Quantum entanglement, which Einstein dubbed “spooky action at a distance”, would be the perfect way to communicate data – if technical hurdles could be overcome.

The method involves linking the quantum properties of two objects such that a change to one is instantly reflected in the other – offering a whole new way to transmit information from opposite sides of the globe.

Entanglement has already been exploited as a way to securely share pass phrases for secret communications, but only over distances of less than 200 kilometres. The inability of the gas-based quantum computer memory used to hold onto information for more than a fraction of a second is to blame.

Now a way to have that memory store quantum information for longer opens up the possibility of entangled communication over 1000 kilometres.


Short memory

While regular computer DRAM memory – which stores information as 1s and 0s, or digital bits – is also short lived, it is repeatedly rewritten every 9 to 70 nanoseconds to keep the data fresh.

But quantum information, stored in quantum bits called qubits, cannot be simply refreshed. The rules of quantum mechanics mean that reading out the state of a qubit changes that state. This means you cannot recreate the previous piece of data because you don’t know what it was.

Holding onto a memory even for a fraction of a second is difficult for qubits, says Stewart Jenkins at the Georgia Institute of Technology.

That limits the distance over which entanglement can be used because it requires the state of one qubit to be copied to another, distant qubit. The message is carried by photons which, although they travel at light speed, still take time to get there.

If the first qubit has forgotten the quantum state it transmitted by the time the photons reach their destination, entanglement cannot happen. The first qubit must be able to hold onto its memory long enough for the second to match it.

Magnetic shield

Jenkins and colleagues have now succeeded in creating quantum memories that last for 7.2 microseconds – more than two orders of magnitude longer than previously reported, and time enough to transmit quantum information over 1000 kilometres.

While qubits made in other ways can hold memories for longer, they struggle to transfer them to photons.

The team’s qubits are stored in gas atoms, encoded into a magnetic property known as “spin”. The key to lengthening the attention span of gas qubits is to shield them from magnetic fields that can distort their spin and dissolve the stored state.

Jenkins’ team has done just that by encoding the spin information into particular energy levels within the atoms that are relatively immune to magnetic disturbances.

However, there are still “several technical hurdles to jump” before quantum communication over 1000 km is possible, says Jenkins.

Solid rival

John Morton, a quantum information researcher at Oxford University in the UK agrees. Currently, Jenkins’ qubits do not transfer memory well between atoms and photons, he says.

“The efficiency is still of the order of 10%, so that’s going to affect the quality of the quantum entanglement,” says Morton.

Although that is still better than solid-state systems like those he works on, Morton thinks that technology will ultimately catch up. “Things are harder in the solid state, but technology companies do become a lot more interested when they can imagine the technology as solid state,” says Morton.

For example, recent work shows defects inside diamonds can help transfer data to photons, he says (Nature, DOI: 10.1038/455606a).

Journal reference: Nature Physics (DOI: 10.1038/NPHYS1152)