What is a quantum computer?
A quantum computer is a device that exploits the "weirdness" of quantum mechanics to be a more powerful type of computer. It can do certain tasks much better than normal computers, like search large databases, but many possible applications are yet to be found. Quantum simulators are similar but are used more directly to develop new materials.
Why do you want to build one?
Quantum simulators will eventually allow scientists to develop new pharmaceuticals and materials on a computer, rather than laboriously manufacturing them and then testing their properties in the lab or on living organisms. Currently this is done on computers with a limited success, and uses up many years of time on super-computers. A quantum simulator will be many times faster. It will be more accurate, as nature is quantum so our model of it should be as well.
Universal quantum computers can do other things like search large databases (e.g. find pictures of Aunt Maude in your photo collection) or help business save money (work out how to deliver parcels in the most cost-effective way, something an ordinary computer struggles to do)
One of the first "killer apps" was an algorithm that would run on a quantum computer and allow the reading of secure messages on the Internet. However, this is only one application, many more exist. Since then quantum cryptography has been developed that will be secure, even against a quantum computer. Quantum cryptography is already being used by banks and very critical applications, and by the time quantum computers can read secure messages, the Internet may already be using quantum cryptography.
What have you already achieved?
In terms of a truly universal quantum computer, i.e. one that can be programmed to do any task, simple demonstrations have already be carried out in many labs. Typically about 8 or so qubits (quantum bits; the measure of the number of quantum components in the computer) have been achieved. Very simple algorithms have been demonstrated. NQIT has demonstrated all the building blocks needed for a quantum computer.
What is the challenge in building one?
Quantum systems are very fragile. Typically they are sensitive their surroundings: heat, stray light, radio waves, magnetic fields etc. all can affect the information stored in the qubit. To protect against these things requires an enormous engineering effort, but we are getting there.
What about D-Wave? Doesn't Google already have a quantum computer?
Many groups and companies around the world are researching quantum technology. To date, only simple machines have been built, and some are limited in their capabilities. We are working on a universal quantum computer, one that can be programmed for any task. The D-wave machine is a quantum annealer, which is good for certain problems, but can't do everything. Google is also building a quantum computer, but with a different technology. Either could be successful in the future, we will have to see!
What is a networked quantum computer?
We believe the best approach, as is often in technology, is to take a modular approach to building a quantum computer. If we can build simple modules containing just a few qubits, and then link them up, there is no end to the number of qubits we could add to our system. This makes a very scalable system. Other people believe it is best to build as many qubits into one system, but this can become very challenging due to the delicate interactions of the qubits. However, interlinking qubits is also hard, and we have to invest a considerable engineering effort to make sure it works. If it works, it will bring other advantages such as being able to link two remote quantum computers in different places.
Does this mean the end of the silicon computer?
Probably not for a long time. Silicon computers, such as your smart phone or desktop are very good at what they do. For playing movies, reading emails, etc. these devices are already incredibly good. The first quantum devices will initially be research machines used by labs and businesses. Eventually, quantum co-processors may find their way in to the home, enabling faster searches of your photo library for instance. They would initially exist side-by-side with the normal computers we have.
Are quantum computers good for the environment?
Quantum computers promise to use less power in the long run. By using the smallest systems possible, e.g. individual atoms, the power used could be a lot less than with anything else. However, the first generation of machines will be quite clunky, like all new technology, and will typically use similar power to a server farm, such as those already operated by Internet companies. In the end though, the hope is to dramatically reduce power used by computers, which is a significant concern.
Is this just a smaller computer?
Quantum computers are fundamentally different from conventional computers, and allow possibilities that are unimaginable on a conventional computer. The qubits used in them are typically very small, perhaps even just single atoms. The drive to smaller and smaller devices is described by Moore's law. But eventually the chips used in computers won't be able to get any smaller. This might happen as soon as 2025 when transistors may only have 10 or so atoms in them. http://www.nature.com/news/the-chips-are-down-for-moore-s-law-1.19338]. At this point, everything will be quantum anyway, so the challenge is to exploit this rather than be defeated by it.
Is it a form of parallel computer?
Yes and no. It is true that all the qubits are involved in an operation at once. Many possibilities are explored, and one outcome is returned. But to compare a quantum computer with normal parallel computers is to miss the point. Both have a role to play in high-end computing, but there are technicalities why one approach might be better than another. See : www.scottaaronson.com/talks/npphys-cmu.ppt for more details
Are we playing with technology we don't understand?
Quantum effects have been understood and researched for over 120 years. A lot of materials we use every day have been made possible due to our understanding of quantum mechanics. To most people quantum mechanics may seem weird and complicated, but this is because we live in a world where we can not see the quantum nature of the universe directly. Scientists have tested and used the theories of quantum mechanics very thoroughly, and it is considered the most well-tested and accurate theory we have. This knowledge has lead to a recent explosion of quantum technology, in fact the 21st century is being called the quantum century, in the way that the late 20th century was dominated by silicon chips.
What is the history of quantum computers?
The history of quantum computers started in the 1980s when scientists realised some problems in chemistry and physics are so hard to do on a super-computer, that even the most powerful computer might take decades or longer than the lifetime of the universe to complete. These include problems like how protein's fold or how new materials behave. Rather than give up hope, physicists such as Richard Feynman realised that nature solves these problems for free. i.e. you can use one system of atoms to act like another. At the time no way of doing it actually existed, but by the 90s new technology involving lasers meant that it became possible. Over the last 20 years this technology has matured, and people are already demonstrating small scale quantum computers.