Quantum computers are cool right now, but they won't be in the future.
Room temperature phosphorous atoms have been placed in a fragile quantum state called a superposition and survived for a leisurely 39 minutes – long enough for each one to perform 2 million calculations if they were to form the guts a quantum computer. The finding could herald the widespread use of these futuristic machines.
Ordinary computers store data as bits that are either 1s or 0s, but quantum computers have qubits, which are in a state of superposition, and can be both at the same time. In principle, this strange property has the power to crunch numbers much more rapidly than ordinary computers – though a quantum computer big enough to take advantage of this potential has yet to be built.
Unfortunately, superpositions are very delicate, meaning most current quantum computers need to be chilled to near absolute zero to work properly. That limits their use to highly specialised buildings and laboratories.
To create a room temperature superposition, which would be much more useful, Mike Thewalt of Simon Fraser University in Burnaby, Canada, and colleagues focused on a component of a quantum computer – the qubit storage system, which holds quantum information to be processed and is analogous to an ordinary computer's hard drive.
The team used a small slab of silicon infused with phosphorus atoms, which serve as the qubits. Each atom's nucleus has a property called spin that can point in different directions – spin up corresponds to a 0, while spin down is a 1, and any other angle is a superposition of the two.
First they placed the silicon in liquid helium, chilling it to just 4.2 degrees above absolute zero, before using a magnetic field to place each qubit in a superposition.
Suspecting that electrons whizzing around in the phosphorous atoms might play a role in destroying the superposition, the team zapped the atoms with a laser first, removing an electron from each. "We recognised the electron is a big source of noise," says Thewalt's colleague Stephanie Simmons at Oxford University.
The technique seemed to work. The superposition lasted for three hours at this low temperature, when previous similar experiments had only lasted for three minutes. What's more, when they raised the temperature to 25 °C the normally fragile quantum state survived for 39 minutes – more than enough to perform 2 million quantum calculations, says Simmons.
For this experiment the team had to cool the system down again in order to read off the state of the qubits, but that is a limitation of their current measurement technique rather than a fundamental hurdle, says Simmons.
In order to actually store information, they will also need to develop ways to put each qubit in its own an individual state. For the current experiment all the qubits were in the same state, so the demonstration is only a proof of principle.
Improving the survival time of fragile atomic states is also a challenge for experiments that probe antimatter – but the record time for the survival of antihydrogen atoms is a mere 15 minutes.
Journal Reference: Science DOI: 10.1126/science.1239584
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