Boffins pack more info onto photon for faster quantum key distro

Qubits are old hat: this uses 'qudits' - a photon with more than two simultaneous states

Alice in Alice's Adventures Underground. Pic credit: Jason Joyce

Researchers have packed extra information onto single photons to speed up quantum key distribution (QKD) systems.

QKD uses a characteristic of quantum mechanics to protect keys used to encrypt data using classical crypto schemes: if Eve tries to snoop on the key Alice is sending Bob, the quantum state/s a photon carries are destroyed. Alice and Bob know there's an eavesdropper, and the key Eve eavesdropped is useless.

However, compared to conventional telecommunications systems, QKD is slow: most systems based on photon-by-photon transmission of crypto keys run at speeds of hundreds of kilobits per second.

Research from Duke University's Nurul Taimur Islam, with collaborators from Ohio State University, Oak Ridge National Laboratory, and the National University of Singapore, achieved megabit key distribution rates using off-the-shelf components, meaning existing photonic QKD systems could be adapted to use it their work.

In a paper based on research funded by the United States Navy and Defense Advanced Research Projects Agency (DARPA), published in Science Advances and available as pre-press at arXiv, the researchers explained that to get faster key distribution rates, they worked to overcome the limits on photon detectors' speed.

The paper explained that if keys are distributed as one-qubit-per-photon, the system speed is limited by how fast states can be generated.

The solution is to encode more than one state on the photon – to turn it into a “qudit” (more than two dimensions per photon) instead of a qubit – in this case using time-of-arrival as one dimension, and the phase of the photon as the other.

“The secret key is calculated using the sifted photon time-of-arrival data, and the amount of extractable secret data is determined using the noise level observed in the sifted phase measurement data,” the paper explained.

“Using a four dimensional (d = 4) state space represented by four distinct time bins and its conjugate state space in the Fourier transform domain, we realise a QKD that generates an ultra-high secret key rate.”

The researchers note that their technique is robust enough to be used to send photons across free space, although they expect its first application would be in metropolitan-scale fibre networks. ?

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