Spooky interaction: quantum computing will take advantage of the entangled relationship between pairs of particles.
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Entangled pages
Quantum computing works in way analogous to classical computers, except that information is processed through qubits – quantum bits, rather than classical bits. Unlike classical bits, quantum bits can exist as either one state or another – such as 0 or 1 in binary language, the language used by computers – but also in a superposition of states. So a qubit may be a 1 or a 0, but its state is uncertain until it is measured.
Another advantage of quantum computers is that the information they use is contained not in the bits themselves, but in the correlation between the bits. The NSF uses the analogy of a book, where classical information is contained on the pages on the book, but quantum information in the relationship between the pages themselves. Thus if you remove a 'page' of the book, classical information is lost, but quantum information is unaffected.
What allows this to happen is a phenomenon occurring between quantum particles called entanglement. This property, described by Albert Einstein as 'spooky', is a non-local relationship between two particles such that an action undertaken on one particle – such as a measurement of its quantum state – affects the other particle, even if this particle isn't near its entangled partner. White describes it as "apparently nonsensical correlations" between particles.
It's entanglement which facilitates the ability of quantum information to 'teleport' — one of the more useful properties of quantum communication.
What's it all about?
All of this curious complexity allows quantum computers to process information exponentially faster than ordinary computers in certain applications, such as decrypting information.
"You can divide all problems into solvable and unsolvable. Of the solvable problems, some are efficiently computable and some are not," says White. Non-efficiently computable problems include factoring large numbers, the basis for cryptography behind, for example, online financial transactions.
In principle, quantum computers can solve these problems, hugely increasing the capacity for privacy and security, and also playing a role in detecting any attempt to 'break into' the code.
Quantum computing processes are also of interest to theoretical physicists like White, who are looking at fundamental problems in nature, such as how some semi-conductors work, or obtaining a full quantum simulation of molecules containing more than 30 atoms (caffeine, for example).
"If you have a full quantum model, you have a better understanding of a molecule works and reacts," says White. "[Qubits also represent] a controllable system that may shed more light on how quantum mechanics itself operates."
Making qubits
What's exciting scientists now are advances towards the components necessary for quantum computing: qubits and quantum gates. Over 20 different physical architectures are currently being investigated as contenders for components, the most successful being ions and photons.
Ions have been used to create the most qubits (eight), while photons have yielded the fastest quantum gates (quantum circuits analogous to classical logic gates in digital computers), and have been most comprehensively characterised, allowing researchers to detect crucial errors inherent in quantum processes.
Quantum information processing is prone to error because of the way components interact with the environment, known as decoherence. Up to seven qubits and several quantum gates may be needed to encode information reliably from one qubit, says White. To work out the factors for the number 15, for example (5 and 3), would take an enormous 4,000 qubits, each prone to error.
