Quantum cryptography: 200Gbit/s over 100km fibre

Toshiba’s Cambridge Research Lab has demonstrated 200Gbit/s quantum key protected data transmission over a single 100km long cable – a world record.

It used six wavelength channels in total, two each at 100Gbit/s for the data and four including data and clock channels for the quantum keys.

One of the things that makes the scheme difficult is cross-talk: the conventionally encoded data bits are each represented by over a million photos, while bits in the quantum key are received on a single photo. Raman scattering spills data photons into the key channel.

“Lots of scattered light makes detecting key photons difficult,” Dr Andrew Shields, assistant MD of Toshiba Research Europe, told Electronics Weekly. “We filter the arriving key channel light in wavelength and time.”

Wavelength filtering is through a optical bandpass filter – narrower than those used in normal WDM (wavelength division multiplexed) fibre networks, and time filtering is based opening the receiver only when a key photon might be expected – a key photon is sent every nanosecond, signalled by the clock channel, and is around ~100ps long at the receiver.

Single photons

The key is encoded into the phase of single photons. “We could to polarisation, but we chose to use phase,” said Shields.

Detecting the phase of single photo at high speed is tricky to say the least.

For this a 1Gphoton/s receiver has been developed that uses a semiconductor-based avalanche photodiode followed by an interferometer with two outputs – one for each expected phase state.

Like many cryptography techniques, the key is sent using a snoop-resistant process while the encrypted data is sent in a more open way.

In the case of quantum cryptography, the way the key is sent is snoop-proof – as far as anyone can tell.

To send the key, pulses of photons are modulated with a key bit then, before they are launched into the fibre, they are attenuated and attenuated until there may or may not be a surviving single photo.

Once in the fibre, this remnant, but still phase-modulated, photon either makes it to the far end or is scattered on the way.

Sender and receiver exchange information about which photons actually arrived, but not about the their states – ‘the first photon, the tenth photon, the……’, for example.

In this way, both ends know which photons arrived, and what their polarisation was, but a snooper can only watch the back-channel and learn which photons arrived, not their polarisation.

Schrödinger’s cat

If the snooper tries to measure the phase as the photos pass, the phase will be corrupted – It is a Schrödinger’s cat situation, said Shields: “A man in the middle will change encoding. There is nothing they can do to gain knowledge without changing the encoding. They will always reveal themselves.”

Being able to send whole keys in less than 1ms means the key can be changed at more than 1kHz, cutting the length of data stream encoded with a single key, therefore further reducing the chance that statistical analysis will reverse-engineer the key from the data. For a fibre length of 36km, the secure key rate exceeds 1.9Mbit/s, sufficient for over 5,000 encryption keys per second.

For the record-breaking trial, which broke the Lab’s own 40Gbit/s record, Toshiba’s Cambridge Research Lab worked with ADVA Optical Networking (which provided the transmitter) and BT’s R&D hub at Adastral Park near Ipswich where the trial took place.

Secure the genome

Toshiba is also involved in a system to secure genome data using quantum cryptography in Sendai, Japan.

This kind of data is unique in that it might have to stay secure for many human generations. “If it is encrypted using today’s technology, someone might save it [snooped data] and decrypt it when computers are more powerful,” said Shields. “You can’t do this with quantum encrypted data, even with quantum computers.”

Governments and banks are other potential users.

The next step in Cambridge is to build a network and demonstrate end-to-end quantum cryptography through that. According to Shields, his team has already demonstrated it working through an optical switch. “It the network is all-optical, we should be able to do it end-to-end,” he said. “If it has electrical switching, there has to be in physically-secure area, for example at a telco. There are ways to mitigate security at these intermediate nodes: doing different data blocks at different nodes, for example.”

The Cambridge team is also collaborating in an EPSRC-funded long-term trial involving building a Cambridge-London-Bristol quantum cryptography network with metro networks at the Cambridge and Bristol nodes. Potential users will be able to use it for application development.

steve bush

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