Originally published on 19/11/10 in felix, the student newspaper of Imperial College London.
Do you think you could make sense of this sentence if every fourth word was missing? How about trying to hold a conversation when you can only hear three quarters of what the other person is saying? Cutting out a fraction of the information being transferred in a given situation may make life slightly difficult, but it certainly doesn’t stop the meaning being conveyed in most cases. This is because of the redundancy built into language. However, redundancy is not only useful for conversations on a dodgy phone line – it can also come in handy in the world of quantum computing, as two researchers explained in a paper published in Physical Review Letters last week.
The research was carried out by Sean Barrett, of Imperial College, and Thomas Stace, at the University of Queensland in Brisbane, Australia. They found that if a quarter of the qubits (the quantum equivalent of bits, which store information in a classical computer) are lost, the computer can still function as normal. Barrett and Stace looked at the remaining information and used a code that could check for errors to decipher what was missing. “It’s surprising, because you wouldn’t expect that if you lost a quarter of the beads from an abacus that it would still be useful,” said Dr Barrett.
One of the main differences between a classical bit and its quantum equivalent is that the latter can exhibit entanglement. This means that, no matter how far away two entangled qubits are, if one changes so will the other – instantaneously. Quantum computers take advantage of this effect, as well as another property of quantum systems known as superposition, to perform complicated calculations much faster than classical computers. At the moment, though, the largest quantum computers have only two or three qubits.
It had previously been thought that large quantum computers would be very sensitive to missing information, but this research shows that they should be much more robust than we’d imagined. At this stage, the work is theoretical and scientists must do a lot more in order to make quantum computers bigger than a few qubits in the lab.
When large quantum computers are a reality, they may have the potential to revolutionise fields as far apart as drug modelling, electronics and code breaking. However, we won’t know exactly what applications quantum computers will be best suited to until we’re able to make one.
“At the moment quantum computers are good at particular tasks, but we have no idea what these systems could be used for in the future,” said Dr Barrett. “They may not necessarily be better for everything, but we just don’t know. They may be better for very specific things that we find impossible now.”
Sean D. Barrett, & Thomas M. Stace (2010). Fault tolerant quantum computation with very high threshold for loss
errors Phys. Rev. Lett. 105, 200502 (2010) arXiv: 1005.2456v2