Near-Perfect Quantum Clones

Producing quantum clones: Beam of light passing through splitter (Source: L. Henderson / UNSW)

Producing quantum clones: Beam of light passing through a splitter (Source: L. Henderson / UNSW)

Physicists at The Aus­tralian National University ANU and University of Queens­land (UQ) have produced near-perfect clones of quantum infor­mation using a new method to surpass previous cloning limits. The new cloning method uses high perf­ormance optical amplifiers to clone light encoded with quantum infor­mation. It is possible this technique could allow quantum encryp­tion to be implemented with existing fibre optic infra­structure.

A global race is on to use quantum physics for ultra-secure encryp­tion over long distances according to Ping Koy Lam, node director of the ARC Centre of Excellence for Quantum Compu­tation and Communi­cation Techno­logy (CQC2T) at ANU. “One obstacle to sending quantum infor­mation is that the quantum state degrades before reaching its destination. Our cloner has many possible appli­cations, and could help overcome this problem to achieve secure long distance communi­cation,” said Lam.

The laws of physics prevent high quality clones being produced with a 100 percent success rate. The team uses a proba­bilistic method to demons­trate that it’s possible to produce clones that exceed theo­retical quality limits. The method was initially proposed by CQC2T researchers led by Timothy Ralph at UQ. “Imagine Olympic archers being able to choose the shots that land closest to the target’s centre to increase their average score,” said Ralph.

“By designing our experiment to have proba­bilistic outputs, we sometimes get lucky and recover more information than is possible using existing deter­ministic cloning methods. We use the results closest to a bullseye and discard the rest,” he said. A distinct difference between archery and quantum infor­mation information is that the proba­bilistic method is permitted, and is useful in many crypto-communi­cation situations, such as gene­rating secret keys.

“Our proba­bilistic cloning method generates higher quality quantum clones than have ever been made before, with a success rate of about five percent. We can now create up to five clones of a single quantum state,” said Jing Yan Haw, ANU PhD researcher. “We first encode infor­mation onto a light beam. Because this infor­mation is in a fragile quantum state, it is difficult to observe or measure,” said Haw.

“At the heart of the demons­tration is a noiseless optical amplifier. When the ampli­fication is good enough, we can then split a light beam into clones. Amplify-then-split allows us to clone the light beam with minimal distortion, so that it can still be read with exqui­site precision,” said Ralph.

Quantum cloning opens up important experi­mental possi­bilities as well as having appli­cations in ultra-secure long distance quantum networks. “One of the problems with quantum encryption is its limited communi­cation range. We hope this techno­logy could be used to extend the range of communi­cation, and one day lead to impe­netrable privacy between two communi­cating parties,” said Lam. (Source: ANU)

Reference: J. Y. Haw et al.: Surpassing the no-cloning limit with a heralded hybrid linear amplifier for coherent states, Nat. Comm. 7, (2016); DOI: 10.1038/ncomms13222

Link: Centre for Quantum Computation and Communication Technology, Research School of Physics and Engineering, The Australian National University, Canberra, Australia

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