Neutrinos are all over the place, passing by planets, stars, and our our bodies by the trillions each second. But these elusive particles are notoriously tough to manage.
Beforehand, physicists proposed a option to produce a concentrated, laser-like beam of neutrinos by cooling radioactive atoms to extraordinarily low temperatures.Â
Nonetheless, two new theoretical research from MIT researchers present that basic quantum mechanics prevents the proposed neutrino laser from working—not due to an engineering drawback, however as a result of nature itself blocks the method.
The “imaginative and prescient of neutrino superradiance runs up towards basic constraints—ones imposed not by engineering challenges however by quantum mechanics itself,” the research authors note.
A laser fabricated from ghost particles
The unique laser proposal referred to as for about a million radioactive rubidium-83 atoms to be cooled right into a Bose-Einstein condensate, a state during which atoms behave collectively as a single quantum system.
The concept relied on superradiance, a phenomenon during which many particles emit radiation collectively fairly than independently. As a substitute of the emission price rising with the variety of atoms, N, it may ideally scale as N².
For the proposed neutrino laser, rubidium-83 would decay into krypton-83 whereas releasing a neutrino. The researchers estimated that collective emission might shorten rubidium’s 86-day half-life to roughly 2.5 minutes—an almost 50,000-fold enhance within the attribute neutrino emission price.
Nonetheless, the brand new calculations present that this situation runs into a number of basic roadblocks.
The atom kicks itself out of the sport
The primary drawback is recoil. A neutrino produced by nuclear decay carries roughly a million-electronvolt vitality, vastly greater than a visual photon. Resulting from momentum conservation, the newly produced krypton atom receives a recoil velocity of some thousand meters per second.
That is quick sufficient for the atom to cross the condensate in lower than a microsecond—far too rapidly for collective emission to construct up. Its movement successfully reveals which atom produced the neutrino, destroying the quantum coherence wanted for superradiance.
Nonetheless, even when researchers might by some means take away this recoil drawback, a second impediment stays.
The researchers thought-about an idealized case during which each rubidium and krypton atoms behaved as bosons. Underneath these situations, successive decays might transfer the condensate by collective quantum states, permitting the emission price to achieve N² scaling.
Nonetheless, the true krypton-83 atoms are fermions. The Pauli exclusion precept prevents similar fermions from occupying the identical quantum state.Â
Consequently, the collective decay chain is blocked after the primary decay, leaving the utmost emission price proportional to N—the identical scaling anticipated from impartial atoms.
A lifeless finish that also teaches physics
The second research discovered one other drawback even after briefly treating krypton as if it had been bosonic.
In contrast to an optical cavity, free house doesn’t present a hard and fast emission mode. The extraordinarily quick, picometer-scale wavelength of nuclear-decay neutrinos makes the potential collective emission cone terribly slender.Â
Calculations present that the fraction of neutrinos that might take part collectively is so tiny that even a million atoms fall far under the brink wanted for superradiance. The proposed setups would offer a achieve of solely about 10-16 or smaller—successfully no helpful amplification.
Collectively, the 2 research present why the proposed neutrino laser just isn’t merely tough to construct. The mechanism itself is basically blocked by recoil, quantum statistics, and the dearth of enough collective coupling.
“These two papers are type of punch one and punch two. Every paper would have killed the proposal,” Wolfgang Ketterle, one of many research authors and a physics professor at MIT, said.
The end result might sound like the top of an concept, however it additionally reveals why failed theoretical proposals may be precious. By revealing precisely the place collective quantum conduct breaks down, the research supply a clearer image of what future quantum systems would wish to make comparable results potential.
