A telltale signature of the theoretical particle was discovered last year. Now the Army wants to use it to build a quantum computer.
Last July, a group of researchers from UC Irvine, UCLA, and Stanford announced they’d found the first strong evidence of the Majorana fermion, an elusive particle first theorised over 80 years ago that acts as its own antiparticle. The evidence of the Majorana fermion was hailed as a major “landmark” in physics, but it didn’t take long for the US Army, which funded the research, to begin considering wartime applications for the particle.
As detailed in an Army statement released Monday, the US military is considering how to deploy the Majorana fermion as a robust defense against cyber attacks and a key component of the quantum computers it hopes will soon be used to analyze military data. The military is placing its bets on the Majorana particle to be the basis of a virtually unhackable quantum computer because it will be able to store data without succumbing to electromagnetic interference. Moreover, any intruder in a quantum system controlled by the Army would be unable to access the system’s data without corrupting it and alerting the Army to the intrusion.
First theorized by the Italian physicist Ettore Majorana in 1937, the Majorana fermion has zero electrical charge. For decades, evidence of the particle’s existence eluded physicists and most considered the neutrino to be the leading candidate for the Majorana fermion. A few years ago, however, the group of California researchers hypothesized that Majorana particles might be created by manipulating exotic materials like superconductors, which conduct electricity with 100 percent efficiency.
As detailed in a paper published last July in Science, the researchers were looking for Majorana quasiparticles, which arise from the collective behavior of particles in a physical medium and exhibit some properties of particles—such as momentum—without actually being a particle. A way to think about this is like bubbles in a glass of beer, which aren’t really independent things, but the result of the displacement of the beer by carbon dioxide. Nevertheless, the bubbles retain certain observable characteristics as they rise.
The researchers found evidence of the Majorana quasiparticles when they stacked two thin films of superconducting materials on top of one another, placed them in a vacuum chamber, and applied electricity to the films, which caused electrons to glide along the edges of the superconductors. When the researchers applied a magnet to the films, the electrons would stop and switch directions, but this also caused pairs of Majorana quasiparticles to emerge in pairs. Although they exhibited the same behavior as the electrons insofar as they would stop and reverse directions along the superconducting films, they did so at half the rate of the electrons, which was the telltale sign that they were Majorana quasiparticles.
It was a major milestone in physics, but as far as the Army is concerned, the discovery also holds a lot of promise for beefing up military cybersecurity. The US military hopes to use the Majorana particles as qubits in topological quantum computers, a theoretical type of machine that uses quasiparticles for computation and is robust against decoherence. A qubit is the fundamental unit of data in a quantum computer, similar to the bit in a normal computer, but instead of being limited to binary calculations—where the bit represents a one or a zero—the qubit can be in a superposition of both these states at the same time. This drastically increases the computing power of quantum computers compared to their classical counterparts.
Although a number of research organizations have developed arrays that have up to 72 qubits, they’re still error prone. This is because most approaches to error correction, such as the surface-code approach, involve encoding a single fault-tolerant qubit across dozens of other qubits. This means that as you add more fault-tolerant qubits to a system, the total number of qubits rapidly increases, leading to a large, unwieldy system.
The idea with topological quantum computers, on the other hand, is to use qubits that are already fault tolerant on their own, rather than having to actively correct errors.
"Imagine that bits of data in standard computers are like cars traveling both ways on two-lane highways," Kang Wang, a professor of electrical engineering at UCLA and lead author of the Science paper, said in a statement. "A quantum computer could have many lanes and many levels of 'traffic,' and the cars could hop between levels and travel in both directions at the same time, in every lane and on every level. We need stable, armored quantum 'cars' to do this and the Majorana particles are those supercars."
The Army considers the Majorana fermion to be an ideal candidate for a fault tolerant qubit in a topological quantum computer because its lack of charge would make it almost perfectly immune to external electromagnetic interference. It would, in a sense, be a virtually unhackable computing device that would also allow the Army to process vast amounts of data with unprecedented efficiency.
“While conventional quantum systems have sophisticated schemes to correct errors, information encoded in a topological quantum computer cannot be easily corrupted," Lei Pan, a UCLA doctoral student and co-author of the Science paper, said in a statement. "What's exciting about using Majorana particles to build quantum computers is that the system would be fault-tolerant."
For now, however, the dream of a topological quantum computer is just that—a dream. But now that the researchers have got the hard part out of the way (that is, finding evidence that Majorana fermions actually exist), the next step is to figure out how to “braid” the Majorana quasiparticles together so that they can store and process information at high speeds, which would form the basis of a topological quantum computer.