Recoil and fermion statistics sink last year's ultracold neutrino laser

Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT, with postdocs Hanzhen Lin and Yu-Kun Lu, presents a two-part analysis in two companion papers in Physical Review Letters.

The MIT papers show that the neutrino laser concept, and a similar proposal for gamma rays, are physically and fundamentally impossible.

Ketterle: "These two papers are sort of punch one and punch two. Each paper would have killed the proposal."

No one has ever produced a Bose-Einstein condensate from radioactive atoms.

Formaggio, who sees the new results as a convincing and constructive challenge, says: "When a new idea, such as the one we proposed, is shared, it is the duty of the community to scrutinize it. Such is the scientific process."

Ketterle received the Nobel prize for creating some of the first ever Bose-Einstein condensates in the 1990s.

Ketterle heard a lecture about the neutrino laser idea and immediately worried about it being too good to be true.

Getting clarity required hours of discussion, including with Jones and Formaggio, according to Ketterle.

Formaggio and Jones did not respond to New Scientist's request for comment.

The key to the proposal was a memory effect: when an atom in the condensate emitted a neutrino it would be more likely to keep emitting in the same direction, because the quantum state shared by the ultracold atoms would retain a trace of that first emission.

Ketterle and colleagues showed that this memory, although present, would be about 10,000 billion times too brief to affect the neutrinos as intended.

The team found the memory would have the opposite effect from that intended, which Ketterle calls an anti-memory: "If I am an atom and I have emitted a neutrino, I am not allowed to [immediately] emit a neutrino again," an effect stemming from neutrinos being fermions rather than particles of light.

In a Bose-Einstein condensate the atoms recoil in sync, so the rate at which they scatter pumped photons in the same direction grows exponentially, producing an observed superradiant photon laser; a cloud of atoms at room temperature would scatter the photons in random directions, generating at best a soft glow.

Superradiance, the quantum amplifying effect the proposal was based on, had been observed only for photons.

To illustrate the proposal, Formaggio and Jones outlined a scenario in which a cloud of radioactive rubidium atoms, once cooled into a condensate, would accelerate its radioactive decay from a half-life of 86 days to one minute.

Kyle Leach at Queen's University in Canada says the new analysis does not categorically rule out every possible way to build a neutrino laser, but shows that the most conventional scenario, where each atom emits one neutrino, cannot work; if each atom emitted two neutrinos at a time, he says, the analysis may be different.

Leach: "I think these papers sharpen where the real difficulty lies," adding that for nuclear-scale energies the requirements become extraordinarily demanding.

Leach says the interesting scientific question becomes more precise rather than disappearing, namely what kinds of nuclear or neutrino processes, if any, avoid the limitations identified, and that it is most likely to be definitively answered through experiments.

Ben Jones was an associate professor of physics at the University of Texas at Arlington when the proposal was made and is now at the University of Manchester.

Neutrinos are produced when radioactive atoms undergo nuclear decay.

A shortfall of 10,000 billion is about thirteen orders of magnitude.

The illustrated rubidium scenario implies a decay speed-up of roughly 124,000-fold.