Quantum dot boost for quantum computing
One of the key features of the devices is a two by two quantum dot lattice on a commercial foundry service. The mass production of the array is a key step for improving error correction achitectures in quantum computers.
“What we have shown is that we can realize single electron control in every single one of these quantum dots. This is very important for the development of a qubit, because one of the possible ways of making qubits is to use the spin of a single electron,” said Fabio Ansaloni, a researcher at the centre for Quantum Devices at NBI. “Reaching this goal of controlling the single electrons and doing it in a 2D array of quantum dots was very important for us.”
Extending quantum computers processors with 2D quantum dot arrays is key for a more efficient implementation of quantum error correction routines. Quantum error correction will enable future quantum computers to be fault tolerant against individual qubit failures during the computations.
“The original idea was to make an array of spin qubits, get down to single electrons and become able to control them and move them around. In that sense it is really great that Leti was able to deliver the samples we have used, which in turn made it possible for us to attain this result,” said Anasua Chatterjee, assistant professor at NBI.
“A lot of credit goes to the pan-European project consortium, and generous funding from the EU, helping us to move from the level of a single quantum dot with a single electron to having two electrons, and now moving on to the two dimensional arrays. Two dimensional arrays is a really big goal, because that’s beginning to look like something you absolutely need to build a quantum computer. So Leti has been involved with a series of projects over the years, which have all contributed to this result.”
Next: Extending quantum dot research
In 2015, researchers in Grenoble succeeded in making the first spin qubit, but this was based on holes, not electrons. The progress is threefold say the researchers.
“First, producing the devices in an industrial foundry is a necessity,” said Chatterjee. “The scalability of a modern, industrial process is essential as we start to make bigger arrays, for example for small quantum simulators. Second, when making a quantum computer, you need an array in two dimensions, and you need a way of connecting the external world to each qubit,” she said.
“If you have 4-5 connections for each qubit, you quickly end up with a unrealistic number of wires going out of the low-temperature setup. But what we have managed to show is that we can have one gate per electron, and you can read and control with the same gate. And lastly, using these tools we were able to move and swap single electrons in a controlled way around the array, a challenge in itself,” she added.
Controlling errors occurring in the devices is key. State-of-the-art physical qubits do not have low error rate yet, but if enough of them are combined in the 2D quantum dot array, they can monitor each other.
The result at NBI shows that it is now possible to control single electrons, and perform the experiment in the absence of a magnetic field. So the next step will be to look for spins – spin signatures – in the presence of a magnetic field.
This will be essential to implement single and two qubit gates between the single qubits in the array. Theory has shown that a handful of single and two qubit gates, called a complete set of quantum gates, are enough to enable universal quantum computation.
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