Superconducting quantum computers typically use voltage measurements and complex amplifier circuits to read the state of qubits, making the design very cumbersome and hampering performance gains. The solution may be to measure the energy of the electromagnetic field of the qubit, but this requires an extremely sensitive sensor with the highest speed of energy measurement. Theoretically, bolometers are suitable for this role, but so far no sensor with suitable parameters has been proposed for this.
A group of Finnish scientists from Aalto University proposed using graphene as a sensitive element of the bolometer. Graphene has a very low heat capacity. Even a small change in energy leads to a change in the temperature of graphene and changes its resistance, and this is how bolometers work. The electromagnetic radiation falling on them leads to heating of the active element and, through a change in its resistance, makes it possible to measure the radiation energy.
The device created by Finnish scientists turned out to be able to measure the radiation energy in 200 nanoseconds or so, which by a huge margin overlaps the coherence time of qubits in modern superconducting quantum computers. The sensitivity of the device also turned out to be a record for similar solutions. The device recorded a change of several megahertz when the energy of several attowatts hit it.
Researchers from the United States and South Korea also proposed a similar device in action, as reflected in the second article in Nature. A team from Harvard University, Massachusetts Institute of Technology, Institute of Photonic Sciences, Pohang University of Science and Technology (South Korea), and Raytheon BBN Technologies said its graphene sensor is capable of “100,000 times better than commercial counterparts. “
The bolometer of American scientists, like the bolometer of Finnish researchers, uses graphene as an active (recording) element. But it is also based on the so-called Josephson junction, which makes it possible to further improve the measurement accuracy. It is argued that the system invented by scientists is capable of fixing the energy of a single photon. Such sensitivity, by the way, is not only the path to quantum computers but also the ability to create extremely sensitive radars, lidars, and other location solutions. Therefore, for example, all the research of the American group is funded by the military.