'Uncrackable' codes set for step up

Zeroes and ones Quantum cryptography is a way to share secret digital keys

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A system that allows electronic messages to be sent with complete secrecy could be on the verge of expanding beyond niche applications.

A team of British scientists has discovered a way to build communications networks with quantum cryptography at a larger scale than ever before.

Quantum cryptography has the potential to transform the way sensitive data is protected.

Details appear in Nature journal.

The system is based on a communication system, where information is carried by individual photons - single particles of light.

Once these single photons of light are observed, they change. That is, they cannot be intercepted by an "eavesdropper" without leaving a detectable trace.

Quantum key distribution

  • QKD is a method to share secret digital keys - random combinations of 0s and 1s - securely on a communication network
  • Those keys can then be used to encrypt or authenticate data to stop other people reading or altering it
  • Once two parties have swapped a key that they know to be safe they can be sure that the messages they are sending each other are secure
Secret communication

Until now, implementing a quantum cryptography network had required a new fibre and an elaborate photon detector for each additional user that was added to the network, at considerable expense.

The team says they have now extended the way to send uncrackable codes - referred to as "quantum key distribution" (QKD) - beyond very niche applications.

Andrew Shields of Toshiba's Cambridge Research Laboratory and colleagues, have demonstrated that up to 64 users can share a fibre link and detector.

The network works on standard fibres that allows information from multiple users to be combined and transmitted on a single fibre.

Encoding information on individual photons of light has the "unique virtue that it allows the secrecy of the communication to be tested", said Dr Shields.

"Now we can connect multiple users up to one single fibre and allow them to share a connection to a quantum network.

"The advantage of that is we can now build quantum networks with many more users than has been possible in the past, which also reduces the cost per user," he told BBC News.

Optical fibre spool and man Existing optic fibres can be used to carry quantum codes

The team said their work could make QKD more practical and was now closer to being a widespread technology that could be used by businesses, banks and government organisations.

Mobile cryptography

Hannes Huebel of Stockholm University, Sweden, said the new work was a breakthrough finding that demonstrated that QKD could soon be used more widely.

He said in the next decade people could even have a laser in their smartphones which would allow them to send encrypted information to others.

This is already one step closer to happening as this week a team from the University of Bristol, say they have developed a way of sending secret quantum messages on handheld devices.

The team, writing in a paper published on Arxiv.org, report: "This opens the way for quantum enhanced secure communications between companies and members of the general public equipped with handheld mobile devices, via telecom-fibre tethering."

Dr Huevel explained that at the moment the technology was still mainly lab-based with highly specialised people operating the technology.

"The aim is to go away from this to make it much user friendly and cheaper. This new research is one step closer, it's the last step between the end user and a proper network," Dr Huebel added.

'Middleman attacks'

Some however disagree that total security can be achieved with quantum cryptography.

Karl Svozil, a theoretical physicist at the Vienna University of Technology, Austria, said the protocol used in the current work was not secure against all eavesdropping methods and required that the classical channel must be uncompromised for quantum cryptography to work.

If there were active "middleman attacks", there could be "active eavesdropping", he said.

"The condition of quantum cryptography relies on certain rules that need to be obeyed - only then is it unconditionally safe. The newly proposed protocol is 'breakable' by middlemen attacks."

The quantum access network

Quantum key distribution system
  • Each bit of the secret key is encoded on a single particle of light (photon)
  • Quantum theory dictates that single photons cannot be intercepted without changing their encoding, this means an eavesdropper will always leave a trace
  • The signals from multiple users can be combined and transmitted on a single fibre
  • The receiver can be used to form a secret key with each transmitter
  • Can detect up to one billion photons per second allowing up to 64 users on one link

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