# To what quantum mechanical 'discoveries' have retroreflectors on the Moon contributed?

In the Washington Post article This company’s one small step may be a giant leap for private space travel the placement of retroreflectors on the Moon is mentioned as a benefit of landing spacecraft (manned or not) on the Moon.

The launch by Moon Express Inc., a Florida-based firm started in 2010, marks the first time a commercial entity will be granted permission to leave Earth's orbit for a destination in outer space. Moon Express — or MoonEx for short — plans to send a robotic lander to the moon, where it will drop scientific instruments that will help researchers study the mysteries of space. For example, the lander will be carrying several retroreflectors designed to reflect light back to earth; in the past, these devices have led to discoveries about relativity and quantum mechanics and in the future, could yield insights into dark matter, said MoonEx co-founder Bob Richards in an interview.

edit: As mentioned by @rob in the answer below retroreflectors are definitely used in quantum mechanical experiments. A quick search for example yielded the 2008 proposal with an astronomical number of co-authors Space-QUEST: Experiments with quantum entanglement in space and the 2015 Physics World article Quantum cryptography set for lift-off where the caption states "Qubits in space: NASA's Jason-2 satellite is a quantum reflector".

I understand that the Apollo-era retroreflectors are still actively used and have set limits in tests of relativistic gravity that are consistent with existing predictions, but I'm not aware of any relativity-related discoveries.

But my question is on the quantum mechanics part of the quote. To what (if any) quantum mechanical 'discoveries' have retroreflectors on the moon contributed (in the past)? There may be future experiments planned, but I'm most interested in work that has been done to date.

The images below are copied from the linked question for reference - example of the use of the Apollo-era retroreflectors on the Moon.

Above: Lunar libration image from here

Above: Lunar Laser Ranging (LRR) images from here

This review by Tom Murphy has a nice section on "science from lunar laser ranging." He reiterates the fundamental problem that relativistic quantum mechanics and relativistic gravitation are incompatible, and that a solution to that problem might be expected to explain the accelerating expansion of the universe. However the results he attributes to lunar laser ranging --- equivalence principle tests at the AU length scale, tests for nonzero rate of change for $G$, measurements of gravitomagnetism and precession, inverse-square-law tests, measurements which reveal properties of the lunar interior, and measurements which reveal fine details of Earth's motion --- are decidedly non-quantum-mechanical.

The APOLLO folks beat the Gravity Probe B folks to the first definitive observation of gravitomagnetism, which is an important discovery.

The efficiency of the APOLLO laser-ranging system is about $10^{-18}$: that is, sending a laser pulse to the moon with $10^{18}$ photons means you typically get about one photon back. It's pretty tough to prepare any sort of a state quantum state when you have to decrease your signal intensity by a factor of a billion squared.

I'm not aware of any quantum-mechanical discoveries that have come from the lunar laser ranging program, and I don't expect any. I think inclusion of "quantum mechanics" in your pullquote is probably just sloppiness on the part of the reporter, his interviewee, or both. (If you're willing to do surgery on the pullquote, retroreflectors are important in terrestrial optics laboratories too, where they have certainly contributed to studies of quantum information; I don't think that was the intended meaning, nor do I think it's a fair interpretation.)

• OK I leaned two new words today; gravitomagnetism and pullquote! I appreciate you taking the time to lay out the "landscape" here, and now I have a lot more to read about. Thanks!
– uhoh
Aug 16, 2016 at 2:39
• Surgery complete, edits/suggestions welcomed. Also, I've asked this follow-up question.
– uhoh
Aug 16, 2016 at 4:06