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TECHNOLOGYREVIEW.COM MIT TECHNOLOGY REVIEW

VOL. 120 | NO. 2 BREAKTHROUGH TECHNOLOGIES

By Russ Juskalian

Advances at Google, Intel, and several research groups indicate that computers with previously unimaginable power are finally within reach.

Breakthrough The fabrication of stable qubits, the basic unit of quantum computers.

Why It Matters Quantum computers could be exponentially faster at running artificial-intelligence programs and handling complex simulations and scheduling problems. They could even create uncrackable encryption.

Key Players - QuTech - Intel - Microsoft - Google - IBM

Availability 4 to 5 years

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One of the labs at QuTech, a Dutch research institute, is responsi- ble for some of the world’s most advanced work on quantum computing, but it looks like an

HVAC testing facility. Tucked away in a quiet corner of the applied sciences build- ing at Delft University of Technology, the space is devoid of people. Buzzing with resonant waves as if occupied by a swarm of electric katydids, it is cluttered by tan- gles of insulated tubes, wires, and control hardware erupting from big blue cylinders on three and four legs.

Inside the blue cylinders—essen- tially supercharged refrigerators—spooky quantum- mechanical things are happen- ing where nanowires, semiconductors, and superconductors meet at just a hair above absolute zero. It’s here, down at the limits of physics, that solid materials give rise to so-called quasiparticles, whose unusual behavior gives them the potential to serve as the key components of quantum com- puters. And this lab in particular has taken big steps toward finally bringing those computers to fruition. In a few years they could rewrite encryption, materials sci- ence, pharmaceutical research, and arti- ficial intelligence.

Every year quantum computing comes up as a candidate for this Breakthrough Technologies list, and every year we reach the same conclusion: not yet. Indeed, for years qubits and quantum comput- ers existed mainly on paper, or in fragile experiments to determine their feasibil- ity. (The Canadian company D-Wave Sys- tems has been selling machines it calls quantum computers for a while, using a specialized technology called quantum annealing. The approach, skeptics say, is at best applicable to a very constrained set of computations and might offer no speed advantage over classical systems.) This year, however, a raft of previously theoretical designs are actually being built. Also new this year is the increased avail- ability of corporate funding—from Google,

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What Is a Quantum Computer?

At the heart of quantum computing is the quantum bit, or qubit, a basic unit of information analogous to the 0s and 1s represented by transistors in your computer. Qubits have much more power than classical bits because of two unique properties: they can represent both 1 and 0 at the same time, and they can affect other qubits via a phenomenon known as quantum entanglement. That lets quantum computers take shortcuts to the right answers in certain types of calculations.

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IBM, Intel, and Microsoft, among others— for both research and the development of assorted technologies needed to actually build a working machine: microelectron- ics, complex circuits, and control software.

The project at Delft, led by Leo Kouwenhoven, a professor who was recently hired by Microsoft, aims to over- come one of the most long-standing obsta- cles to building quantum computers: the fact that qubits, the basic units of quantum information, are extremely susceptible to noise and therefore error. For qubits to be useful, they must achieve both quantum superposition (a property something like being in two physical states simultane- ously) and entanglement (a phenomenon where pairs of qubits are linked so that what happens to one can instantly affect the other, even when they’re physically separated). These delicate conditions are easily upset by the slightest disturbance, like vibrations or fluctuating electric fields.

People have long wrestled with this problem in efforts to build quantum com- puters, which could make it possible to solve problems so complex they exceed the reach of today’s best computers. But now Kouwenhoven and his colleagues believe the qubits they are creating could eventually be inherently protected—as sta- ble as knots in a rope. “Despite deform-

ing the rope, pulling on it, whatever,” says Kouwenhoven, the knots remain and “you don’t change the information.” Such stability would allow researchers to scale up quantum computers by substan- tially reducing the computational power required for error correction.

Kouwenhoven’s work relies on manipulating unique quasiparticles that weren’t even discovered until 2012. And it’s just one of several impressive steps being taken. In the same lab, Lieven Vandersypen, backed by Intel, is showing how quantum circuits can be manufac- tured on traditional silicon wafers.

Quantum computers will be particu- larly suited to factoring large numbers (making it easy to crack many of today’s encryption techniques and probably pro- viding uncrackable replacements), solv- ing complex optimization problems, and executing machine-learning algorithms. And there will be applications nobody has yet envisioned.

Soon, however, we might have a bet- ter idea of what they can do. Until now, researchers have built fully programma- ble five-qubit computers and more frag- ile 10- to 20-qubit test systems. Neither kind of machine is capable of much. But the head of Google’s quantum comput- ing effort, Harmut Neven, says his team

is on target to build a 49-qubit system by as soon as a year from now. The target of around 50 qubits isn’t an arbitrary one. It’s a threshold, known as quantum suprem- acy, beyond which no classical supercom- puter would be capable of handling the exponential growth in memory and com- munications bandwidth needed to sim- ulate its quantum counterpart. In other words, the top supercomputer systems can currently do all the same things that five- to 20-qubit quantum computers can, but at around 50 qubits this becomes physi- cally impossible.

All the academic and corporate quan- tum researchers I spoke with agreed that somewhere between 30 and 100 qubits— particularly qubits stable enough to per- form a wide range of computations for longer durations—is where quantum com- puters start to have commercial value. And as soon as two to five years from now, such systems are likely to be for sale. Eventually, expect 100,000-qubit systems, which will disrupt the materials, chemistry, and drug industries by making accurate molecular- scale models possible for the discovery of new materials and drugs. And a million- physical-qubit system, whose general computing applications are still difficult to even fathom? It’s conceivable, says Neven, “on the inside of 10 years.”

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