# Q & A: Can entanglement be used to synchronize clocks outside the light cone?

Q:
Can entanglement be used to synchronize clocks outside the light cone?
- Bill Chestnut (age 69)
A:

Hi Bill,

You didn't exactly define which light-cone you are referring to, so just to be clear, here's what I assume you meant: you have two clocks, and you want to zero them at the same time. The act of zeroing a clock is an event in space time, and you are wondering if two distant observers can use entanglement to ensure that they press the zero at the same time. These two events would then be outside each other's light cone.

Such distant, instantaneous synchronization is very easy to achieve, and can be done classically without entanglement. There are two main protocols, Einstein Synchronization (ES) and Slow Clock Transport (SCT). In ES, the two observers share a light pulse, as described here http://en.wikipedia.org/wiki/Einstein_synchronisation. Basically, light is sent from clock A to clock B and then immediately back to A. Clock B is zeroed as soon as it receives the pulse, while clock A is set so that it measured the pulse at a time equal to half the transit time. In SCT, a third clock is infinitely slowly transported between the first two clocks, and then used to zero the second clock.

The first idea is fairly practical; the second not so much. So you don't need to use any fancy quantum effects to achieve your goal. There may be some advantages to using entanglement, however. For example, recent papers on this topic have suggested that, unlike classical synchronization schemes, entanglement-based setups could work even if neither clock's owner knows where the other person is or what type of medium is in between.

An example of an entanglement-based scheme* can be found here: http://arxiv.org/pdf/quant-ph/0407204v1.pdf. Basically, an entangled pair of particles would be shared by the two clock owners (Alice and Bob). They would then measure a specific property of the photons (specifically, the second-order correlation function) which depends on the time difference between detection events. By shifting the zero of time of one clock until this function was maximized, the observers would have accurately synchronized the clocks.