Could radioactive atoms in a Bose–Einstein condensate (BEC) form a “superradiant neutrino laser”? According to a paper published last year by Benjamin Jones and Joseph Formaggio of the Massachusetts Institute of Technology the answer is “yes”. We were intrigued by this proposal and commissioned the freelance physics writer Tim Wogan to write an article for us.
As part of his reporting, Wogan sought the opinion of a BEC expert – James Thompson of NIST, JILA and the University of Colorado in Boulder. Thompson told us that he was sceptical of the viability of such a device – in particular, he pointed out that the de Broglie wavelength of the emitted neutrinos is much too short for superradiance to occur.
Now, another physicist (Wolfgang Ketterle, who shared a Nobel prize for his pioneering work on BECs) has published two papers that cast doubt on Jones and Formaggio’s proposal. All three papers are published in a very prestigious journal, so this is a scientific debate of note.
So what is superradiance, and how could it be used to create a source of coherent neutrinos?
Coherent burst
Superradiance was originally envisioned in an ensemble of excited atoms, each of which can decay by emitting a photon. Normally, the rate at which photons are emitted from such an ensemble is simply proportional to the number of atoms present. Double the number of atoms, and the rate of emission will double. However, if the ensemble is dense enough that the wavelength of the emitted photons is much longer than the separation between neighbouring atoms, the emission of a single photon will involve all of the atoms. What is more, the superradiant emission rate is proportional to square of the number of atoms. The result is short, intense burst of coherent radiation. It is this coherence that has caused some to refer to superradiant lasers – although the physics of lasers is different.
Jones and Formaggio’s proposal involves radioactive atoms that decay with the emission of a neutrino, rather than a photon. Their idea is to cool an ensemble of these atoms to near absolute zero so that it forms a BEC. This is a unique form of matter that can be described as a macroscopic quantum state. Under this condition, the duo argued that neutrinos would be emitted rapidly from a superradiant state.
Radioactive BEC could form a ‘superradiant neutrino laser’
Now, Ketterle, Yu-Kun Lu and Hanzhen Lin at the Massachusetts Institute of Technology argue in two papers that such a superradiant neutrino laser is an impossibility. As well as backing up Thompson’s concerns about the short wavelength of the neutrinos, they point out that the atoms produced by the neutrino decays are fermions, and this precludes superradiance. They also showed that the recoil of atoms after neutrino emission would not allow the coherent emission of neutrinos to occur.
I think this is a fascinating subject and I hope that Jones and Formaggio can revive their proposal, because a BEC-based neutrino laser would be very cool!
You can read more about the debate in an article in Physics by Thompson and colleagues.
