Quantum computers are capable of performing some calculations exponentially faster than ordinary machines. But bringing the technology to practical application requires a huge number of repeated tests. Reading quantum states at high speed has until now been difficult.
A group of scientists from Tohoku University and the National Institute for Materials Science (NIMS) has proposed a solution. A method has been developed that makes it possible to measure charge in quantum devices based on 2D materials - thin films one or several atoms thick - with ultrahigh sensitivity and speed. The work was published in the journal npj 2D Materials and Applications.
The main difficulty was the high electrical resistance of such devices. Because of it, they could not be measured at high frequencies. To get around the limitation, the researchers added a variable capacitor made of strontium titanate (SrTiO3) to a high-frequency resonant circuit (an electrical circuit tuned to a specific frequency).
The scheme provided almost ideal impedance matching - tuning the circuit so that the signal is transmitted without losses - with high-resistance quantum devices. This made it possible to record Coulomb oscillations: signals that arise when electrons enter a quantum dot (a tiny region that holds charges) one at a time and leave it. The system remains stable in a perpendicular magnetic field - a mandatory condition for the operation of spin-valley qubits (quantum bits that use the spins and valleys of electrons).
According to Associate Professor Tomohiro Otsuka of the Tohoku University Advanced Institute for Materials Research (WPI-AIMR), the use of variable capacitors based on strontium titanate gave almost ideal impedance matching even in perpendicular magnetic fields. This opens up charge sensitivity equal to or exceeding traditional quantum-dot sensors and provides a new platform for high-speed qubit readout in 2D materials.
Two-dimensional materials such as bilayer graphene are considered promising for creating stable qubits with exceptionally long coherence times - the period during which a quantum state is preserved. However, high resistance hindered precise high-frequency measurements. Without impedance matching, fast and accurate state readout was difficult.
Otsuka explains: fast evaluation of quantum states is necessary, but the high resistance of 2D materials got in the way. Since qubit development requires massive repeated statistical measurements, a fast readout method becomes a crucial catalyst. This basic technology will significantly accelerate the entire research and development cycle for quantum devices based on 2D materials.
The authors emphasize that so far this is a laboratory method that can accelerate the creation of qubits, not a finished quantum computer. Nevertheless, the development brings closer the emergence of the next generation of quantum machines capable of potentially transforming medicine, logistics and cybersecurity.