Researchers at Qutech have shown that qubits can simultaneously be read out quickly, reliably and scalably. Being able to do these three things simultaneously is a prerequisite for building an effective quantum computer.
The research at the group led by Lieven Vandersypen focuses on quantum dots, ie individual electrons, confined in silicon. A single electron spin signal is very weak, so the spin state is read out via its charge. In one of the spin states, the electron can escape from the quantum dot, changing the charge on the quantum dot. In the other state, it cannot escape.
The change in charge is measured by an electrometer, but these are located next to the dot, and therefore obstruct any physical expansion of the number of dots and scaling up to more qubits. “That’s why, for a number of years, research has been conducted into an alternative readout method in which a resonator is connected to an electrode – which anyway is already present to make the dot. The significant advantage of this approach is that scaling up is a lot easier, provided that the readout method functions properly,” explains Qutech researcher Guoji Zheng.
“But this method also had its limitations,” continues Zheng. “The technique was not fast enough to read out before quantum states decay, and the signal-to-noise ratio was poor.” However, the researchers have now significantly improved the quality of the resonators by placing them on the chip itself. “This means that we can read out a hundred times faster than before. I believe we’re currently the fastest for this type of qubit in silicon.”
No additional sensor is required for this novel readout method, which means the quantum chip design can be simpler. What’s more, the researchers think that they can further improve the quality of the readout. “For example, by using a better amplifier of the electrical signal,” Vandersypen points out.
“Fast readout is important for the application of quantum error correction, without which a quantum computer simply cannot function. This is the process by which we quickly detect and correct a faulty qubit, before the quantum state is lost,” Vandersypen explains. “If we’re able to improve the quality a little more, we can confidently scale up the number of qubits in two dimensions, on our way towards a quantum computer.”