An Introduction to Quantum Computing

When asking the question "how does quantum computing work?" think of that uncertainty you see while the coin is spinning — it's like the uncertainty we capture and use in quantum computing. We put many processing elements — qubits — into a state of uncertainty. Then we program the qubits, run the program, and capture the results — just like when the coin lands.

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How does a quantum computer work?

Quantum computing is complementary to classical computing, the kind of computing we use today, not a replacement for it. By working with uncertainty, we can take on some of the biggest, most complex problems that humanity faces, in a new and powerful way. Quantum computing will solve problems for which today’s computing falls short — problems in areas such as modeling the climate, drug discovery, financial optimization, and whether or not it’s a good morning to launch a rocket.

And this technology is just getting started. Many advanced quantum computers run only for a fraction of a second at a time. However, steady progress is being made. Even now, at this early stage, quantum computing is inspiring us to, as a sage once said, “think different” about the way we use existing computing capabilities.

Those betting on the success of these machines see many potential quantum computing applications, including in the fields of medical science and health care, cryptology, climate change abatement, insurance risk assessment, finance, and more.

Understanding why quantum computing is so strange

There are two main reasons. The first reason is people’s fundamental misunderstanding of the nature of matter, which quantum mechanics explains. The second is the incredible power that quantum computing, when mature, is expected to deliver to humanity.

How does quantum mechanics change people’s view of the world? The world we live in, where rocks fall down and rockets go up, seems to be dominated by solid matter, with energy as a force that acts on matter at various times. Yet matter can simply be seen as congealed energy.

Most of the mass of the protons and neutrons inside the nucleus of an atom, for instance, is simply a bookkeeper’s description of the tremendously powerful energetic fields that keep these particles in place. One of the most important kinds of particles in quantum computing, photons, have no mass at all; they are made up of pure energy.

And it was Einstein himself who told us that matter and energy are equivalent, with his famous equation, E=mc². To translate: The energy contained in solid matter equals its mass times the speed of light squared.

The speed of light is a very large number — 300,000 km/second, or 186,000 miles/second. Squaring the speed of light yields a far larger number. Plug this very large number into Einstein’s famous equation and you'll see that there is a lot of energy in even small amounts of matter, as demonstrated by nuclear power plants and nuclear weapons.

The second reason that quantum computers get such a strong emotional reaction is the tremendous power of quantum computing. The best of today’s early-stage quantum computers are not much more powerful, if at all, than a mainstream supercomputer. But future quantum computers are expected to deliver tremendous speedups.

Over the next decade or two, we expect quantum computers to become hundreds, thousands, even millions of times faster than today’s computers for the problems at which they excel.

People can’t really predict, nor even imagine, what it’s going to be like to have that kind of computing power available for some of the most important challenges facing humanity. That future is very exciting, yes. But it’s also a bit, as Einstein described quantum mechanics, “spooky.”

Grasping the power of quantum computing

• Qubits: Qubits are the quantum computing version of bits — the 0s and 1s at the core of classical computing. They have quantum mechanical properties. Qubits are where all the magic happens in quantum computing.
• Superposition: While bits are limited to 0 or 1, a qubit can hold an undefined value that is neither 0 nor 1 until the qubit is measured. The capability to hold multiple values at once is called superposition.
• Entanglement: In classical computing, bits are carefully separated from each other so that the value of one does not affect others. But qubits can be entangled with each other. When changes to one particle cause instantaneous changes to another, and when measuring a value for one particle tells you the corresponding value for another, the particles are entangled.
• Tunneling: A quantum mechanical particle can instantaneously move from one place to another, even if there’s a barrier in between. (Quantum computing uses this capability to bypass barriers to the best possible solution.) This behavior is referred to as tunneling.
• Coherence: A quantum particle, such as an electron, that is free of outside disturbance is coherent. Only coherent particles can exhibit superposition and entanglement.

These five terms are at the heart of the promise of quantum computing and are involved in many of the challenges that make quantum computing difficult to fully implement. In this section, we describe each of these crucial concepts.

Classical computing describes the computers we use every day, which includes not only laptop and desktop computers but also smartphones, web servers, supercomputers, and many other kinds of devices.

The term classical computing is used because classical computers use classical mechanics, the cause-and-effect rules of the road that we see and use in our daily lives, for information processing.

Quantum computing uses quantum mechanics — which is very different, very interesting, and very powerful indeed — for information processing.

Introducing Puff, the magic... qubit?

In a computer, bits are stored in tiny, cheap electromechanical devices that reliably take in, hold, and return either a 0 or a 1 — at least until the power is turned off. Because a single bit doesn’t tell you much, bits are packaged into eight-bit bytes, with a single byte able to hold 256 values. (2⁸ — all possible combinations of 8 binary digits — equals 256.)

A qubit is a complex device that has, at its core, matter in a quantum mechanical state (such as a photon, an atom, or a tiny piece of superconducting metal). The qubit includes a container of some kind, such as a strong magnetic field, that keeps the matter from interacting with its environment.

A qubit is much more complex and much more powerful than a bit. But qubits today are not very reliable, for two reasons:

• They’re subject to errors introduced by noise in the environment around them. A result of 0 can be accidentally flipped to a result of 1, or vice versa, and there’s no easy way to know that an error has occurred.
• It’s hard to keep qubits coherent, that is, capable of superposition, entanglement, and tunneling.

In quantum computers, qubits are much more complex and far more expensive than bits. Nor are they as easy to manage — but they are far more powerful.

The photo below shows a quantum computing module from IBM, suspended at the bottom of a cooling infrastructure that keeps the superconducting qubits at a temperature near absolute zero.

©Lars Plougmann / Flickr
A quantum computing processor from IBM

Until it's measured, each qubit can represent an infinite range of values between 0 and 1. How does the qubit hold all these values? At the core of the qubit is a quantum particle — a tiny piece of reality in the form of a photon, an electron, an ionized atom, or an artificial atom formed using a superconducting metal.

IBM is not the only technology company developing this new technology. Here are some other quantum computing companies: Google, D-wave, Microsoft, Amazon, Intel, Alibaba Group, Atos Quantum, Toshiba, and Rigetti.

Much of the power of qubits comes from the fact that they behave in a probabilistic manner; a given qubit, running the same calculation multiple times without errors, may produce a 0 on some runs and a 1 on another. The final result consists of the number of times each qubit returns a 0 or a 1. So the result of most quantum calculations is a set of probabilities rather than a single number.

Qubits are hard to create and hard to maintain in a state of coherence; they also tend to interfere with nearby qubits in an uncontrolled fashion. Taming qubits is one of the biggest challenges to overcome in creating useful quantum computers.

A popular approach to building quantum computers involves the use of superconducting qubits, which must be kept at a temperature very close to absolute zero to minimize interference due to heat and, in many cases, to maintain superconductivity.

Classical computers are designed to work at room temperature, but they tend to generate heat and to stop working properly as the temperature rises. The need to dissipate heat prevents device makers from packing components as tightly as they would like without resorting to expensive and clumsy solutions such as water-cooling or refrigerating the components.

In quantum computing, each additional qubit adds exponentially to the power of the computer. But because qubits tend to interfere with each other, adding more is difficult.

IBM, a leader in quantum computing, has published a roadmap showing past and future increases in the number of qubits that power its current and upcoming quantum computers.

If you're interested in staying up to date on the development of this technology, here are some places to find quantum computing news: Phys.org; The Quantum Insider; MIT News; Quantum Zeitgeist.