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What is quantum computing?

Quantum computing is essentially harnessing and exploiting the amazing laws of quantum mechanics to process information. A traditional computer uses long strings of “bits,” which encode either a zero or a one. A quantum computer, on the other hand, uses quantum bits, or qubits. What's the difference? Well a qubit is a quantum system that encodes the zero and the one into two distinguishable quantum states. But, because qubits behave quantumly, we can capitalize on the phenomena of "superposition" and "entanglement."

Superposition and entanglement? Pardon?

It’s OK to be a bit baffled by these concepts, since we don’t experience them in our day-to-day lives. It’s only when you look at the tiniest quantum particles – atoms, electrons, photons and the like – that you see intriguing things like superposition and entanglement.

Superposition is essentially the ability of a quantum system to be in multiple states at the same time — that is, something can be “here” and “there,” or “up” and “down” at the same time.

Entanglement is an extremely strong correlation that exists between quantum particles — so strong, in fact, that two or more quantum particles can be inextricably linked in perfect unison, even if separated by great distances. The particles remain perfectly correlated even if separated by great distances. The particles are so intrinsically connected, they can be said to “dance” in instantaneous, perfect unison, even when placed at opposite ends of the universe. This seemingly impossible connection inspired Einstein to describe entanglement as “spooky action at a distance.”

#### Why do these quantum effects matter?

First of all, they’re fascinating. Even better, they’ll be extremely useful to the future of computing and communications technology.

Thanks to superposition and entanglement, a quantum computer can process a vast number of calculations simultaneously. Think of it this way: whereas a classical computer works with ones and zeros, a quantum computer will have the advantage of using ones, zeros and “superpositions” of ones and zeros. Certain difficult tasks that have long been thought impossible (or “intractable”) for classical computers will be achieved quickly and efficiently by a quantum computer.

#### What can a quantum computer do that a classical computer can’t?

Factoring large numbers, for starters. Multiplying two large numbers is easy for any computer. But calculating the factors of a very large (say, 500-digit) number, on the other hand, is considered impossible for any classical computer. In 1994, a mathematician from the Massachusetts Institute of Technology (MIT) Peter Shor, who was working at AT&T at the time, unveiled that if a fully working quantum computer was available, it could factor large numbers easily.

#### But I don’t want to factor very large numbers…

Nobody wants to factor very large numbers! That’s because it’s so difficult – even for the best computers in the world today. In fact, the difficulty of factoring big numbers is the basis for much of our present day cryptography. It’s based on math problems that are too tough to solve. RSA encryption, the method used to encrypt your credit card number when you’re shopping online, relies completely on the factoring problem. The website you want to purchase from gives you a large "public" key (which anyone can access) to encode your credit card information.

This key actually is the product of two very large prime numbers, known only to the seller. The only way anyone could intercept your information is to know those two prime numbers that multiply to create the key. Since factoring is very hard, no eavesdropper will be able to access your credit card number and your bank account is safe. Unless, that is, somebody has built a quantum computer and is running Peter Shor's algorithm!

#### Wait… so a quantum computer will be able to hack into my private data? That’s not good.

Don’t worry — classical cryptography is not completely jeopardized. Although certain aspects of classical cryptography would be jeopardized by quantum computing, quantum mechanics also allows for a new type of highly secure cryptography.

Let’s look at a common cryptographic protocol called the one-time pad: Say party A and party B (let's call them Alice and Bob) share a long string of random zeros and ones — the secret key. As long as they only use this key once and they are the only ones who know this key, they can transmit a secret message such that no eavesdropper (we’ll call her Eve) will be able to decipher the message. The main difficulty with the one-time pad is the actual distribution of the secret key. In the past, governments sent people to exchange books full of random data to be used as keys. That, of course, is impractical and imperfect. This is where quantum mechanics comes in very handy once again: Quantum Key Distribution (QKD) allows for the distribution of completely random keys at a distance.

#### How can quantum mechanics create these ultra-secret keys?

Quantum key distribution relies on another interesting property of quantum mechanics: any attempt to observe or measure a quantum system will disturb it.

The Institute for Quantum Computing (IQC) is home of one of the few QKD prototypes in the world. “Alice,” a device located at IQC headquarters, receives half of the entangled (highly correlated) one of the photons generated by a laser on the roof of a building at the University of Waterloo. “Bob” is housed at the nearby Perimeter Institute, and receives the other half of the entangled photons.

Photons have a unique measurable property called polarization (which should sound familiar to any connoisseur of sunglasses).

Since the polarization of each individual photon is random, there’s no way of knowing the unique properties of each photon in advance. But here is where entanglement becomes interesting: if Alice and Bob measure the polarization of the entangled photons they receive, their results will be the same (remember, “entangled” means the particles are highly correlated with each other, even at great distances). Depending on the polarization of each photon, Alice and Bob ascribe either a “one” or a “zero” to each photon they receive. Therefore, if Alice gets a string like 010110, Bob also gets a 010110. Unless, that is, an eavesdropper has been attempting to spy on the signal. This will disturb the system, and Alice and Bob will instantly notice that their keys don’t match.

Alice and Bob keep receiving photons until their identical keys are long and identical enough and, presto, they’ve got ultra-secure keys for encrypting communications.

So harnessing the quantum world can break and make codes. Anything else?

Plenty. For example, quantum computers will be able to efficiently simulate quantum systems, which is what famous physicist Richard Feynman proposed in 1982, effectively kick-starting the field. Simulation of quantum systems has been said to be a "holy grail" of quantum computing: it will allow us to study, in remarkable detail, the interactions between atoms and molecules. This could help us design new drugs and new materials, such as superconductors that work at room temperature. Another of the many tasks for which the quantum computer is inherently faster than a classical computer is at searching through a space of potential solutions for the best solution. Researchers are constantly working on new quantum algorithms and applications. But the true potential of quantum computers likely hasn’t even been imagined yet. The inventors of the laser surely didn’t envision supermarket checkout scanners, CD players and eye surgery. Similarly, the future uses of quantum computers are bound only by imagination.

#### Sounds great! Where can I get a quantum computer?

Not so fast. While quantum computers have been theoretically demonstrated to have incredible potential, and scientists are working at IQC and around the world to realize that potential, there is much work to be done before quantum computers hit the market.

#### What is required to build a quantum computer?

Simply put: we need qubits that behave the way we want them to. These qubits could be made of photons, atoms, electrons, molecules or perhaps something else. Scientists at IQC are researching a large array of them as potential bases for quantum computers. But qubits are notoriously tricky to manipulate, since any disturbance causes them to fall out of their quantum state (or “decohere”). Decoherence is the Achilles heel of quantum computing, but it is not insurmountable. The field of quantum error correction examines how to stave off decoherence and combat other errors. Every day, researchers at IQC and around the world are discovering new ways to make qubits cooperate.

#### So when will there be a real quantum computer?

It depends on your definition. There are quantum computers already, but not of sufficient power to replace classical computers. A team of researchers from IQC and MIT hold the current world record for the most number of qubits used in an experiment (12). While practical quantum technologies are already emerging — including highly effective sensors, actuators and other devices — a true quantum computer that outperforms a classical computer is still years away. Theorists are continually figuring out better ways to overcome decoherence, while experimentalists are gaining more and more control over the quantum world through various technologies and instruments. The pioneering work being done today is paving the way for the coming quantum era.

#### So quantum technology is still years away?

No, quantum technologies are already in use! QKD is already commercially available, and will greatly benefit from new research (scientists at IQC are currently pursuing quantum encryption through free space via satellite). Although a fully functioning quantum computer is a longer-term goal, many fundamental and practical discoveries have been made in the name of quantum computing. Quantum sensors and actuators will allow scientists to navigate the nano-scale world with remarkable precision and sensitivity. Such tools will be invaluable to the development of true quantum information processors. The quantum revolution is already under way, and the possibilities that lie ahead are limitless.

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