So far, two main quantum computing technologies have been commercialized. One type of material, called a transmon, involves loops of superconducting wire connected to a resonator; it is used by companies like Google, IBM and Rigetti. Companies like Quantinuum and IonQ have instead used individual ions held in light traps. At the moment, the two technologies are in a delicate position. They have been clearly shown to work, but they need significant scaling and quality improvements before they can perform useful calculations.
It may be a little surprising to see that Microsoft has committed to an alternative technology called “topological qubits”. This technology is far enough behind other options that the company has just announced that it has worked out the physics to create a qubit. To better understand Microsoft’s approach, Ars spoke with Microsoft engineer Chetan Nayak about the company’s progress and plans.
The base of a qubit
Microsoft starts behind some competitors because the basic physics of its system has not been fully understood. The company’s system relies on the controlled production of a “Majorana particle”, something that has only been demonstrated to exist in the last decade (and even then its discovery has been controversial) .
The particle takes its name from Ettore Majorana, who came up with the idea in the 1920s. Simply put, a Majorana particle is its own antiparticle; two Majorana particles that differ in spin would annihilate if they met. So far, none of the known particles appear to be Majorana particles (all but neutrinos definitely are not). But the concept has endured because of the prospect of making Majorana quasiparticles, or a collection of particles and fields that, in certain contexts, behave as if they were a single particle.
Probably the most important quasiparticle is the Cooper pair, in which two electrons are paired in a way that changes their behavior. Cooper pairs are necessary for superconductivity to work.
Nayak said Microsoft’s system involved superconducting wire and its Cooper pairs. Under normal circumstances, having an extra unpaired electron incurs a cost on the total energy of the system. But in a sufficiently small wire in the presence of magnetic fields, it is possible to stick an electron to the end of the wire without energy cost. “In a topological state and a topological superconductor, you end up having states that can, at no energy cost, absorb an extra electron,” Nayak told Ars.
This being quantum mechanics, the electron is not located at the end of the wire where it is inserted; instead, it is delocalized at both ends. “The two ends are the real and imaginary parts of this quantum wave function, essentially,” Nayak said. These end states are called Majorana zero modes, and Microsoft now claims to have created them and measured their properties.
From quasiparticle to qubit
Alone, Majorana’s zero modes are not usable as qubits. But Nayak said it was possible to link them to a nearby quantum dot. (Quantum dots are pieces of a material sized to be smaller than the wavelength of an electron in that material.) He described a U-shaped wire with Majorana zero modes at each extremity and these extremities near a quantum point.
“You can effectively, as a virtual process, have an electron tunnel from the quantum dot on one Majorana zero mode and an electron tunnel from the other Majorana zero mode and on the quantum dot,” Nayak said. in Ars. These exchanges alter the quantum dot’s ability to store charge (ie its capacitance), a property that can be measured. Nayak also said that the connections between the wire and the quantum dots can be controlled, potentially allowing Majorana’s zero modes to be disconnected, which would help preserve their state.
Microsoft hasn’t gotten to the point of connecting a quantum dot. But he did a tremendous job of getting the topological state to work in the thread. The materials used by the company are relatively unusual: aluminum as the superconducting wire and indium arsenide as the semiconductor that surrounds it. Microsoft manufactures all the devices itself.