New proof-of-concept demonstration beats the best known classical protocol

Thursday, October 29, 2015

Professor Norbert LütkenhausWaterloo physicists Norbert Lütkenhaus, Juan Miguel Arrazola and Shihan Sajeed, along with colleagues at University of Toronto, University of British Columbia and the University of Hong Kong, experimentally demonstrated a quantum fingerprinting system that can transmit less information than the best known classical protocol.

Their results were published this week in the latest issue of Nature Communications.

Quantum fingerprinting is one of the most appealing protocols in quantum communication complexity because quantum mechanics allows an exponential reduction in the transmitted information.

Imagine Alice and Bob receive inputs and based on these inputs, they send a message to a third party – the referee. The referee uses this received information to determine whether the inputs to Alice and Bob are equal, with the goal of transmitting as little information as possible.

Last year, Arrazola and Lütkenhaus proposed a practical version of a quantum fingerprinting protocol by Waterloo computer scientists and IQC faculty Richard Cleve, John Watrous and collaborators.

In this practical protocol, Alice and Bob’s inputs are fed to an error-correcting code that is used to modulate a sequence of very weak laser pulses. These quantum signals transmit very little information, but still allow the referee to distinguish whether the inputs are equal or not.

In this paper, the research team focused on how to implement the protocol. The practical laser pulse protocol was improved with a better decision rule by the referee. A careful simulation was also performed that considered experimental imperfections, and was used to design the experiment.

They then developed a new error-correcting code so that they could implement the system using commercial devices. By modifying a version of a commercial Quantum Key Distribution (QKD) system using optical components, the researchers were able to transmit messages up to 100 Mbits with 66 per cent less information than the best known classical protocol over a five-kilometre standard fibre operating at telecom wavelengths.

This doesn’t mean that the telecom industry is going to switch over to this protocol and offer that service,” said Lütkenhaus, a professor in the Department of Physics and Astronomy and a faculty member at the Institute for Quantum Computing. “It’s a first exiting step that opens the door for experimental quantum communication complexity.”

Their work has attracted other groups eager to collaborate to do more work in the field.

For a long time, the protocols in quantum communication complexity were believed to not be implementable and so no one was working on them. This proof-of-concept experiment is a first step, but there is a lot of room for improvement,” said Arrazola, a recent PhD graduate from the Physics and Astronomy Department. “We are excited about the prospect of developing these results further as there’s a lot of interest in moving to new protocols.”