Last month, Microsoft announced a new quantum computing chip called Majorana 1 that it expected would “realise quantum computers capable of solving meaningful, industrial-scale problems in years, not decades”. Independent scientists soon raised doubts about this claim — grandiose as it is — but also acknowledged Microsoft had taken on a great challenge to build such a chip and that its efforts in this direction couldn’t or shouldn’t be written off altogether.
Microsoft named the chip “Majorana 1” because it consists of Majorana particles, which is a particular type of subatomic particle with unusual properties. One is that a Majorana particle is its own anti-particle. The particles that make up matter, called fermions, have anti-particles with distinct identities. For example, the electron’s anti-particle is a positron, not another electron. The proton’s anti-particle is the anti-proton, not another proton. But uniquely among fermions, a Majorana particle’s anti-particle is yet another Majorana particle. If two of them meet, they will annihilate each other in a flash of energy.
One of the major open questions in contemporary physics is whether neutrinos are Majorana particles.
Neutrinos, neutrinos everywhere
Neutrinos are the second-most abundant subatomic particle in the universe, after photons, the particles of light. They were produced in copious amounts during the Big Bang event. They are produced in radioactive decay, when massive stars explode, and when cosmic rays strike the earth’s atmosphere. They are also made during nuclear fusion: the sun alone is responsible for flooding every square centimetre on the earth with 60 billion neutrinos each second. These particles are also extraordinarily hard to catch because they interact very weakly and very rarely with matter.
Yet it is crucial physicists study them: neutrinos may just be the key to answering many of the open questions about our universe. Their tremendous numbers are a sign that they’re involved in many, many subatomic processes. Thus a clear view of their properties will also afford physicists a clear view of these processes, and the as-yet unresolved questions they can answer.
We don’t know many things about neutrinos. Perhaps the biggest unknown is how much a neutrino weighs. We know neutrinos come in three flavours, or varieties, and we know the differences between the squares of their masses, but not the individual masses themselves. If neutrinos are found to be Majorana particles, the process that reveals them to be can be easily used to reveal their masses as well. This process is called neutrinoless double beta decay, or 0vßß for short.
Chilling with beta decay
Every atom has some energy, which it bears in its particles and the forces acting between them. Sometimes an atom’s nucleus may have too much energy, rendering it unstable and looking for opportunities to shed the excess. This notion of stability comes from the fact that for every set of protons and neutrons in the nucleus, there is a number that allows the particles to arrange themselves in a way that leaves the nucleus with the bare minimum of energy.
For example, the nucleus of the actinium-227 atom contains 89 protons and 138 neutrons, forcing the nucleus to exist in a highly unstable configuration. To shed the ‘excess energy’, it undergoes a process called beta decay: it emits an electron and an anti-neutrino and changes to the thorium-227 nucleus. Th-227 also isn’t stable and decays further, but since the beta decay process releases energy, the nucleus is better off than it was before.
In nature, beta decay is a common way for an unstable nucleus to decay. It can happen in one of two forms depending on whether a nucleus has too many neutrons or too many protons. In the first case, a neutron is converted to a proton and releases an electron and an anti-neutrino. In the second, a proton is converted to a neutron and releases a positron and a neutrino. A third form exists where two beta decays happen simultaneously, i.e. two neutrons are simultaneously converted to two protons, emitting two electrons and two anti-neutrinos.
The conversion ability stems from the weak interaction, which is one of the four ways in which subatomic particles can interact with each other. (The others are the strong, electromagnetic, and gravitational interactions.) The weak interaction is characterised by the appearance of particles called W or Z bosons. For example, during the Ac-227 beta decay, a neutron emits a W– boson and turns into a proton, and the W– boson decays to an electron and an anti-neutrino.
A sign in the difference
As common as beta decay is, scientists are currently on the hunt for an extremely rare variant: 0vßß. It may not even exist, but just in case it does, it would prove neutrinos are Majorana particles.
In 0vßß, a nucleus emits two electrons instead of an electron and an anti-neutrino. This can happen only when the neutrino emitted by one neutron is absorbed as an anti-neutrino by the other neutron, which in turn can only happen if neutrinos and anti-neutrinos are the same thing. Each of the emitted electrons also has more energy because it ‘includes’ the energy of the missing anti-neutrino. Experiments looking for evidence of 0vßß can thus use this energy difference to tell whether a nucleus has undergone beta decay or 0vßß.
This is precisely what the AMoRE experiment in South Korea has been doing, with sensitive particle detectors pointed at a crystal containing 3 kg of molybdenum-100 nuclei, cooled to fractions above absolute zero. Mo-100 nuclei are known to undergo double beta decay.
The search continues
In a paper published in Physical Review Letters on February 27, the AMoRE team reported it hadn’t observed evidence of 0vßß. Because the process is already hypothesised to be rare, not observing it could just as easily mean we didn’t look long enough. This is why the team reported in the paper that a population of Mo-100 nuclei would decay to half their number through 0vßß in no less than 1024 years. It could also mean 0vßß might show itself in a larger sample. In a future iteration of AMoRE, the physicists plan to look for it in 100 kg of Mo-100.
Meanwhile, they’ve also estimated the mass of each neutrino would have to be lower than 0.22-0.65 billionths of a proton. This is an extremely low mass ceiling, but it’s not the same as saying the neutrinos have zero mass. The distinction is crucial. The current theory of all subatomic particles, called the Standard Model of particle physics, says neutrinos should be massless. The presence of even a small amount of mass thus vexes the theory and indicates it has a gap somewhere. The trouble is physicists don’t yet know where. So AMoRE looks forward to its upgraded form and the search continues.
Published – March 18, 2025 05:30 am IST