MIT physicists show why a laser made of neutrinos is impossible

A laser created from a number of the universe’s most elusive particles sounded virtually too unusual to be doable. Based on two new theoretical analyses from MIT, it’s.

Physicists have discovered {that a} lately proposed neutrino laser can’t obtain the collective amplification wanted to show radioactive decay right into a concentrated beam. Two companion papers printed in Physical Review Letters determine separate issues with the idea: the large recoil produced when a neutrino escapes an atom, and the basically fermionic nature of the neutrino itself.

“These two papers are type of punch one and punch two,” MIT physicist Wolfgang Ketterle stated. “Every paper would have killed the proposal.”

The work was carried out by Ketterle, postdoctoral researchers Hanzhen Lin and Yu-Kun Lu, and colleagues at MIT’s Department of Physics, Analysis Laboratory of Electronics and MIT-Harvard Heart for Ultracold Atoms. The findings don’t present that each conceivable methodology of manufacturing directional neutrinos is unimaginable. As a substitute, they rule out the particular superradiant mechanism proposed for radioactive Bose-Einstein condensates.

Neutrinos barely work together with atypical matter. Trillions move via the human physique each second, but detecting even a small fraction requires monumental, extremely delicate experiments. Additionally they come in several varieties, or flavors, and may change from one taste into one other whereas touring.

In 2025, physicists Ben Jones and Joe Formaggio proposed an uncommon method to flip radioactive decay into one thing resembling a laser. They instructed cooling radioactive atoms till they fashioned a Bose-Einstein condensate, or BEC, a state through which many atoms behave as a coherent quantum system.

Their proposal centered on superradiance, a phenomenon through which collective quantum conduct causes radiation from many emitters to strengthen itself. As a substitute of particular person atoms emitting randomly, correlated atoms can launch radiation in a extremely amplified, directional means.

Jones and Formaggio calculated {that a} condensate containing about a million radioactive rubidium-83 atoms may speed up a traditional radioactive lifetime of roughly 86 days to about one minute. The emitted neutrinos may then type an intense, laser-like beam.

No radioactive BEC has but been created. Nonetheless, the proposal provided a tantalizing risk for controlling a particle that usually passes virtually untouched via planets, stars and other people.

The primary main drawback seems on the instantaneous a neutrino leaves an atom.

Superradiance relies on the emitting atoms retaining details about earlier emissions. In optical experiments, when an atom emits or scatters a photon, the recoil can stay a part of the coherent movement of the condensate. That shared response helps subsequent radiation construct in the identical course.

Neutrino-producing nuclear reactions function at vastly increased energies. Seen photons carry energies round an electron volt, whereas neutrinos produced in radioactive decay can emerge with energies roughly 1,000,000 instances bigger. The daughter atom due to this fact receives an unlimited kick.

Ketterle in contrast the recoil velocity to Mach 10. At that velocity, the recoiling atom successfully leaves the ultracold condensate virtually immediately. Reasonably than preserving a helpful quantum imprint of the emission, the system loses that data earlier than it could amplify one other occasion.

The companion Physical Review Letters evaluation concluded that proposed neutrino and gamma-ray schemes face extraordinarily quick coherence instances and multimode emission. As a substitute of manufacturing helpful amplification, the calculated achieve can fall to about $10^{-20}$ or smaller.

Even eliminating the recoil drawback wouldn’t rescue the proposed neutrino laser.

Photons are bosons, particles whose quantum conduct permits giant numbers to occupy the identical state. That attribute makes the constructive buildup behind superradiance doable. Below the correct circumstances, the emission charge from $N$ atoms can scale roughly with $N^2$.

Neutrinos are fermions. Fermions obey completely different quantum statistics and can’t pile into an identical quantum states in the identical means.

Lu, Lin and Ketterle solved an idealized model of the issue through which recoil and different sensible obstacles have been eliminated. Even beneath these favorable assumptions, neutrino emission couldn’t generate the $N^2$ enhancement required for superradiance. The utmost emission charge scaled solely with $N$, akin to atypical incoherent radiation.

The primary emitted neutrino successfully leaves what Ketterle describes as an “anti-memory” within the collective system. As a substitute of encouraging one other neutrino to observe the identical path, the fermionic excitation blocks additional emission into that state. Within the easiest coherent mannequin, the method reaches a darkish state after one neutrino is emitted.

The underlying concept dates to physicist Robert Dicke, who confirmed in 1954 that teams of quantum emitters may radiate collectively moderately than independently. Below appropriate circumstances, their emission amplitudes intervene constructively, tremendously rising the radiation charge.

Bose-Einstein condensates later offered an distinctive platform for exploring such collective results. Ketterle shared the 2001 Nobel Prize in Physics for work involving Bose-Einstein condensation in dilute gases, following his crew’s 1995 commentary of a sodium condensate at MIT.

That historical past partly explains why the neutrino proposal was intriguing. BECs actually can produce placing collective phenomena involving atoms and photons. The error was assuming that the identical quantum amplification may merely be transferred to high-energy radioactive decay and fermionic radiation.

“In superradiance, it’s a few reminiscence impact, or quantum correlations within the condensate,” Ketterle stated. However for emitted fermions, the crew discovered that the reminiscence works in the other way, suppressing moderately than amplifying repeated emission.

Formaggio, one of many physicists behind the unique proposal, has welcomed the problem moderately than treating it as a setback.

“When a brand new concept — such because the one we proposed — is shared, it’s the obligation of the neighborhood to scrutinize it. Such is the scientific course of,” he stated. “Certainly, it was nice to see how our paper generated a variety of pondering outdoors of our authentic idea. We suspect that can proceed.”

The brand new work does go away room for extra restricted collective results involving fermions. The theoretical paper finds that sure states can nonetheless exhibit an $N$-fold enhancement, regardless that the highly effective $N^2$ superradiant scaling wanted for the proposed laser is forbidden.

Formaggio additionally expects the argument could ultimately face an experimental check. “Nature, as at all times, is the ultimate arbiter of such issues,” he stated.

For Ketterle, nonetheless, the central conclusion is already clear. “Artistic concepts and discussions amongst scientists are wanted to uncover nature’s surprises,” he stated. “However within the case of neutrino lasers, the shock was too good to be true.”

These sources hint the unique neutrino-laser proposal and the physics of superradiance and Bose-Einstein condensates that impressed it.