So far, two basic quantum computing technologies have been commercialized. One type of device, called a transmitter, includes superconducting wire loops attached to a resonator. It is used by companies such as Google, IBM, and Rigetti. Companies such as Quantinuum and IonQ have instead used single ions trapped in light traps. At the moment, both technologies are in a critical situation. They are clearly proven to be effective, but they need some significant quality and quality improvements before they can perform useful calculations.
It may be a bit surprising to see that Microsoft is committed to an alternative technology called “topological qubits”. This technology is far enough away from the company’s other options just announced I’ve worked in physics to form qubits. To better understand Microsoft’s approach, Ars spoke to Microsoft engineer Chetan Nayak about the company’s progress and plans.
Microsoft started behind some competitors because the underlying physics of its system were not fully defined. The company’s system relies on the controlled production of the “majorana particle”, something whose existence has only been proven over the past decade (and even then, its discovery It was controversial).
The particle got its name from Ettore Majorana, who proposed the idea in the 1920s. In simpler terms, Majorana particle is its antiparticle. Two majorana particles that differ in their rotation will perish if they meet. So far, none of the known particles appear to be a Majorana particle (all but neutrinos are not). But this concept has persisted because of the possibility of making Majorana quasiparticles, or collections of particles and fields that, in certain contexts, behave as if they were a single particle.
The most notable quasiparticle is probably the Cooper pair, in which two electrons are paired in a way that changes their behaviour. Cooper pairs are necessary for superconductivity to work.
Nayak said the Microsoft system includes a superconducting wire and its accompanying Cooper pairs. Under normal conditions, the presence of an additional non-paired electron imposes a cost on the total energy of the system. But in a wire small enough in the presence of magnetic fields, it is possible to stick an electron at the end of the wire without energy cost. “In the topological state and the topological superconductor, you end up having states that can, at no energy cost, absorb an extra electron,” Nayak told Ars.
Being quantum mechanics, the electron is not localized to the end of the wire where it is inserted; Instead, it is passed on to both ends. “The two extremes are the real and imaginary parts of a quantum wave function, basically,” Nayak said. These end states are called Majorana zero modes, and Microsoft now says it created them and has measured their properties.
From quasiparticles to qubits
Majorana zero modes alone cannot be used as qubits. But Nayak said it could be linked to a nearby quantum dot. (Quantum dots are pieces of a material whose size is smaller than the wavelength of the electron in that material.) Describe a U-shaped wire with majorana zero modes at each end and those ends close to a quantum dot.
“You could, as a hypothetical process, have an electron tunnel away from the quantum dot in Majorana’s zero mode and an electron tunnel away from the other Majorana’s zero mode and onto the quantum dot,” Nayak told Ars. These exchanges change the quantum dot’s ability to store charge (its capacity, in other words), a property that can be measured. Nayak also said that the connections between the wire and the quantum dots could be controlled, which could allow Majorana zero patterns to be separated, which would help preserve their state.
Microsoft has not yet reached a quantum dot tethering point. But it did quite a bit of work to make the topological state work in the wire. The materials the company uses are relatively unusual: aluminum as the superconducting wire and indium arsenide as the semiconductor that surrounds it. Microsoft makes all the same hardware.