‘The quantum computer, a tantalising but distant prospect’

Trapped ions, superconducting electric circuits, electron spins in diamonds. The list of promising building blocks for a quantum computer is growing fast. Majorana particles are the latest addition, thanks to a recent breakthrough by researchers from Delft and Eindhoven (see textbox Majoranas: the new robust building blocks).

These developments prompted Nature magazine in an editorial earlier this year to state that 2017 is a year of crossroads. “Quantum computing has long seemed like one of those technologies that are 20 years away and will always remain so. But 2017 will be the year that the field of quantum computing sheds its research-only image.”

Scalability and programmability

Dr Julia Cramer, outreach coordinator of QuTech, believes there is truth in Nature’s statement. “You see now that not only research institutes, but also companies, are investing in quantum technology. Take QuTech for instance. This consortium of TU Delft and TNO collaborates with companies like Intel and Microsoft. It is not only about the fundamental science anymore, it is about scalability and programmability.”

Still in 1962

Pinpointing exactly where we are in the quest for quantum computing is difficult of course, but QuTech roadmap-leader Dr Leonardo di Carlo, who works on superconducting quantum circuits, can give us something to hold on to. According to him, if you make the analogy between the developments that led to current day computers with the strides towards a quantum computer, we are still in 1962.

In the early sixties, when the Beatles where still an obscure rock band and the Cuba crisis broke out, computers took their first baby steps. The first integrated circuit was disclosed in 1958 and it contained only a few transistors. Similarly, the first quantum processors with a few qubits were demonstrated around 2012-2014, and we are now at a few dozens of qubits.

Open to debate

“Nineteen sixty-two, yes that makes sense,” says Richard Warburton, a quantum information researcher from the University of Basel’s department of physics. “But the question assumes that the development of a quantum computer will follow a similar trajectory to the development of the integrated circuit. It is not at all clear to me if this will be the case. Back in the 1960s, it was clear that semiconductors were the way forward. This turned out to be the case and digital computing has been based on semiconductors ever since. In quantum computing, the basic hardware choice is still open to debate. Who knows – perhaps one platform will be good for a while, but will eventually be overtaken by another.”

“The quantum computer remains a tantalising but distant prospect in my view,” Warburton concludes. “In the realm of quantum computing, it should retain its strong ‘research-only’ image. Nothing wrong with this: building a quantum computer is a grand challenge of our time.”

Crossroads

John Martinis, quantum computing researcher at the University of California Santa Barbara, is partnering with Google to build a quantum computer. Like the editorial in Nature, he also believes that 2017 is a year of crossroads.

“We are now at an interesting time in the field as researchers are beginning to make complex quantum systems,” he says. “For our group at Google, we are working on superconducting qubits and are now able to build systems at modest scale.” Martinis’ plan is to make a 49 qubit quantum computer before the end of the year.

‘I think the quantum computer is still largely in its infancy, but it’s definitely walking on its own feet”’

What hurdles need to be overcome before quantum computing becomes of practical use? “There is a range of things we need to improve at the same time in order to get all the systems engineering and physics to work well. This can be summarised as improving qubit coherence, control, and scalability. Thinking about the fundamental physics, these issues tend to compete against each other, hence the difficulty.”

In the next 10 years

Andrea Morello, quantum physicist at the University of New South Wales (Australia) says: “I think the quantum computer is still largely in its infancy, but it’s definitely walking on its own feet. I think a key turning point will be to use quantum computers to design quantum systems. My hope is that we will see this happen in the next 10 years.”

“The research-only image of quantum computing technology is already gone, there’s no doubt about that. There is plenty of research left to do, but the technologists, engineers, and investors have already entered the playground over the last two years.”

‘We approach the point where it will be difficult for classical computers to keep up’

Daniel Lidar, quantum computation researcher at the University of Southern California, agrees with Morello. “Quantum computers are still several years away from being useful for solving practical problems, but significant advances are being made all the time. In particular, special-purpose quantum information processors designed for optimization or for simulations of quantum mechanics are approaching the point where it will be difficult for classical computers to keep up. However, quantum cryptography and communication has already been commercialized and is used in certain niche applications where a very high degree of security is required at relatively low data transmission rates.”

Majoranas: the new robust building blocks

The newest building block in the quantum engineer’s toolbox is Majorana Fermions. These particles, first demonstrated in 2012 by Leo Kouwenhoven’s (QuTech/TU Delft & Microsoft), group are their own antiparticle at one and the same time.

In Nature last month, an international team of researchers from Eindhoven, Delft and the University of California presented an advanced quantum chip comprised of ultrathin networks of nanowires in the shape of ‘hashtags’, which has all the qualities needed to allow Majorana particles to exchange places. This would allow for a type of quantum computing that depends on excitations of matter that encode information by tangling around each other like braids.

Microsoft, that is collaborating with the QuTech consortium, is betting on this technique, called topological quantum computing. Majorana particles are likely to be quite robust. They are not very sensitive to external perturbations nor prone to errors. But this technology is still in its infancy.