Toshiba claims breakthrough in 'unhackable' quantum networks

New technique enabled researchers to send quantum information over a distance of 600 km via optical fibres

A team of researchers working at the Cambridge Research Laboratory of Toshiba Europe claims to have set a new quantum communication record by successfully sending quantum information over a distance of 600 km via optical fibres.

The researchers said that their experiments showed that it was possible to transmit quantum bits (or qubits) over hundreds of kilometres of optical fibres without disturbing the fragile quantum data encoded in the particles.

The team expects the breakthrough to pave the way for large-scale quantum networks that are able to send information securely between cities and even countries.

Quantum communication systems are considered to be more secure than conventional networks as they rely on the quantum properties of photons, rather than computer code that can be cracked by hackers. However, such systems are very complex and expensive to make.

In order to communicate, quantum devices need to send and receive qubits that exist in a special, but extremely fragile, quantum state.

"One of the most difficult technological challenges in building the quantum internet, is the problem of how to transmit quantum bits over long optical fibres," the researchers said.

"Small changes in the ambient conditions, such as temperature fluctuations, cause the fibres to expand and contract, thereby scrambling the fragile qubits, which are encoded as a phase delay of a weak optical pulse in the fibre."

To address the instable conditions inside optical fibres, the researchers developed a novel 'dual band' stabilisation technique, in which two optical reference signals were sent along with the qubits themselves, which are encoded as a phase delay of a weak optical pulse.

The first reference signal is sent at a wavelength that cancels out fluctuations from the environment, while the second wavelength operates at the same wavelength as the optical qubits themselves and is used for fine adjustment of the phase.

In simple words, the two wavelengths when used in combination help cancelling environmental fluctuations inside the fibre in real time.

Using the technique, Toshiba team was able to hold the optical phase of a quantum signal constant to within a fraction of a wavelength, with a precision of 10s of nanometers, even after propagation through 100s of km of fibre. This allowed them to transmit the data over a distance of 600 km via optical fibres, which was about six times further than the previous record.

According to researchers, they have already used the new technique to test quantum-based encryption - one of the most well-known applications of quantum networks.

Known as Quantum Key Distribution (QKD), this protocol is used to establish a secure communication channel during the transmission of light particles over a network. With QKD, it becomes possible for two communicating parties to agree on an encryption key. In QKD, quantum properties are used to exchange the encryption key between two parties. The encryption key is then used to encrypt the messages transmitted over an insecure channel.

The QKD protocol can provide security against even the most arbitrarily powerful eavesdroppers. It is based on the fact that any attempt by an attacker to measure a quantum system will eventually disturb the entire system.

"This is a very exciting result," said Mirko Pittaluga, research scientist at Toshiba Europe.

"With the new techniques we have developed, further extensions of the communication distance for QKD are still possible and our solutions can also be applied to other quantum communications protocols and applications," he added.

The details of the research are published in the journal Nature Photonics.