Microsoft quantum computing scientists have captured a Majorana quasiparticle

The nanowire (green) where the Majorana quasiparticle was captured and measured. Source: Hao Zhang, QuTech.

Quantum computers hold the promise of the so-called quantum supremacy, which is the ability to solve problems that classical computers cannot. One of the reasons why scientists believe quantum computers could outperform classical computers on some tasks is the existence of superposition, a quantum mechanics phenomenon that allows a qubit to be an arbitrary combination of zero and one at the same time, instead of just one or zero at a given time, as conventional bits.

But for now no one knows what architecture a quantum computer should have, particularly because most of those currently under research are not stable and, therefore, highly prone to errors during computation. Among those architectures are the gate-based superconducting systems, which are being researched by Microsoft, Intel, IBM and Google. Recent developments in this approach include IBM's announcement of a 50-qubit prototype quantum computer last year and the recently announced Google Bristlecone quantum processor, with 72 qubits.

Both companies are expected to reach quantum supremacy later this year, with Google's Quantum AI Lab team even stating they "are cautiously optimistic that quantum supremacy can be achieved with Bristlecone". This optimism is mainly due to the amount of coupled qubits on Bristlecone, which is located just after the classically simulatable area highlighted in purple on the graph above. But even if Google or IBM succeed and some near-term applications of quantum computing are achieved by this architecture, gate-based superconducting processors would still be subject to a high error rate until about one to ten million qubits can be coupled together, if correctly predicted.

Topological quantum systems

So enters another architecture under development and being spearheaded by Microsoft: topological quantum systems. The building blocks of such systems are two-dimensional quasiparticles which, accounting for time as another dimension, live in a three-dimensional spacetime where their paths, or worldlines, pass around one another in such a way that forms braids - I know, mindblowing.

These braids are the logic gates in a topological quantum system, and because their topological properties do not change due to small but cumulative perturbations, this architecture is much more stable than those that make use of trapped quantum particles, such as the superconducting systems. Of course, errors induced by thermal fluctuations are still a problem for topological quantum systems, but this can be circumvented by dramatically decreasing the temperature and separating the quasiparticles by a reasonable distance.

What this all means is that a topological quantum computer could become fault-tolerant with far less qubits than other architectures. So why searching for other architectures if this one is so promising? That is because it requires a specific kind of two-dimensional quasiparticle that was theoretically predicted but never found - until now.

The Majorana particle

Back in 1937, physicist Ettore Majorana theoretically predicted the existence of a particle that is its own antiparticle, now known as the Majorana particle. Besides the fact that it is a singular particle in the Standard Model (it is the only fermion known to be its own antiparticle, which is cool by itself), it was found that bound Majorana particles behave as quasiparticles when in solid state and, more important, are exactly of the kind of two-dimensional quasiparticles required by topological quantum computers.

But to create and capture a Majorana particle has been an immensely difficult task for researchers. In order to accomplish the task, the experiment must be performed at a temperature of almost absolute zero, colder than deep space. Also, strong magnetic fields are required to allow alternative ways for electrons to organize themselves. So, that is what a team of scientists from Microsoft Research and universities in the Netherlands and the United States have done.

Experimental setup. Source: Technical University of Delft.

Capturing the Majorana particle

In a paper to be published in the April 5 issue of the scientific journal Nature, named Quantized Majorana conductance, the team describes how it carefully measured the nanowire's electrical response (the experimental setup is depicted in the image above) and successfully detected, for the first time, the full effect predicted by theory. The experiment also included variations in some parameters, such as the magnetic field, in order to verify that the signal was indeed the expected for a Majorana particle. As stated in the paper abstract:

The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau.

It is important to note that the same team has been working on this project for several years and, back in 2012, they were able to detect a weaker electric signal of the Majorana particle. Unfortunately, the weak signal was not enough to rule out the possibility of non-Majorana particles being responsible for its generation, what was ruled out in the new experiment:

We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence.

An e-print of the full paper is currently available for free here, on the arXiv open access library.

In a statement to BBC, Prof. Charles Marcus, from the Niels Bohr Institute and Principal Researcher at the Microsoft Quantum Research project at the University of Copenhagen, said:

What's really astounding with this activity compared with what everybody else is doing is that we have to invent a particle that's never existed before and then use it for computing.

The detection of the Majorana particle paves the way for new experiments that could soon lead to a topological quantum computer. Also, due to the increased stability of this architecture's qubits, it is also expected that scaling it up to a full, useful and error corrected quantum computer should be far easier than for other architectures. This may be the reason why Dr. Julie Love, director of quantum computing business development at Microsoft stated the company expects to have "a commercially relevant quantum computer - one that's solving real problems - within five years".

Of course, Microsoft is still to provide a public demonstration of the computation capabilities and scalability of its topological quantum computer, as both IBM and Google have been doing recently. But if everything goes as planned, Microsoft may have just become the front runner in the race for quantum supremacy.

Sources: Nature & arXiv via BBC