Short answer: Quantum fingerprinting is a protocol from communication complexity where two parties send tiny quantum messages to a referee, who checks whether their inputs match. Theory says quantum communication can cut the information transmitted exponentially. Experiments in 2015, 2016 and 2019 report proof-of-principle demonstrations that transmit less information than the best known classical protocol, in the 2016 case below the proven classical limit. They are laboratory demonstrations.
The communication complexity setting
A 2015 Nature Communications paper explains that communication complexity studies the minimum information that must be transmitted to solve distributed tasks. It states that for several problems quantum mechanics has been proven to allow exponential reductions in communication compared with classical, with practical relevance for communication system design, very-large-scale integration circuit design and data structures. It calls quantum fingerprinting arguably the most appealing protocol in the area.
The 2015 demonstration
The authors report a proof-of-concept fingerprinting system that transmits less information than the best-known classical protocol. It is based on a modified commercial quantum key distribution system with off-the-shelf optical components at telecom wavelengths, and is practical for messages as large as 100 Mbits even with experimental imperfections. They describe it as a first step toward experimental quantum communication complexity.
The 2016 demonstration
A 2016 Physical Review Letters paper reports a proof-of-principle fingerprinting protocol that for the first time surpasses the ultimate classical limit on transmitted information. It uses ultralow-noise superconducting single-photon detectors and a stable fibre-based Sagnac interferometer, and transmits less information than the proven classical lower bound over 20 km of standard telecom fibre for input sizes up to 2 Gbits. The authors say the results open a window for research in communication complexity and tests of the foundations of physics.
A different task in 2019
A 2019 Nature Communications paper defines the Sampling Matching problem, inspired by Hidden Matching, which has an exponential quantum-classical gap in the one-way communication model. It allows a photonic implementation using phase-encoded coherent states, a fixed-size linear optics circuit and single-photon detection. The authors demonstrate in a proof-of-principle experiment an advantage in transmitted information over the best known classical protocol, which they say is impossible to reach for the original Hidden Matching problem. They note implications for quantum verification and cryptographic settings, and say that while quantum computation advantage remains elusive on current systems, communication complexity offers a place to showcase it.
How the three experiments differ
The 2015 system leans on commercial-style hardware and modest message sizes, while the 2016 system uses superconducting detectors and an interferometer to reach inputs of up to 2 Gbits over fibre. The 2019 experiment changes the task rather than the hardware, choosing a problem whose structure suits a simple photonic circuit. Taken together, the papers show the advantage appearing in different forms and under different experimental choices, each with its own caveats about how the classical comparison is defined.
What this does not tell you
These advantages are about the amount of information transmitted, not speed or cost, and the experiments are small proofs of principle. They do not show a deployed communication system.
FAQ
What is quantum fingerprinting?
A protocol where parties send short quantum states so a referee can check whether their inputs are equal.
What advantage does it offer?
Theory shows an exponential reduction in transmitted information over classical protocols.
Has it been shown experimentally?
Yes, in proof-of-principle form in 2015, 2016 and 2019.
How far did the 2016 experiment reach?
Over 20 km of standard telecom fibre, for inputs up to 2 Gbits.
Is it in use?
These papers describe laboratory demonstrations only.
All field notes