges that more work will need to be done to surpass the quality of signals produced using superconductors. They say they are optimistic about their chances though and believe their amplifier will one day be amplifying signals in a quantum computer.
More information: Microwave amplification with nanomechanical resonators, Nature 480, 351–354 (15 December 2011)doi:10.1038/nature10628
The sensitive measurement of electrical signals is at the heart of modern technology. According to the principles of quantum mechanics, any detector or amplifier necessarily adds a certain amount of noise to the signal, equal to at least the noise added by quantum fluctuations1, 2. This quantum limit of added noise has nearly been reached in superconducting devices that take advantage of nonlinearities in Josephson junctions3, 4. Here we introduce the concept of the amplification of microwave signals using mechanical oscillation, which seems likely to enable quantum-limited operation. We drive a nanomechanical resonator with a radiation pressure force5, 6, 7, and provide an experimental demonstration and an analytical description of how a signal input to a microwave cavity induces coherent stimulated emission and, consequently, signal amplification. This generic scheme, which is based on two linear oscillators, has the advantage of being conceptually and practically simpler than the Josephson junction devices. In our device, we achieve signal amplification of 25 decibels with the addition of 20 quanta of noise, which is consistent with the expected amount of added noise. The generality of the model allows for realization in other physical systems as well, and we anticipate that near-quantum-limited mechanical microwave amplification will soon be feasible in various applications involving integrated electrical circuits.