Quantum computing
Quantum computing harnesses the phenomena of quantum mechanics to deliver a huge leap forward in computation to solve certain problems. It could transform medicine, break encryption and revolutionise communications and artificial intelligence. To speed computation, quantum computers tap directly into an unimaginably vast fabric of reality—the strange and counterintuitive world of quantum mechanics. Companies like IBM, Microsoft and Google are racing to build reliable quantum computers. Technologies like quantum sensors, quantum computers and quantum information security are transforming how we live, work and play. But what is quantum computing? And how does it work?
Quantum computing is the exploitation of collective properties of quantum states, such as superposition and entanglement, to perform computation. The devices that perform quantum computations are known as quantum computers. Ever since Peter Shor discovered in 1994 that a quantum computer could break most of the encryption that protects transactions on the internet, excitement about the technology has been driven by more than just intellectual curiosity. Instead of bits, quantum computers use qubits. Rather than just being on or off, qubits can also be in what’s called ‘superposition’ – where they’re both on and off at the same time, or somewhere on a spectrum between the two.
If you ask a normal computer to figure its way out of a maze, it will try every single branch in turn, ruling them all out individually until it finds the right one. A quantum computer can go down every path of the maze at once. It can hold uncertainty in its head.
The other thing that qubits can do is called entanglement. Normally, if you flip two coins, the result of one coin toss has no bearing on the result of the other one. They’re independent. In entanglement, two particles are linked together, even if they’re physically separate. If one comes up heads, the other one will also be heads.
WHAT CAN WE DO WITH QUANTUM COMPUTER
Unlike current classical computers, quantum computers will be able to perform calculations and tasks at a far faster rate and a greater level of complexity than even the most powerful of today’s supercomputers. Yes, they might someday solve a few specific problems in minutes that (we think) would take longer than the age of the universe on classical computers. The goal of quantum computers is to be able to ask questions that can’t be answered by classical computers.
Cryptanalysis: Most online security currently depends on the difficulty of factoring large numbers into primes. While this can presently be accomplished by using digital computers to search through every possible factor, the immense time required makes “cracking the code” expensive and impractical. Quantum computers can perform such factoring exponentially more efficiently than digital computers, meaning such security methods will soon become obsolete.
Simulation: Quantum computers, which are built to exploit the physical phenomena of very small systems, are unsurprisingly well suited to simulate those kinds of systems.
One example that Professor Kouwenhoven cited is the discovery of new treatments against diseases. Presently, we commonly experiment with different molecules to create drugs which are then tested to confirm their effectiveness against certain diseases. “We don’t know how to predict or simulate processes in nature or biological systems,” Kouwenhoven explained. “We just try and see if it works or not – that’s just trial and error. If we had a quantum computer on hand, then we could use it to help solve these problems vastly more quickly and at a very fundamental level.”
This would be a powerful tool for researchers in chemistry, pharmaceuticals, and materials science, to name a few. However, it would then be up to those researchers to come up with the new products, safer car materials, more effective medicines, etc., all of which would impact the lives of consumers.
Optimization and Machine Learning: Quantum algorithms are more efficient at optimizing some function over a large search space, which means that a quantum computer could improve things like supply chain and delivery routes. Once again, this could lead to real changes!
And Beyond: What else could a quantum computer do? The possibilities are vast, but that is all we know from the 26 years since the first quantum algorithm (Shor’s) was published.
WHAT IS QUANTUM COMPUTING?
Quantum and classical computers both try to solve problems, but the way they manipulate data to get answers is fundamentally different. Classical computers carry out logical operations using the definite position of a physical state. These are usually binary, meaning its operations are based on one of two positions. A single state - such as on or off, up or down, 1 or 0 - is called a bit. Quantum computers use qubits instead of traditional bits (binary digits). Qubits are different from traditional bits because until they are read out (meaning measured), they can exist in an indeterminate state where we can’t tell whether they’ll be measured as a 0 or a 1. Such algorithms would be useful in solving complex mathematical problems, producing hard-to-break security codes, or predicting multiple particle interactions in chemical reactions.
The concept of superposition is infamously hard to render in everyday words. What superposition really means is “complex linear combination.” Here, we mean “complex” not in the sense of “complicated” but in the sense of a real plus an imaginary number, while “linear combination” means we add together different multiples of states. So a qubit is a bit that has a complex number called an amplitude attached to the possibility that it’s 0, and a different amplitude attached to the possibility that it’s 1. These amplitudes are closely related to probabilities, in that the further some outcome’s amplitude is from zero, the larger the chance of seeing that outcome; more precisely, the probability equals the distance squared.
But amplitudes are not probabilities. They follow different rules. For example, if some contributions to an amplitude are positive and others are negative, then the contributions can interfere destructively and cancel each other out, so that the amplitude is zero and the corresponding outcome is never observed; likewise, they can interfere constructively and increase the likelihood of a given outcome.
Superposition makes qubits interesting, but their real superpower is entanglement. Entangled qubits can interact instantly. Entanglement is a phenomenon in which quantum entities are created and/or manipulated such that none of them can be described without referencing the others. Individual identities are lost. This concept is exceedingly difficult to conceptualize when one considers how entanglement can persist over long distances. A measurement on one member of an entangled pair will immediately determine measurements on its partner, making it appear as if information can travel faster than the speed of light. This apparent action at a distance was so disturbing that even Einstein dubbed it “spooky” (Born 1971, p. 158).
HOW DO QUANTUM COMPUTERS WORK?
The goal in devising an algorithm for a quantum computer is to choreograph a pattern of constructive and destructive interference so that for each wrong answer the contributions to its amplitude cancel each other out, whereas for the right answer the contributions reinforce each other. If, and only if, you can arrange that, you’ll see the right answer with a large probability when you look. The tricky part is to do this without knowing the answer in advance, and faster than you could do it with a classical computer.
In reality a quantum computer leverages entanglement between qubits and the probabilities associated with superpositions to carry out a series of operations (a quantum algorithm) such that certain probabilities are enhanced (i.e., those of the right answers) and others depressed, even to zero (i.e., those of the wrong answers). When a measurement is made at the end of a computation, the probability of measuring the correct answer should be maximized. The way quantum computers leverage probabilities and entanglement is what makes them so different from classical computers.
HOW TO BUILD A QUANTUM COMPUTER?
Building quantum computers is incredibly difficult. In the world of atoms and molecules, the rules that govern their behaviour are quantum. In our world – the world of falling balls and apples – the rules are classical. What we don’t know, just yet, is the connection between the two sets of rules. What we need to do – to build a quantum computer – is use our classical understanding to build and control a quantum system. And that’s not easy.
First, qubits need to be protected from the environment because it can destroy the delicate quantum states needed for computation. To make functional qubits, quantum computers have to be cooled to near absolute zero (zero Kelvin). At these very low temperatures, we can make things quantum. Even at such a low temperature, qubits are only stable (retaining coherence) for a very short time. The longer a qubit survives in its desired state the longer its “coherence time.”
Second, however, for algorithm execution qubits need to be entangled, shuffled around physical architectures, and controllable on demand. The better these operations can be carried out the higher their “fidelity.” Balancing the required isolation and interaction is difficult, but after decades of research a few systems are emerging as top candidates for large-scale quantum information processing.
Superconducting systems, trapped atomic ions, and semiconductors are some of the leading platforms for building a quantum computer. Each has advantages and disadvantages related to coherence, fidelity, and ultimate scalability to large systems. It is clear, however, that all of these platforms will need some type of error correction protocols to be robust enough to carry out meaningful calculations, and how to design and implement these protocols is itself a large area of research. For an overview of quantum computing, with more detail regarding experimental implementations, see Ladd et al. (2010).
In nature, physical systems tend to evolve toward their lowest energy state: objects slide down hills, hot things cool down, and so on. This behavior also applies to quantum systems.
CONCLUSIONS AND OUTLOOK
Quantum computers have the potential to revolutionize computation by making certain types of classically intractable problems solvable. While no quantum computer is yet sophisticated enough to carry out calculations that a classical computer can't, great progress is under way. A few large companies and small start-ups now have functioning non-error-corrected quantum computers composed of several tens of qubits, and some of these are even accessible to the public through the cloud. Additionally, quantum simulators are making strides in fields varying from molecular energetics to many-body physics.
