Australian scientists have used artificial atoms called quantum dots to make a quantum silicon chip they say is surprisingly stable. The quantum dots make quantum bits, or qubits, and the qubits’s instability has been a bottleneck in designing this kind of chip.
The University of South Wales has made news for previous qubit developments like a suggested periodic table, and the same team is working on the new silicon qubit chips. The quantum dots themselves are made of silicon, and in an artificial atom structure, electrons still swirl and act, but just aren’t surrounding an elemental nucleus.
For this circuit, the research team called on its artificial atoms that have higher numbers of electrons, which the researchers found made for “much more robust qubits” in the resulting chip. These artificial atoms are made by stimulating silicon with electricity in a specific way that attracts electrons from the surrounding silicon material. The electrons begin to arrange themselves right away. The center is a “gate electrode,” and the electrons surrounding it are in 2D orbits rather than the 3D orbits found in nature.
All of this happens in a space of just 10 nanometers, and the extreme tininess and precision required means that even slightly, microscopically impure silicon materials could throw off the whole process. The group found that as more electrons were attracted to the gate electrodes, a bigger, more densely packed outermost shell protected the innermost layers from instability. Suddenly, the same exact sample of silicon could foster more stable, more robust qubits.
Naturally occurring silicon has an atomic number of 14, with 14 electrons that are arranged in valence layers of 2, 8, and 4. In this experiment, researchers found that artificial atoms also were most stable with three valence layers. These layers create the stability. “Their utility indicates that it is not necessary to operate quantum dot qubits at single-electron occupancy, where disorder can degrade their reliability and performance,” the team writes.
Think of a quantum dot in this scenario as a kid jumping rope. It takes concentration and effort to keep jumping and spinning the rope. But the same kid also has to watch for people walking into their space and disrupting their activity, and when that happens, they’re expected to handle it while still jumping rope. Now, there’s a circle of enforcers preventing interference, letting the jumper keep jumping.
Within their setup, the configurations that result in qubits depends on how many valence electrons there are.
“We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. Various fillings containing a single valence electron—namely 1, 5, 13 and 25 electrons—are found to be potential qubits,” the team writes. These artificial setups match real elements, too. “When we create the equivalent of Hydrogen, Lithium and Sodium in the quantum dot, we are basically able to use that lone electron on the outer shell as a qubit,” participating Ph.D. student Ross Leon says.
The extra electron that spills over into a new shell tilts the artificial atom’s energy, allowing it to be positioned to carry data. Once electrons are in pairs, they cancel each other out by design, which is why the “qubit values” are all odd.
The team says its next step is to examine how these neighboring quantum dots can interact or even bond, and how that will affect their computing.