An Amplifier That’s Quantum Quiet
An exotic new amplifier might be quiet enough for quantum computing
14 December 2011—Quantum computers have the potential to solve seemingly intractable problems in no time flat. But a big stumbling block on the path to practical quantum computing is figuring out how to observe the tiny quantum signals that drive computation. In an advance that may make that observation easier, a group at Aalto University, in Finland, has created a new kind of microwave amplifier based on a mechanical resonator—essentially a nanometer-scale tuning fork.
“When you have microwave signals on the level of a single quantum, you can’t manipulate them with your bare hands—you need to amplify the signal,” says Francesco Massel, a postdoctoral researcher at Aalto who worked on the device. But today’s amplifiers boost those signals at a cost, sometimes drowning them out with noise from the amplifiers themselves. “Our device adds, at least in principle, the minimum possible amount of noise dictated by the laws of quantum mechanics,” says Massel.
Other designs for such amplifiers rely on rarefied superconducting circuits called Josephson junctions, but they can’t promise as clean a signal boost as the Finnish amplifier. The Aalto group is the first to use a mechanical resonator, an idea they stumbled upon by chance. About a year ago, some group members were trying to cool a mechanical resonator, and while tweaking the system they noticed that it amplified microwave signals under certain conditions.
In the new amplifier described tomorrow in Nature, two elements are coupled together: a microwave cavity (a sort of mirrored, walled maze for electromagnetic waves) and a mechanical resonator (a suspended flexible aluminum beam). Only a tiny gap separates the mechanical resonator and the optical cavity; etched by a focused beam of ions, the gap is just 6 to 13 nanometers wide. “It’s one of the keys to the amplification,” says Massel. “The smaller the separation, the higher the coupling between optics [the cavity] and mechanics [the resonator].”
When the microwave cavity is hit with a strong microwave signal (called the pump), radiation pressure from the microwaves reduces the damping of the resonator, allowing it to vibrate with less resistance. When a second microwave signal—the one intended for amplification—hits the cavity, the resonator’s reduced damping lets the beam amplify the signal.
One advantage of the mechanical amplifier over competing designs, says Massel, is that its simple structure isn’t susceptible to “electrical flicker,” a major source of noise found in other kinds of amplifiers. Flicker, normally a low-frequency disturbance, is found in almost all electrical devices, including Josephson junctions. The best amplifier would eliminate all sources of noise except for those from quantum fluctuations—the uncertainty about an object’s position or momentum that arises when it’s measured. The Aalto mechanical amplifier would leave only thermal noise and those unavoidable quantum blips to interfere with measurements. “This is the selling point of this concept,” says IEEE Fellow Fadhel Ghannouchi, a microwave researcher at the University of Calgary, in Alberta, who was not involved in the research.
The Aalto team was able to reduce noise down to 20 quanta, but that isn’t a clear demonstration that a mechanical resonator can reduce noise to the unavoidable quantum limit, says Ghannouchi. “They’re replacing what is electronic with mechanics, and you don’t get flicker noise with mechanics,” he says. “We anticipate that we can therefore measure low-level signals, but I’m not seeing any clear demonstration of that yet.”
The group is now working on a version of the system that would reduce thermal noise by cooling the resonator, work that Massel says is currently “a whiteboard with nothing on it.” But he hopes that his amplifiers will soon be able to approach the quantum noise limit. “This is the beginning of using mechanical resonators to actually do something,” he says, “including manipulating and storing quantum information.”