optics - Recently publicized experiment on destructive interference between two laser beams

Recently I've had several non-physicist friends ask me, independently of each other, about an experiment where two collinear laser beams destructively interfere along a certain length. Everybody wants to know "where does the energy go?"

Answering that question is not the problem, but I would be more convincing if I knew what experiment they were talking about! None of them can recall where they read it, but it seems to have made the rounds of popular science websites sometime in the last year. I am especially surprised that this has been published now, since similar experiments have been done for decades, so I'm guessing there must be some other twist to the experiment that didn't register in my friends' memories.

Can anybody point me towards a published paper or a popular science article?

Answer

If we have the same popular press in mind, the official name of this device - informally known as anti-laser - is a "coherent perfect absorber". See

http://en.wikipedia.org/wiki/Coherent_perfect_absorber

It was proposed in early 2010 by A. Douglas Stone and collaborators:

http://arxiv.org/abs/1003.4968
http://www.pqeconference.com/pqe2011/abstractd/170.pdf

It was already observed in late 2010, see e.g.

http://www.opticsinfobase.org/abstract.cfm?URI=FiO-2010-PDPC11

The anti-laser is just a time-reversal of a laser, so whatever problem with energy conservation you could see exists for both devices. Obviously, there is no problem - the energy of light is being taken from, or deposited to, the atomic excitation energies in the cavity.

There are roughly 10 papers about this subject only

http://scholar.google.com/scholar?q=%22coherent+perfect+absorber%22

so one shouldn't buy some popular media's hype suggesting that it's one of the hottest things in physics.

If your question were not about anti-laser but about ordinary interference between two laser beams, well, I would leave it as no comment. Of course that the energy is conserved at the end. One may get destructive interference in a big region but this is always compensated by the existence of constructive interference in other regions - e.g. on the boundary of the beams. Maxwell's equations may be showed to imply energy conservation so the total energy can never get lost.