By Andre Saraiva, UNSW

Noise is a qubit killer. This is the main challenge faced by quantum hardware makers and the reason why Microsoft decided to chase topological particles called Majoranas for their qubits, instead of the usual two-level systems. It is a “three-little-pigs” situation: you can choose the material that will give you a qubit faster, such as superconducting circuits, but you will build a fragile quantum processor. On the other end of the spectrum are the hard-to-build yet all-enduring topological qubits. The problem that topological-qubit advocates are facing now is that it seems that they are even harder to build than first thought. This creates a major setback to Microsoft’s main quantum hardware program, as discussed by Quantum Computing Report in a previous article.

Topological properties are believed to be impervious to noise because they are not easily affected by stray electromagnetic fields. As an analogy, think of a sailboat taking laps around a lake. While the changes in wind might make the path of the sailboat wiggle around its intended route, the number of laps that the boat takes is unaffected (unless you have catastrophic winds). In this analogy, the wiggly path would be the noise that affects traditional qubits, while the number of laps is the robust topological property. But topological states are not easy to synthesize – Majorana particles, for instance, do not occur naturally.

These Majorana particles were theorised to emerge in complex devices made from superconductors and special semiconducting nanowires. In 2018, a team led by Professor Leo Kouwenhoven from QuTech, in the Netherlands, developed ingenious fabrication techniques and brought together all the necessary ingredients (they had partial results before, but their main result only came in 2018). They then measured currents through these nanowires and concluded that they saw signs of these topological particles, kickstarting Microsoft’s global effort to make Majorana-based quantum computers. In a surprising twist, the authors of the original work decided now to retract the paper from Nature and publish extended data that reveals that their main conclusion was incorrect. Majoranas were not measured in those nanowires.

Why did it take two years for an extremely well-funded, globally-reaching scientific team to notice something was off? Firstly, it should be noted that the “extended data” published now is not new data acquired through more recent measurements – it is data that had been cut out from the original paper. This data was analysed by experts in the field and it contains enough information to discard the main conclusion in the original paper. If this information were made public before (as dictated by good scientific conduct), this issue could have been identified by trained eyes earlier.

But on top of that, the collective understanding of the physics behind these complex devices is in its infancy, and effects of unavoidable disorder in the nanowires are only now starting to be understood. That is the risk that Microsoft took for itself when deciding to embark on a journey to create quantum computers out of particles that had not yet been observed in laboratory.

This opened a Pandora’s box of reactions. The specialised media was fast to capitalise on the “Microsoft vs Google vs IBM vs Intel quantum race”, claiming that the result shows how far behind Microsoft is when compared, for instance, with Google’s 53 qubit superconducting chip. But anyone that has been paying close attention knows that Google is not the world leader in quantum computing – 53 qubits are an impressive display of engineering skills, but these are very faulty qubits that cannot be used for any real-life applications. While we wait for Google to show how its Sycamore processor performs when trying to perform quantum error corrections, it is hard to gauge whether Google really has something that could move forward as a viable universal quantum processor.

On the topic of quantum error correction, that could well be where other technologies stall and Microsoft’s Majoranas catch up. One of the theoretical masterminds behind the idea of topological qubits and co-author in the retracted paper, Professor Sankar Das Sarma from University of Maryland fired on Twitter that the “…idea of using surface codes to do error correction is in some crude sense trying to produce approximate topological qubits…”. Indeed, the surface code is largely based on the ideas from 2016 Nobel laureate Duncan Haldane. By organising qubits in a two-dimensional array and repetitively performing operations and measurements, one tries to force the faulty qubits into collectively maintaining bits of information that are protected from the local stray fields acting on each separate qubit. This is, once again, leveraging the idea that global properties (like the number of laps of a sailboat around a lake) are more resistant than local properties (such as the position of the sailboat at any given moment, as affected by winds and waves).

Professor Das Sarma goes on about the shortcomings of the usual two-level-system approach in his Twitter account (called Condensed Matter Theory Center after the UMD-based institution that he directs). He says most of the mediatic noise regarding this episode is the result of “…total ignorance of how a quantum computer works—you must have LOGICAL qubits which NOBODY is even close to having”. This is arguably incorrect – the group of Professor Christopher Monroe from the same University of Maryland posted to an online preprint repository a manuscript showing a logical qubit based on ion traps with fault-tolerant operation levels (as covered by Quantum Computing Report here).

One of the key findings by Professor Monroe is that a significant improvement is achieved when, unlike in the surface code, long range coupling between qubits is attained. The surface code assumes that a qubit can only be entangled with its immediate neighbours, but some technologies allow for qubits to be moved around or even to achieve pairwise interactions mediated by the collective movement of all qubits (which is the case for ions in a trap). Hard to believe that Professor Das Sarma is unaware of the work by Professor Monroe – it is unclear whether he thinks there is something wrong with Professor Monroe’s conclusions or if he is (perhaps ironically) waiting for the manuscript to be published in a peer-reviewed journal.

Declaring the topological qubits dead prematurely might be a mistake. But undoubtedly this retraction reveals how ignorant we still are about these ethereal topological particles and how Microsoft is not ready to firmly progress in harvesting them for technological applications. Scientists are not nearly ready to pass this baton to engineers.

Dr. Saraiva has worked for over a decade providing theoretical solutions to problems in silicon spin quantum computation, as well as other quantum technologies. He works on MOS Silicon Quantum Dot research and commercially-oriented projects at the University of New South Wales (UNSW).

February 16, 2021