A team of research engineers from Australia previously created artificial atoms on a silicon chip for use in quantum computing. Now, further advancements in their technology have been published in Nature Communications.
The artificial atoms are known as quantum dots, which are nanoscale semiconducting crystals with a space that traps electrons, thus confining their movement in three dimensions and holds them in place within electric fields.
The team found a way to minimize the error rate of the previous studies that had over 99% accuracy. This time around they have found a way to make the artificial atoms more stable, giving them the ability to produce more quantum bits (qubits).
Quantum Engineer Andrew Dzurak, from the University of New South Wales (UNSW) in Australia, talks about their latest achievement:
What really excites us about our latest research is that artificial atoms with a higher number of electrons turn out to be much more robust qubits than previously thought possible, meaning they can be reliably used for calculations in quantum computers. This is significant because qubits based on just one electron can be very unreliable.
The previous imperfections were caused by silicon. Now, the scientists developed a way to attract spare electrons from the silicone into the artificial atoms using a “metal surface gate electrode,” which applies voltage to the silicon.
Andre Saraiva, a Solid State Physicist from UNSW, explains the difference in how a real atom works compared to this new artificial atom:
In a real atom, you have a positive charge in the middle, being the nucleus, and then the negatively charged electrons are held around it in three-dimensional orbits.
In our case, rather than the positive nucleus, the positive charge comes from the gate electrode which is separated from the silicon by an insulating barrier of silicon oxide, and then the electrons are suspended underneath it, each orbiting around the center of the quantum dot. But rather than forming a sphere, they are arranged flat, in a disc.
As reported in Science Alert; hydrogen, lithium, and sodium are elements that can have just one electron in their electron shell and this is the model that’s used in quantum computing. With these new artificial atoms, which are equivalent to hydrogen, lithium, and sodium, the single electron can be used as a qubit.
These qubits become much more powerful because they can be in a state called superposition, giving them the ability to perform parallel computations based on their spin states.
After achieving these results the first time around, imperfections with the silicon at atomic levels were enough to throw off the qubits, leading to errors.
To fix this problem they needed to make the shells predictable and well organized like real atoms. The team was able to mimic heavier atoms that have multiple electron shells, by turning up the voltage on the gate electrode, it allowed more electrons to enter.
Dzurak explains:
When the electrons in either a real atom or our artificial atoms form a complete shell, they align their poles in opposite directions so that the total spin of the system is zero, making them useless as a qubit. But when we add one more electron to start a new shell, this extra electron has a spin that we can now use as a qubit again. Our new work shows that we can control the spin of electrons in the outer shells of these artificial atoms to give us reliable and stable qubits. This is really important because it means we can now work with much less fragile qubits. One electron is a very fragile thing. However, an artificial atom with 5 electrons, or 13 electrons, is much more robust.
