Quantum memory has been extended to six hours in an advance that brings the spirit of the Pony Express to quantum communications, raising the prospect of physical transport of ‘read once’ quantum ‘memory sticks’.

Researchers at the ARC Centre of Excellence for Quantum Computing and Communications Technology have also shown that quantum physics promises uncrackable communication, as any attempt to read the message will be immediately detectable. 

“This new, very stable memory is also incredibly steady for the hundred or so milliseconds required to transmit a signal around the globe. It’s the first valid candidate for quantum repeater stations supporting a global network,” says the Centre’s Matt Sellars.

Quantum information can be carried along optical fibres only a couple of hundred kilometres before too much of the signal is lost. A proposed solution is regular ‘repeater’ stations to maintain the signal—similar to Pony Express way stations providing riders with fresh horses every 10 miles. But quantum states are extremely fragile—any interaction with the outside world causes them to collapse, usually within a few milliseconds.

To date, even the best quantum memories have not shown the resilience needed by repeater stations in a global quantum network. The ARC Centre’s researchers have extended quantum memory operating time by a factor of 100 times, achieving memory lifetimes of up to six hours. The scientists coded quantum information onto atoms of europium, then isolated the atoms from any external interaction via precisely aligned magnetic fields, preserving the fragile quantum information. The memory time raises the potential of physical, quantum ‘memory sticks’ that could be used to courier information, and the fragility of quantum-encrypted information means any attempt to read it in transit would be immediately obvious. “It’s a ‘read once’ device,” says Matt.

Couriers could carry such secure memory devices from city to city; modern Pony Express riders with quantum-encrypted
communications in their saddlebags.

For more information: ARC Centre of Excellence for Quantum Computation and Communication Technology, University of New South Wales, Tony Raeside, admin-cqc2t@unsw.edu.au, www.cqc2t.org