Anyone who has tried to lead a group of tourists through a busy city knows the problem. How do you keep the group together when they are constantly jostled, held up and distracted by the hubbub around them?
It's a problem the designers of quantum computers have to tackle. In some future quantum computers, information will be encoded in the delicate quantum states of groups of particles. These face jostling by noise and disorder within the materials of the processor. Now, an international team has proposed a scheme that could help protect groups of particles and enable them to move together without any getting lost or held up.
The proposal, published 17 November in Physical Review Letters, comes from researchers at the National University of Singapore (NUS), Technical University of Crete, University of Oxford and Google. Their paper puts forward a scheme that can reliably transport quantum states of a few photons along a line of miniature quantum circuits. Simulations show that it should efficiently move a three-photon state from one circuit site to the next over dozens of sites: the particles jump together throughout and finally appear at the other end undisturbed, with no spreading out.
The scheme is based on the ideas of physicist David J. Thouless, who won half the 2016 Nobel Prize in physics for his work on topological effects in materials. Topological effects are to do with geometry, and their use in quantum computing can help protect fragile quantum states during processing.
One of Thouless' major contributions was the invention of 'topological pumping'. This works something like Archimedes' screw pump for water. The Ancient Greek's screw spins around, but the water within it travels in a straight line up a hill. "Even though the motion of the machine is cyclical, the motion of the particles is not, they move in a line," explains Jirawat Tangpanitanon, first author on the paper and a PhD student in the group of Dimitris Angelakis at the Centre for Quantum Technologies (CQT) at NUS.
In the quantum scheme, the screw thread is not a physical structure but an oscillating external field imposed on the particles by electronic control over the device that contains them.
Angelakis started his group looking into topological pumping after others in 2015 demonstrated the effect for individual, non-interacting, particles. Angelakis, Tangpanitanon and Research Fellow Victor Bastidas wanted to find out if it would be possible to move groups of particles coherently too.
The answer is yes. What's more, unlike Archimedes' pump, which can only move water one way, the quantum particles can even be sent into reverse by changing the initial conditions. "It's like a moonwalk," jokes Tangpanitanon. It looks like everything should be moving forward, but instead the particles go backwards due to quantum effects.
Co-author Pedram Roushan - part of the Google group in Santa Barbara, California building superconducting circuits for quantum computing - and the team hopes to see the idea implemented in similar hardware. "This paper is almost a blueprint. We developed the proposal to match existing devices," says Angelakis, who is a Principal Investigator at CQT and a faculty member at the Technical University of Crete.
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Jirawat Tangpanitanon et al, Topological Pumping of Photons in Nonlinear Resonator Arrays, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.213603 , https://arxiv.org/abs/1607.04050