Special relativity combines forces with quantum physics to secure communication

Relativistic bit commitment between points on opposite sides of Earth can keep a bit secret for up to 21ms. Researchers came close to this maximum in a first test of the idea between Singapore and Geneva. Image: NASA, Visible Earth

Agents positioned in Singapore and Geneva have performed a demonstration of 'bit commitment' protected by the combined forces of quantum physics and special relativity.

The experiment is described in a paper in Physical Review Letters by CQTians Stephanie Wehner, Jed Kaniewski and Marco Tomamichel, along with collaborators at the University of Geneva in Switzerland, University of Cambridge in the UK and the Perimeter Institute for Theoretical Physics in Canada. Various media also reported the work.

Bit commitment is a protocol for secure communication between two mistrustful parties, typically called Alice and Bob. Bob 'commits' some information that is later revealed to Alice, such that he can't change his message nor Alice peek early. The classical equivalent is a sealed auction bid.

In the new demonstration, the bit is kept 'sealed' for about 15ms. The researchers write in the paper that "this kind of system could potentially be useful in the high-speed trading stock market where short term commitments are sufficient". They provide a security proof that takes experimental imperfections into account.

It has been shown that guaranteed security for bit commitment is possible only under some assumptions. In 2012, CQT researchers performed the world's first demonstration of a secure bit commitment. This used quantum communication under the assumption of 'noisy storage' for quantum information, which allows the bit to be committed for an unlimited time.

In the new work, the assumption that provides security is rooted in physics – specifically, the tenet of special relativity that nothing can travel faster than the speed of light. The experiment adapts a protocol first proposed by theorist Adrian Kent of the University of Cambridge. (A team in China has implemented his original proposal, see arXiv:1306.4413.)

Alice and Bob each have two agents, which they send to some far off place. In this experiment, Alice and Bob put one agent each in Singapore, and one agent each in Geneva. Bob's agents give Alice's co-located agents an encrypted version of the bit at the commitment time, and the key to decode it at the unveiling time. After the commitment, the bit is protected from cheating for as long as one can guarantee that Bob's two far-off agents haven't been able to fiddle the decoding information – a time determined by special relativity's speed limit on communication between them. After the unveiling, Alice's agents can take their time to cross-check the results.

For Geneva and Singapore, separated by a straight-line distance through Earth of about 9354km, the commitment duration is 15.6ms. The theoretical maximum for points on opposite sides of Earth is about 21ms.

The quantum part happens between Alice and Bob before their agents get involved. At the outset, Alice sends Bob quantum bits to measure. This step generates the coding information for the bit commitment. Because quantum information cannot be copied, Bob's agents cannot keep copies of the bits that might allow them to fiddle the decoding key. The separation of the quantum communication from the rest of the protocol is one of the modifications made to Kent's original proposal. It offers the advantage that quantum data can be accumulated anywhere in advance.

For this experiment, the quantum bits were exchanged using a modified commercial quantum key distribution system from ID Quantique in an office at the University of Geneva. The agents in Geneva and Singapore were standalone computers equipped with field-programmable gate arrays programmed to execute the protocol.

For more details, see "Experimental Bit Commitment Based on Quantum Communication and Special Relativity" Phys. Rev. Lett. 111, 180504 (2013); arXiv: 1306.4801.

Selected media coverage:

• The Register: Quantum-classical crypto sends secret vote from Switzerland to Singapore
• University of Cambridge: Quantum 'sealed envelope' system enables 'perfectly secure' information storage
• Physics Focus: Information Exchange for the Mistrustful