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Quantum Computing Continues to Advance

The late theoretical physicist Richard Feynman once marveled, “Our imagination is stretched to the utmost, not, as in fiction, to imagine things which are not really there, but just to comprehend those things which are there.”1 It was Feynman who developed the concept of nanotechnology, and it was Feynman who first dreamed of the potential of quantum computing.

If there ever existed a technology that challenged the imagination, it is quantum computing. In traditional silicon computers, data is represented in binary bits that are always in one of two states: either a 1 or a 0. However, in a quantum computer each quantum bit, or “qubit,” can represent both a 1 and a 0 at the same time through a principle called superposition.

What this means is that a quantum computer can perform multitudes of calculations simultaneously. A two-qubit system can perform the operation on four values. A three-qubit system performs it on eight values. The performance of quantum computers explodes as the number of qubits increases.

This means that a quantum computer harnessing millions of qubits could, in a matter of minutes, process data and solve problems that would tie up today’s fastest supercomputers for a century.

While the potential is dazzling, the path from concept to reality has been difficult. The major challenges have included finding a cost-effective way to connect more than one qubit, and the need to control errors in a large-scale system.

But now, according to a study reported in the journal Nature, a team of scientists has made an important breakthrough:2 They’ve succeeded in building quantum logic gates in silicon for the first time, making calculations between two qubits of information possible.

A two-qubit logic gate is the central building block of a quantum computer. By using the same technology as existing computer chips, the team at Australia’s University of New South Wales (UNSW) has made it easier to manufacture a full-scale processor chip. The building of a quantum computer based on silicon should be much more feasible, since it is based on the same manufacturing technology as today’s computer industry.

As they explained in Nature, they reconfigured the “transistors” that are used to define the bits in existing silicon chips, and turned them into qubits.3 They morphed silicon transistors into quantum bits by ensuring that each has only one electron associated with it. They then stored the binary code of 0 or 1 on the “spin” of the electron, which is associated with the electron’s magnetic field.

However, it isn’t enough to develop qubits in silicon. To scale up to a fully operational quantum computer, what is needed is the ability to arrange millions of qubits so they can be precisely controlled, while correcting for errors in calculations.

To solve that challenge, another UNSW team designed a new silicon architecture. As they reported in Science Advances, the team, working with researchers from the University of Melbourne, created a blueprint for a quantum computer using atomic-scale qubits aligned to wires inside a 3D design.4

In the team’s conceptual design, they have moved from a one-dimensional array of qubits, positioned along a single line, to a two-dimensional array, positioned on a plane that is far more tolerant to errors. This qubit layer is “sandwiched” in a three-dimensional architecture, between two layers of wires arranged in a grid.

By applying voltages to a sub-set of these wires, multiple qubits can be controlled in parallel, performing a series of operations using far fewer controls. Importantly, with their design, they can perform the 2D surface code error correction protocols in which any computational errors that creep into the calculation can be corrected faster than they occur.

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According to Professor Sven Rogge, Head of the UNSW School of Physics, “Ultimately, the structure is scalable to millions of qubits, required for a full-scale quantum processor.”

Meanwhile, physicists at the Universities of Bonn and Cambridge have succeeded in linking two completely different quantum systems to one another. This breakthrough is yet another important step forward on the way to making a quantum computer a reality.

As they explained in Physical Review Letters, they combined the strengths of two components:5

• Quantum dots (qDots)

• Charged atoms (ions)

qDots are incredibly fast at disseminating quantum information. However, they “forget” the result of the calculation just as quickly. In contrast, ions are slow at processing so they perform poorly at fast calculations, but they store quantum information much longer than qDots can.

The physicists from Bonn and Cambridge developed a system that enables these two completely different quantum systems—qDots and ions—to work in tandem.

qDots can be produced using the same techniques as normal computer chips. To do so, it is only necessary to miniaturize the structures on the chips until they hold just one single electron, compared to the 10 to 100 electrons in a conventional PC.

The electron stored in a qDot can take on states that are predicted by quantum theory. However, they are very short-lived: They decay within a few picoseconds. This decay produces a small flash of light: a photon.

Photons are wave packets that vibrate in a specific plane—the direction of polarization. The state of the qDots determines the direction of polarization of the photon. According to Professor Michael Köhl from the Institute of Physics at the University of Bonn, “We used the photon to excite an ion. Then we stored the direction of polarization of the photon.”

To do so, the researchers connected a thin glass fiber to the qDot. They transported the photon via the fiber to the ion many meters away. The fiber optic networks used in telecommunications operate very similarly. To make the transfer of information as efficient as possible, they had trapped the ion between two mirrors. The mirrors bounced the photon back and forth like a ping-pong ball, until it was absorbed by the ion.

As Köhl explains, “By shooting it with a laser beam, we were able to read out the ion that was excited in this way. In the process, we were able to measure the direction of polarization of the previously absorbed photon.” In a sense then, the state of the qDot can be preserved in the ion—theoretically this can be done for many minutes.

The result of these breakthroughs means that all of the physical building blocks for a quantum computer have now been successfully constructed, allowing engineers to finally begin the task of designing and building a functioning quantum computer.

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Looking ahead, we offer the following forecasts:

First, by 2030, full-scale quantum computers will surpass the capabilities of today’s most powerful supercomputers, and offer enormous advantages for a range of complex problems.

Quantum computing will have major applications in the finance, security, and healthcare sectors, allowing the identification and development of new medicines by greatly accelerating the computer-aided design of pharmaceutical compounds; the development of new, lighter, and stronger materials spanning consumer electronics to aircraft; faster information searches through large databases; more accurate modeling of financial markets; and the optimization of vast metropolitan transportation networks.

Second, investors who want to get in on the ground floor of the quantum computing revolution should consider IBM, Alphabet, Microsoft, and privately held D-Wave Systems.

• Under the Logical Qubits research program to develop practical quantum computers, IBM recently received a significant research grant from IARPA, the research arm of U.S. intelligence. According to Arvind Krishna, director of IBM Research, “We are at a turning point where quantum computing is moving beyond theory and experimentation to include engineering and applications. Quantum computing promises to deliver exponentially more speed and power not achievable by today’s most powerful computers with the potential to impact business needs on a global scale.”

• Microsoft is funding research into quantum computing at its Station Q laboratory in Santa Barbara and through sponsored external research groups working on fundamental physics, advanced materials, and computer engineering. According to Station Q Director Michael Freedman, the company “could be developing the foundations for a new kind of technology—sort of a post-silicon age.” Microsoft predicts that it will develop a functional quantum computer based on its approach by 2025. It recently released a software emulator for quantum computers as open-source software; it enables computer scientists to use conventional computers to create the algorithms for quantum computers.

• Alphabet Inc-A, the parent company of Google, recently disclosed research findings in MIT Technology Review showing that its quantum computer is significantly faster than traditional computers.6 Google has entered an agreement with D-Wave Systems that gives it access to each new generation of D-Wave computers. According to one report, Google could attempt to purchase D-Wave, but the Wall Street Journal reported that the smaller company may be considering an IPO.7 At the same time, Alphabet is pursuing its own approach to quantum computing. According to Technology Review, the company hired physicist John Martinis to create chips with fewer qubits than the D-Wave chips, but optimized for specific tasks, such as pattern recognition and machine learning, which would improve the targeting of Google’s personalized ads, while facilitating new technologies like self-driving cars.

References

2. Nature, October 15, 2015, Vol. 526, Iss. 7573, “A Two-Qubit Logic Gate in Silicon,” by M. Veldhorts, et al. © 2015 Macmillan Publishers Limited. All rights reserved. http://www.nature.com/nature/journal/v526/n7573/full/nature15263.html

3. Ibid.

4. Science Advances, October 30, 2015, Vol. 1, No. 9, “A Surface Code Quantum Computer in Silicon,” by Charles D. Hill, et al. © 2015 American Association for the Advancement of Science. All rights reserved. http://advances.sciencemag.org/content/1/9/e1500707.full

5. Physical Review Letters, March 23, 2015, Iss. 114, “Direct Photonic Coupling of a Semiconductor Quantum Dot and a Trapped Ion,” by M. Kohl et al. © 2015 American Physical Society. All rights reserved. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.123001

6. MIT Technology Review, December 18, 2015, “Google’s Quantum Dream Machine,” by Tom Simonite. © 2015 MIT Technology Review. All rights reserved. http://www.technologyreview.com/s/544421/googles-quantum-dream-machine

7. Wall Street Journal, January 29, 2015, “D-Wave Systems Raises C$29 Million to Build Quantum-Computing Software,” by Deborah Gage. © 2015 Dow Jones & Company. All rights reserved. http://blogs.wsj.com/venturecapital/2015/01/29/d-wave-systems-raises-c29-million-to-build-quantum-computing-software

8. MIT Technology Review, December 18, 2015, “Google’s Quantum Dream Machine,” by Tom Simonite. © 2015 MIT Technology Review. All rights reserved. http://www.technologyreview.com/s/544421/googles-quantum-dream-machine