Quantum clock synchronization with photon pairs in three papers

Short answer: Quantum clock synchronization uses the timing correlations of photon pairs to line up two distant clocks. Three papers report a 2016 proof-of-principle over a 4 km fibre with 0.44 picosecond timing stability, a 2021 preprint with a field test over 7 km of fibre reaching 1.9 picoseconds, and a 2023 scheme that needs no external timing reference and reaches jitter below 68 picoseconds. These are experiments, not operating timing systems.

How does it work?

The 2016 Scientific Reports abstract describes an algorithm based on second-order quantum interference between frequency-entangled photons produced by parametric down-conversion. In the reference frame of a Hong-Ou-Mandel interferometer, photon correlations define simultaneous events. Once the interferometer is balanced with an adjustable optical delay in one arm, each clock records the arrival times of simultaneously generated photons, and the clock offset is found by correlating the recorded arrival times.

What did the 2016 experiment show?

The authors demonstrate a proof-of-principle experiment that synchronizes two clocks separated by a 4 km fibre link. They report a minimum timing stability of 0.44 picoseconds at an averaging time of 16,000 seconds, with an absolute time accuracy of 73.2 picoseconds. They say the stability is limited by the correlation measurement device and ideally could be better than 10 femtoseconds. That last figure is their estimate and not a measurement.

What did the 2021 field test show?

A 2021 arXiv preprint, which may not have been peer reviewed, says two-way quantum clock synchronization has been shown to provide femtosecond-level synchronization capability and also security against symmetric delay attacks. The authors report a field test between a hydrogen maser and a rubidium clock linked by a 7 km deployed fibre. Limited by the frequency stability of the rubidium clock, the time stability at 30 seconds was 32 picoseconds. With a fibre-optic microwave frequency transfer technology, they improved this by more than an order of magnitude to 1.9 picoseconds, even though only 1440 photon pairs were acquired in 30 seconds because of the low sampling rate of the coincidence measurement system. They say this shows the practicability of the method for field applications, which is their conclusion.

What did the 2023 scheme change?

The 2023 Physical Review Applied abstract notes that event synchronization is needed in applications from 5G technology to industrial automation, and that quantum communication networks will raise the demand for precise synchronization, which usually costs resources such as ultrastable clocks or extra synchronization lasers. The authors show how temporal correlations of photons can be used for synchronization in quantum networks. They achieve stable synchronization jitter below 68 picoseconds with as few as 44 correlated detection events per 100 millisecond data package, and demonstrate feasibility in emulated high-loss links, including atmospheric turbulence. They say this is done without any external timing reference and with only simple crystal oscillators, and that it replaces the transmission of timing signals with classical communication and computer-aided postprocessing. They suggest it could pave the way to entanglement-based secure time transmission, which is an outlook.

How to read this

The three results use different links, clocks and averaging times, so their figures are not directly comparable. Timing stability and absolute accuracy are different quantities, as the 2016 numbers show. This post is based on abstracts, and full texts were not checked. It makes no claim about products or timing networks in use. For work on the clocks themselves, see our posts on optical atomic clocks.

FAQ

What does the Hong-Ou-Mandel interferometer do here?

It defines simultaneous events through photon correlations, per the 2016 abstract.

How long were the fibre links?

4 km in 2016 and 7 km in the 2021 field test.

Was a security benefit claimed?

The 2021 authors say two-way quantum synchronization is secure against symmetric delay attacks, citing earlier work.

Does the 2023 scheme need a timing reference?

The authors say no, only simple crystal oscillators.