Amplifying electrical signals while minimizing noise is crucial to a lot of modern technology. Now, scientists have created a device that amplifies microwaves with almost the minimum amount of noise allowed by quantum mechanics. The device could be used to process the sensitive outputs of quantum computers, which are expected to outperform conventional computers in years to come.
Noise is ever-present in electrical signals. Some of it arises simply because atoms have heat—they jostle around, knocking into one another. But even when there is no heat—that is, when temperatures have dropped close to absolute zero—noise remains. That's because of a fundamental tenet of quantum mechanics, which says that space is always filled with fluctuating energy, or quantum jitters.
Reducing noise to this quantum limit is out of the league of most electronic amplifiers. Some specialized devices do a good job if they contain superconductors, which isolate the devices from the environment. But these devices have other drawbacks, particularly an inability to amplify signals with a large dynamic range—ones that go from faint to very strong.
Now, physicists Francesco Massel and Mika Sillanpää of Aalto University in Finland and colleagues have come up with an amplifier that can processes signals with a large dynamic range while operating close to the quantum noise limit. The basic principle of the amplifier is to boost a microwave signal by stealing photons from a consistent "pump" wave. In practice this is difficult, however, because the pitch or energy of the signal photons varies with time, and so the pump photons won't always match.
Massel and colleagues have a neat trick to match up the photon energies. The researchers send the signal and the pump wave into a zigzag cavity, where they bounce back and forth. Next to the cavity is a flimsy mechanical beam, which begins to resonate under pressure from the bouncing pump waves and in doing so reduces the pump photons' energy. Soon, the energy of the pump photons drops until it matches the energy of the signal photons, at which point the pump wave merges with the signal wave and amplifies it.
As reported online today in Nature, the device can achieve 25 decibels of amplification, with a noise 20 times the quantum limit. This is far better than most conventional microwave amplifiers, although it is not as clean as amplification from superconducting devices, which operate at just two or three times above the quantum noise limit. However, Massel points out that he and his colleagues only discovered the low-noise behavior of their amplifier by accident, and they could reach the quantum noise limit after further development.
For physicist Nicolas Bergeal of ESPCI ParisTech in France, this need for further development makes it hard to foresee potential applications. But the researchers performed "a remarkable experiment," he says, because no one has tried to approach the quantum noise limit using mechanical resonators before. "To date, people [have] focused mainly on superconducting ... devices, which rely exclusively on electrical elements."
Still, Massel believes the device could be used to amplify the faint output signals from quantum computers, which are expected to exploit the inherent fuzziness of quantum mechanics to perform calculations much faster than any of today's machines. "Beyond the general interest of having a quantum-limited amplifier itself," he says, "this is the application where it most likely to be used."