Graphene drum could store quantum information
Devices made from resonating graphene "drums" could be used as microwave amplifiers and memory chips in quantum computers. So say researchers at the Kavli Institute of Nanoscience at the Delft University of Technology in the Netherlands, who are the first to demonstrate optomechanical coupling between a mechanical resonator and a superconducting microwave cavity.
Graphene is a sheet of carbon just one atom thick, and Gary Steele and colleagues created their drum by placing a multilayer sheet of graphene over a 4 μm-diameter hole in a silicon chip. The drum is adjacent to a superconducting microwave cavity that has been created by depositing a metal alloy on the chip, and microwave photons are able to move between the two structures.
Tiny changes in position
The graphene drum behaves like a mirror that reflects microwave photons that are fired at it. By measuring the interference of the reflected photons, the researchers are able to sense tiny changes in the position of the graphene sheet. Indeed, a shift in position of just 17 fm, which is 1/10,000th of the diameter of a single atom, can be measured.
"The microwave light is not only good for helping us to detect the position of the vibrating graphene drum, it can also exert a force," explains team member Vibhor Singh. This force arises because light carries momentum. "If I shine a flashlight at a piece of paper, in principle, the light hitting the paper will exert a force on it, pushing it away from the light source," he says. "The radiation pressure force that light exerts, however, is usually far too small to detect – you cannot push somebody over by shining a laser pointer at them. But, thanks to the graphene sheet weighing so little, and our ability to detect small displacements of the resonator, we can make the graphene 'dance to tune' with the 'beat' set by the incident microwave light."
This "beating" leads to an interference phenomenon known as optomechanically induced transparency. By measuring this effect in their device, the team is the first to show that it has achieved optomechanical coupling between a mechanical resonator and a superconducting microwave cavity.
Amplifying microwave signals
"Now that we have firmly established that optomechanical coupling is taking place here, the consequences of this are enormous," Singh says. Similar devices could be used to amplify microwave signals, or even to store microwave photons for up to 10 ms. This storage capability means that the drum could function as a memory device that can store quantum information in quantum computers.
"One of the long-term goals of our project is to use these 2D crystal drums to study quantum motion," says Steele. "If you hit a classical drum with a stick, it will start oscillating, shaking up and down. This up and down motion can be thought of as the 1 and 0 bit states in a computer chip. With a quantum drum, however, we can not only make the drumhead move up and then down, but also make it move both up and down at the same time by putting it into a 'quantum superposition state'."
Steele adds, "Quantum graphene drums that are shaking up and down at once could be used to store quantum information in the same way as RAM chips in ordinary computers store information today."
More details about the research can be found in Nature Nanotechnology.
Steele and colleagues have made a video showing how microwave photons interact with the drum and you can watch it below.