Q&A: Will Microsoft’s quantum ‘breakthrough’ revolutionize computing?
More than 30 years ago, Penn State physicist Jainendra Jain pioneered the theory of a new state of matter called the fractional quantum Hall effect, whose discoverers were awarded the Nobel Prize in Physics in 1998. Jain described it as a liquid of certain strange particles that he called composite fermions.
Under certain conditions, composite fermions form a superconductor — or a material that can conduct electricity without losing any energy at low temperatures — that theorists predicted would contain an even stranger particle, called a Majorana, which is its own antiparticle — a particle with the same mass but different charge.
Theorists envisioned that the Majorana particles, which mirror themselves as their own antiparticles, could be used to perform fault-tolerant quantum computation. This ability to run calculations while simultaneously correcting errors is essential for advancing quantum computing for real-world, industrial scale applications.
Last week (Feb. 19), Microsoft announced a potential breakthrough in quantum computing based on these long-theorized but experimentally unconfirmed Majorana particles. They also published a paper in the journal Nature on some of the work described in their announcement.
In the following Q&A, Jain, an Evan Pugh University Professor and Erwin W. Mueller Professor in Physics, spoke about his work on the theory of composite fermions, how it relates to Microsoft’s announcement and why skepticism is a valuable element of scientific discovery.
Q: How does theoretical physics support real-world design?
Jain: Quantum physics is the science of how tiny particles — like photons, particles of light, and electrons — behave in ways that seem counter to our everyday experience. Unlike objects we encounter in our daily lives, these tiny particles can pass through barriers and can seemingly exist in multiple places at once, until they are observed.
It turns out that physicists can create new particles in the laboratory, which are unlike any particles nature has given us. And, sometimes, they have incredibly strange properties and do things which the old, familiar particles couldn’t. Such particles are called emergent particles. If we can understand them, then maybe we can use them to develop new materials and technologies for the benefit of humanity.
I am on the theoretical understanding side of this spectrum, but I work closely with scientists who test whether the theories correspond to reality. The news from Microsoft is an example of how basic research at universities could lead to real-world applications that drive innovation — like quantum computers.
Q: What is a quantum computer and how would the Majorana particle help?
Jain: A normal computer works with “bits,” which are binary digits and comprise the smallest data unit. It can code either one or zero, on or off, and many of them together in a certain order convey larger messages. A quantum bit, or qubit, offers additional possibilities: it can be on, off or in any superpositions of the two, including on and off at the same time — just like the famous thought experiment “Schrodinger’s cat” in which a cat in a box is both alive and dead at the same time, until someone opens the box to check, at which time a definitive state of dead or alive is achieved.
When you link many qubits together, the possibilities grow exponentially. This allows a quantum computer to do certain types of calculations, at least theoretically, much faster than a classical computer. One can come up with examples where all the world’s current computers operating together for decades would not be able to perform the calculations of one quantum computer in a day.
One of the biggest hurdles in quantum computing is that information degrades because of interaction with noise, or interference from the environment or elsewhere in the system. This can lead qubits to collapse into a definite state, introducing errors. This is where the rather strange Majorana particles come in.
Two Majorana particles can produce either nothing or a whole fermion, which make the on and off states of a single qubit. Unlike many other qubits, the information here can be stored non-locally in a topological fashion — meaning the two Majorana particles forming a qubit could be far apart. Then, since neither Majorana contains the full information, any local noise cannot switch off to on or on to off. This enables the qubits to hold information without information loss.
Q: Tell me more about your work. What are composite fermions and how do they relate to applications like quantum computing?
Jain: My work had a role in the unlikely sequence of events that led to the idea of Majorana-based quantum computation. I think this is an example of how ideas evolve and mutate and move into unexpected directions.
A phenomenon called the quantum Hall effect was discovered just before I started graduate school. It occurs when experimentalists cool electrons in a two-dimensional material like graphene, apply a strong magnetic field and measure its resistance. Surprisingly, a property called the Hall resistance was found to be precisely quantized — it took on some special values that depend only on certain fundamental constants of nature. While this effect wasn’t predicted, researchers in the field quickly understood it.
Soon after I began my graduate studies, researchers discovered a twist on this effect, called the fractional quantum Hall effect. Here the Hall resistance was quantized at some fractional values instead of integers. This was much more mysterious. And the more the experimentalists probed, the more intricate structures they discovered.
In the late 1980s, it occurred to me that I could explain the observations if I assumed that electrons absorb quantized chunks of the magnetic field to make a new kind of particles, which I called composite fermions. To my pleasant surprise, and gratification, a few years later, several experiments verified composite fermions by direct observation. The composite fermions had many offshoots and led to a host of other predictions by many researchers, including some from my own group.
In particular, it was found that under certain conditions composite fermions pair up to make a superconductor. Theorists predicted that this is actually a special kind of superconductor — a topological superconductor — that harbors Majorana particles. The possibility of such particles was theoretically shown by Italian physicist Ettore Majorana almost a century ago, but the work was largely forgotten since it did not apply to any real particles. It was mind-boggling that Majorana particles could emerge as excitations in the composite-fermion superconductor. In this incarnation, the Majorana particle is, roughly speaking, half of a composite fermion caught in a vortex inside the superconductor. And to make the story line more captivating, theorists further proposed that the Majorana particles could be used to make qubits and to perform fault-tolerant quantum computation.
Countless hours have been spent working to confirm Majorana particles in composite fermion superconductors. A convincing observation has not yet been made. However, once the notion of Majorana as an emergent particle was out there, scientists realized that the conditions for producing them can be created in other systems — for example, by putting together an ordinary superconductor and a semiconductor. Even here, producing Majorana has proved challenging. Such is scientific inquiry — sometimes ideas work and sometimes they don’t. But no matter how compelling, a theory needs confirmation by experiments.
Q: Does Microsoft’s Majorana-1 chip demonstrate composite fermions serving as qubits?
Jain: No, no. Microsoft’s quantum computing program focuses on Majorana particles in superconductor-semiconductor hybrid nanowires, not on Majorana particles in a composite-fermion superconductor. Moreover, they have not shared details of their qubits. The Nature article does not demonstrate a Majorana qubit but instead demonstrates the feasibility of a measurement that would be needed for a future computer based on Majorana particles. I look forward to learning more about their findings at the upcoming March meeting of the American Physical Society.
It is, of course, natural that their claim of the realization of topological qubits will be scrutinized. It would take much work to confirm it, and then even more to explore its full potential.
Q: What do you love most about your work?
Jain: My research is driven by curiosity. It’s the thrill of discovering something new. It’s like solving a mystery — you think you know how things should behave, but every so often, something completely unexpected happens. That’s when you dig deeper and once in a rare while, you feel you have figured out something new, and maybe even something important. It’s thrilling when it pans out. And just when you feel you understand, there are more twists. Many concepts from the study of the fractional quantum Hall effect are so far out that if experiments hadn’t proven them, nobody would have taken them seriously.
I have been truly fortunate to have incredible students who have nurtured the field since its early days, clarifying and advancing the story. Seeing many of my graduate students developing into wonderful, well-rounded scientists has been one of my greatest pleasures.