An artificial atom as qubit

With a pioneering project like the quantum computer, it’s a good idea not to place all your bets on a single horse. In Leiden’s Quantum Optics research group, instead of working on a Majorana-based qubit, people are working on a qubit based on an ‘artificial atom’. If that becomes the basis of the quantum computer, this computer will make calculations using infrared light instead of tiny electric currents.

Electron in a cage

It is already almost possible to make these qubits using techniques that are commonplace in the manufacture of conventional microprocessors. This is significant, because that way, later on you can scale up more quickly from a lab prototype to serial production. These qubits are composed of tiny lumps of a semiconducting material (indium arsenide), embedded in another semiconducting material (gallium arsenide). By precisely controlling conditions during the production of the chip, which has multiple qubits on it, and cooling the chip to 5 degrees above absolute zero (–268 degrees Celsius), you can ensure that each lump contains exactly one extra electron, as if it were trapped in a little cage.

If you shine infrared laser light on one of these cages, the electron can absorb a photon (an individual light particle), putting it in a different state. This closely resembles the way an atom reacts to the entrapment of the photon, which is why this is also called an artificial atom. This is how the mixture of two states (0 and 1) necessary for a qubit is created.

Nano-mirrors

Over the last few years, under the direction of Martin van Exter and Dirk Bouwmeester, important advances have been made in refining these artificial atoms into usable qubits. One problem was that an artificial atom only absorbs a small portion of the incoming photons.

By adding an extra step to the production process, it became possible to place two nano-mirrors around the artificial atom, so to speak. An incoming photon bounces back and forth between the two mirrors more than a thousand times. The chance that it will get absorbed by the artificial atom one of these times is then proportionally greater. The researchers have also mastered the technique of ‘reading’ a qubit or ‘writing’ to it with a single photon, which is necessary for making quantum calculations.

Making this sort of qubit involves a great deal of trial and error, because the properties of the nano-mirrors and the artificial atom need to be a perfect match. Producing a perfect qubit still requires a good measure of good luck, but fortunately not many of them are needed for these experiments. Once a chip with a good qubit has cooled to its operating temperature, it doesn’t wear out. One qubit that turned out exceptionally well has worked for more than a whole year.

Playing quantum games

The next step will be to shoot multiple photons at a qubit in succession, in such a way that they all become entangled. In a quantum computer, qubits need to communicate with other qubits. Sometimes this makes it necessary to entangle qubits and photons with each other. Van Exter calls this ‘playing more quantum games and more complicated games’. This can eventually lead to a quantum chip on which hundreds of artificial atoms communicate with each other using photons. But for the time being the focus is on an intermediary step: ‘What we’re concerned with now is the fundamental physics. We want to show that certain quantum operations in our system work well, making it suitable for an essential building block of a quantum computer.’