Industry collaboration leads to important milestone in the creation of a quantum computer
From: Niels Bohr Institute
December 28, 2020 – For a quantum
computer, one of the obstacles for
progress in the quest for a working quantum computer has been that the working
devices that go into a quantum computer and perform the actual calculations,
the qubits, have hitherto been made by universities and in small numbers. But
in recent years, a pan-European collaboration, in partnership with French
microelectronics leader CEA-Leti, has been exploring
everyday transistors--that are present in billions in all our mobile phones—for
their use as qubits.
The French company Leti makes giant
wafers full of devices, and, after measuring, researchers at the Niels Bohr
Institute, University of Copenhagen, have found these industrially produced
devices to be suitable as a qubit platform capable of moving to the second
dimension, a significant step for a working quantum computer. The result is now
published in Nature Communications.
Quantum dots in two dimensional array is
a leap ahead
One of the key features of the devices
is the two-dimensional array of quantum dot. Or more precisely, a two by two
lattice of quantum dots. “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. So reaching this goal of
controlling the single electrons and doing it in a 2D array of quantum dots was
very important for us”, says Fabio Ansaloni, former PhD student, now postdoc at
center for Quantum Devices, NBI.
Using electron spins has proven to be
advantageous for the implementation of qubits. In fact, their “quiet” nature
makes spins weakly interacting with the noisy environment, an important
requirement to obtain highly performing qubits.
Extending quantum computers processors to
the second dimension has been proven to be essential 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 importance of industry scale
production
Assistant Professor at Center for Quantum Devices,
NBI, Anasua Chatterjee adds: “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. A lot of credit goes to the pan-European project consortium, and
generous funding from the EU, helping us to slowly 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.”
The credit for getting this far belongs
to many projects across Europe
The development has been gradual. In 2015,
researchers in Grenoble succeeded in making the first spin qubit, but this was
based on holes, not electrons. Back then, the performance of the devices made
in the “hole regime” were not optimal, and the technology has advanced so that
the devices now at NBI can have two dimensional arrays in the single electron
regime. The progress is threefold, the researchers explain: “First,
producing the devices in an industrial foundry is a necessity. 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. 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.”
Two dimensional arrays can control
errors
Controlling errors occurring in the devices is a
chapter in itself. The computers we use today produce plenty of errors, but
they are corrected through what is called the repetition code. In a
conventional computer, you can have information in either a 0 or a 1. In order
to be sure that the outcome of a calculation is correct, the computer repeats
the calculation and if one transistor makes an error, it is corrected through
simple majority. If the majority of the calculations performed in other
transistors point to 1 and not 0, then 1 is chosen as the result. This is not
possible in a quantum computer since you cannot make an exact copy of a qubit,
so quantum error correction works in another way: State-of-the-art physical
qubits do not have low error rate yet, but if enough of them are combined in
the 2D array, they can keep each other in check, so to speak. This is another
advantage of the now realized 2D array.
The next step from this milestone
The result realized at the Niels Bohr
Institute 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.
Link to the Scientific article: https://www.nature.com/articles/s41467-020-20280-3
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