Unleashing the potential of quantum computing
Unlike electronic computers which all use transistor based microprocessors, there are many different ways to make a quantum computer, though few if any have yet proved totally practical. One of the most promising candidates is a crystal containing rare earth ions. Data is written in with a laser of incredibly well controlled wavelength and stored in the form of excited ions within the crystal. The quantum calculations are then performed by the interaction between these billions of excited ions.
In an everyday environment those subtle quantum interactions between different ions in the crystal are absolutely swamped by external influences such as the buffeting they receive from thermal vibrations and light shining in. So to make a working computer, scientists need to cool the crystal down close to absolute zero. But even then, problems remain.
Rose Ahlefeldt has just completed her PhD at the Australian national University, during which she’s been working on the growth of crystals for use in quantum computers.
“If the crystal isn’t perfect, in other words if the atoms aren’t all lined up exactly as they should be, any ions that sit near the defects experience different conditions to their neighbours” Rose says, “So the way they store and manipulate the laser light changes and the computer won’t work. So a lot of my work has focused on developing better ways to crystalise the materials we need.”
All crystals grow by adding successive atoms, just like a bricklayer building a wall. And just like the bricklayer, the faster the job is done the worse the quality of the work tends to be. In the case of crystal growth it’s the temperature of the solution that dictates how fast the atoms are laid down. If the solution is cooled too quickly, the atoms go on haphazardly leaving gaps and wrongly seated atoms, creating crooked rows in the lattice.
“The process starts with the creation of a good seed crystal. That’s really important because it will dictate how the main crystal develops. We also have to carefully control the temperature and solution strength so that we get the best possible result,” Rose says, “But even if every atom were in the right place, there can still be problems because of different isotopes.”
An isotope is an atom with fewer or additional neutrons in its nucleus. Because they have the same number of electrons, the chemical properties are usually exactly the same. “With many elements like carbon, there’s one common isotope and a couple of other very rare ones that are present in tiny proportions. However with europium and chlorine, the main ingredients in our crystal, isotopes are far more prevalent.” Rose explains. “They don’t make any difference chemically but the requirements for a working quantum computer are so stringent that the tiny difference that extra neutron makes can become a problem.”
The next step will be to grow crystals from a solution of isotopically pure chlorine. “isotopically pure salts are very difficult to produce and as a result they’re quite expensive. We’ve recently bought about half the world’s current supply which is still only a few grams. Naturally we want to get our growth techniques spot on before we start using that.”
The current world of quantum computing is a strange mixture of ultra high technology and some strangely incongruous every day materials. “Having tried many sophisticated methods to hold the delicate crystals in the near absolute zero temperatures of the cryostat, one of the most successful is common dental floss!” Rose explains. “Science is a bit like that, the functional properties of materials don’t often relate to our human perception of how good or exotic they are. So if dental floss works best, that’s what we use!”