'Dead time' limits quantum cryptography speeds

Quantum cryptography is potentially the most secure method of sending encrypted information, but researchers have warned that the technology is hampered by a "speed limit".

A new paper by researchers at the National Institute of Standards and Technology (Nist) and the Joint Quantum Institute (JQI) suggests that maximum transmission rates will stall at levels comparable to that of a single broadband connection.

This is unless researchers can reduce "dead times" in the detectors that receive quantum-encrypted messages.

The scientists explained that, in quantum cryptography, a sender, traditionally dubbed 'Alice', transmits single photons, or particles of light, encoding 0s and 1s to a recipient known as 'Bob'.

The photons Bob receives and correctly measures make up the secret 'key' that is used to decode a subsequent message.

Because of the quantum rules, an eavesdropper, 'Eve', cannot listen in on the key transmission without being detected.

But it would be possible to monitor a more traditional communication (such as a phone call) that must take place between Alice and Bob to complete their communication.

Modern telecoms hardware easily allows Alice to transmit photons at rates much faster than any internet connection.

But at least 90 per cent (and more commonly 99.9 per cent) of the photons do not make it to Bob's detectors, so that he receives only a small fraction of the photons sent by Alice, the researchers explained.

Alice can send more photons to Bob by increasing the speed of her transmitter, but they will run into problems with the detector's 'dead time', the period during which the detector needs to recover after it detects a photon.

Commercially available single-photon detectors need about 50-100 nanoseconds to recover before they can detect another photon, much slower than the one nanosecond between photons in a 1GHz transmission.

"Not only does 'dead time' limit the transmission rate of a message, it raises security issues for systems that use different detectors for 0s and 1s," the researchers stated.

"In that important 'phone call', Bob must report the time of each detection event. If he reports two detections occurring within the 'dead time' of his detectors, Eve can deduce that they could not have come from the same detector and correspond to opposite bit values.

"Bob can choose not to report the second, closely spaced photon, but this further decreases the key production rate. And for the most secure type of encryption, known as a one-time pad, the key has to have as many bits of information as the message itself."

The speed limit would go up, according to Nist physicist Joshua Bienfang, if researchers reduce the 'dead time' in single-photon detectors, something that several groups are trying to do.

Bienfang believes that higher speeds would also be useful for wireless cryptography between a ground station and a satellite in low-Earth orbit.

Since the two would be close enough to communicate for only a small part of the day, it would be beneficial to send as much information as possible during a short time window.