Major Quantum Computing Strategy Suffers Serious Setbacks

MZMs are a different kind of electron quasiparticle. They are predicted to appear in a material, such as a semiconductor, that has an energy gap when electrons pair up in a different way. One of the simplest systems predicted to show this behavior, first proposed by the physicist Alexei Kitaev in the early 2000s, is a one-dimensional chain of electrons. At very low temperatures the electrons can pair up with their neighbors to become superconducting. But the electrons at each end of the chain have only half the partners. They become two halves of a single quasiparticle, widely separated in space. This quasiparticle can be thought of as existing right in the middle of the energy gap, at precisely zero energy — hence the “zero mode.”

The crucial point about zero-mode quasiparticles is that each one is made up of the electron states at both ends. So you can’t find out anything about it, or disturb it, by probing just one end. If the two end states could somehow be brought together in space, they would fuse to produce either an electron or nothing — a vacuum state. While they are separated, they are in a kind of quantum superposition of both states. So an MZM can act as a two-state bit for encoding quantum information: a qubit.

That the two halves of an MZM quasiparticle can fuse into nothing — an empty vacuum — accounts for their name. This self-annihilation is like that of a particle and antiparticle pair, except that here both members of the pair are identical. Hypothetical fundamental particles that are their own antiparticles were proposed as a possibility by the Italian physicist Ettore Majorana, working in Enrico Fermi’s group in Rome, in 1937. No such particle has ever been seen. MZMs are their quasiparticle equivalent, illustrating how behaviors of particles seen or suggested in high-energy physics are now turning up in materials governed by quantum laws.

The problem is, not only has no one ever made an MZM qubit, no one has convincingly shown the existence of even a single MZM quasiparticle.

The Escape Artist

The first reported sighting of Majoranas in nanowires came in 2012 from Kouwenhoven’s team at Delft. Looking back, it was an early sign of just how difficult it would be to establish unambiguous evidence of these elusive quasiparticles.

To look for MZMs, researchers generally apply a voltage to nanoscale wires made of semiconducting material connected to a superconductor. They carefully plot the conductance through the wire. If it contains MZMs, then as the voltage changes, the conductance should rise in discrete quantized steps and peak at precisely zero voltage — the energy corresponding to the zero mode.

Kouwenhoven’s 2012 paper reported that they discovered this “zero-bias peak,” but other researchers soon began to worry that the signal wasn’t unique to Majoranas. For example, Eduardo Lee, a physicist now at the Autonomous University of Madrid, and co-workers showed in 2013 that such a signal could be mimicked by what are known as Andreev bound states (ABSs) in the energy gap where MZMs are thought to sit. Then earlier this year, Marco Valentini of the Institute of Science and Technology in Klosterneuburg, Austria, and colleagues repeated experiments like those at Delft and showed that this system exhibits a zero-bias peak due to a particular kind of ABS called a Yu-Shiba-Rusinov state — a quasiparticle that also appears at zero energy but can’t be used to make a topological qubit.

Such false positives have dogged the field. In addition to Kouwenhoven’s retracted 2018 paper, skeptics point to other examples where claimed sightings of MZMs have not been replicated. In March 2020, Marcus and colleagues reported evidence in Science that the authors said was “consistent with the emergence of Majorana zero modes.” But then in late July 2021, the journal published an “editorial expression of concern” about whether the data used in the study was wholly representative. The Niels Bohr Institute has launched an independent investigation into the research. Marcus and his colleagues stand behind the original paper.

The trouble, according to critics like Frolov, is that Majorana particles aren’t necessary to produce the signals being seen. “Yet, affirmative papers kept coming out without even mentioning alternative explanations,” he wrote in his Nature critique. He accuses researchers of cherry-picking their data, and calls for more openness about sharing it. He has identified five diagnostic signatures of Majoranas. Most of the papers claiming sightings since 2012 show just one or two of these, he said — but none shows them all.

Yet the real problem, Marcus asserts, is not premature claims but the difficulty of verification. Marcus wants t o do the best experiments possible to see whether Majoranas exist, but “there won’t ever be a day on which you say: It’s proved!” he said. “That’s just not the way experimental physics works. There’s always going to be some smart guy out there who said, ‘I thought of something else! It doesn’t pass all the Majorana tests!’ That goes on forever.”

The question then becomes: How do you build a quantum computer out of something that you can’t say for sure exists?

Braided Information

The key to quantum computing is that, during the computation, you must avoid revealing what information your qubits encode: If you look at a bit and say that it holds a 1 or a 0, it becomes merely a classical bit. So you must shield your qubits from anything that could inadvertently reveal their value. (More strictly, decide their value — for in quantum mechanics this only happens when the value is measured.) You need to stop such information from leaking out into the environment.

That leakage corresponds to a process called quantum decoherence. The aim is to carry out quantum computing before decoherence can take place, since it will corrupt the qubits with random errors that will destroy the computation.

Current quantum computers typically suppress decoherence by isolating the qubits from their environment as well as possible. The trouble is, as the number of qubits multiplies, this isolation becomes extremely hard to maintain: Decoherence is bound to happen, and errors creep in. So for any of the existing quantum computers like those made by IBM, Google and others, scaling up to large systems “will clearly require huge amounts of active error correction,” said Scott Aaronson, a computer scientist and quantum theorist at the University of Texas, Austin. Current proposals for doing that involve wiring up huge numbers of “physical qubits” into a single “logical qubit” with the capacity for error correction. Some estimates suggest that hundreds or even thousands of physical qubits would be needed for each logical qubit — a dizzying technical challenge.

Kitaev realized that Majorana quasiparticles could be made resistant to errors — that is, to decoherence — if the two halves at the ends of an electron chain are jumbled up so that the information encoded in each pair of end states can be kept perfectly hidden and can’t inadvertently leak into the environment to cause decoherence. Because this jumbling weaves together the particle trajectories — their timeline “threads” — it is called braiding.

Here’s how braiding works. If you bring two Majorana quasiparticles together, they collapse to either an electron or a vacuum state. Once that happens, you know what they are — their information is revealed. But now suppose that many Majorana-mode pairs are shuffled around like couples in a square dance, constantly changing partners while maintaining an appropriate social distance. As long as the dance goes on, explains Marcus, “you can never focus on a single Majorana and ask it which state it is in — that is, whether it will fuse to an electron or to vacuum. Because it will say, ‘I don’t know, who am I dancing with?’” In effect, braiding ensures that the information in Majorana pairs is no longer localized on any of them but is made nonlocal. You can never reveal it by interrogating one of them.

“As long as they stay away from each other and change partners, the game is on,” said Marcus. “Kitaev’s genius was to say, if the information is stored nonlocally, then no local measurement reveals the information. That predicts that this thing should be a very good qubit” — because it is protected from incurring errors by the topological nature of the braiding.

To carry out a quantum computation with such MZM qubits, you shift the braided threads around in a specified way, and then bring pairs together to see if they fuse to form an electron or vacuum — that’s the readout. If that’s done in the right way, it performs the computation, free of any errors.

Such error-resistant topological quantum computing using MZMs is “a challenging way to go, but not an impossible one,” said Lee. “Microsoft has gone out on a limb to make this gamble, I think it is one worth taking.”

And if physicists are having an incredibly difficult time confirming the existence of MZM quasiparticles, perhaps they should simply forgo that step. In the end, Marcus thinks that experimentalists might need to stop trying to close all the loopholes and just go ahead and try braiding the system to see if a robust qubit emerges. After all, who cares if it’s definitively an MZM or some other quasiparticle, so long as it’s a good qubit?

Indeed, there are already proposals for using other quasiparticles as qubits too. For example, Christina Psaroudaki of the California Institute of Technology and Christos Panagopoulos of Nanyang Technological University in Singapore have suggested that skyrmions — vortex-like patterns in the orientation of spins in a magnetic material — might offer another way to encode and manipulate topologically protected quantum information.

Even though “the difficulties in finding MZMs, or more generally getting even the most basic building blocks of topological quantum computing to work in the lab, have clearly been a disappointment to many in the field,” said Aaronson, hope survives. He feels sure they’ll be found “sooner or later.” Valentini agrees, despite having shown that MZM signatures could be mistaken for other quasiparticles. “I strongly believe that MZMs will be created and detected,” he said.

But Aaronson adds that “whether this is a viable approach to quantum computing more broadly — and whether it will win over competing approaches — is still anyone’s guess.”

In early 2018, around the time that Nature published the doomed topological computing paper, Todd Holmdahl, the Microsoft vice president then in charge of their quantum computing efforts, predicted that they would have a working topological qubit by the end of the year, and a commercial quantum computer based on the technology in five years’ time. Although those predictions now seem extravagantly optimistic, they mirror the broader hype that has accompanied quantum computing over the past 10 years. More traditional approaches to quantum computers have enjoyed a degree of success, but they still face enormous challenges in scaling up. The best path to the long-promised quantum future remains unclear.

“It would be irresponsible of the physics community to decide now that we already know the only way,” said Ady Stern of the Weizmann Institute of Science in Rehovot, Israel. “We’re on page 10 of a thriller, and we’re trying to guess how it’s going to end.”