MIT team rules out the neutrino laser: recoil and fermionic nature close the path
Wolfgang Ketterle, Hanzhen Lin and Yu-Kun Lu show in two Physical Review Letters papers that last year's neutrino laser proposal cannot be built. An atom that emits a neutrino recoils at Mach 10 and leaves the condensate at once, and the second paper finds that emitting a fermion leaves an opposing imprint telling the condensate which direction not to send the next neutrino. Joe Formaggio, who put the proposal forward, calls the scrutiny part of the scientific process.
Science··Night
The recoiling atom leaves the condensate at once
Wolfgang Ketterle, with the postdoctoral researchers Yu-Kun Lu and Hanzhen Lin, published two companion papers in Physical Review Letters, and both show that a neutrino laser cannot be built. Ketterle says a neutrino carries about 1 million times the energy of a visible light particle. An atom that emits a neutrino at 1 million electronvolts recoils at Mach 10, and an atom recoiling that fast leaves the condensate almost at once, so no quantum imprint builds up behind it. The condensate loses the trace of the emitted neutrino immediately, and the decay continues without enhancement. The team applied a model describing superradiance to radioactive atoms and neutrinos, taking in the range of energies at which the particles are emitted, the recoil of the decaying atom and the dynamics of the condensate; in every scenario the analysis considered, superradiance did not appear.[1], [2]
Emitting a fermion leaves an opposing imprint
The second paper shows that the proposal does not stand even if the recoil problem were solved. Ketterle says the imprint a recoiling atom leaves in the condensate tells it which direction to avoid for the next neutrino. The team calls this an anti-memory and ties the opposing correlation to the neutrino being, fundamentally, a fermion. Fermions and bosons are the two classes of particle that build matter in the universe: bosons such as photons carry whole-integer spins, fermions such as electrons and neutrinos half-integer spins. Ketterle says superradiance rests on a memory effect, that whether the emitted particle is a boson or a fermion had been taken as immaterial, and that describing fermion emission correctly gives an anti-memory which keeps the condensate from lasing.[1], [2]
Where the proposal came from: a nanokelvin cloud and an 86-day half-life
The proposal was put forward last year: a cloud of radioactive atoms would be cooled to nanokelvin temperatures, one-billionth the temperature of interstellar space, the atoms would form a Bose-Einstein condensate, decay in step and give off a laser-like beam of neutrinos. Joe Formaggio of MIT made the proposal with Ben Jones, then at the University of Texas at Arlington and now at the University of Manchester; in their illustrative scenario the half-life of radioactive rubidium atoms fell from 86 days to one minute. No one has so far made a Bose-Einstein condensate out of radioactive atoms. Formaggio says that when a new idea is shared it is the duty of the community to scrutinise it, and that this is how the scientific process works. Ketterle co-discovered Bose-Einstein condensates in 1995 and shared the 2001 Nobel Prize in Physics for that work.[1], [2]
