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In another quantum effect, called entanglement, two particles are connected so that the state of one depends on the other.

You don't often see these quantum effects in the everyday world. Real-world objects contain billions of particles, and they aren't isolated. They are not in a coherent quantum state, so Newton's laws still do a better job of explaining the real world than quantum mechanics.

Irwin Schrodinger imagined a real-world quantum system in his famous cat paradox. Imagine a cat in a box with a cylinder of poison gas, the valve of which is controlled by the quantum state of a single particle. If it's isolated from the rest of the world, it's a quantum system. We don't know what state the particle is in and therefore we don't know if the valve is open or closed, or whether the cat is alive or dead. Until we open the box, there are two 'superposed' cats, one alive and one dead.

This is just a theoretical experiment. In a real box there would always be interaction between the cat and the outside world. This could be as small as a quantum of heat slipping out, or a sound getting in. Any interaction would decohere the particles in the box. Schrodinger's cat is a theoretical puzzle, not a science project.

That is, until the quantum computing idea was floated. At its simplest, quantum computing is a version of Schrodinger's cat experiment.

The basic idea

The idea of quantum computing was suggested by Richard Feynman and Paul Benioff, but its real father was David Deutsch, who spelt

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out the possibility in 1985, and started to work out algorithms that might be useful on a quantum computer. Since then, theoreticians have put meat on the bones, and researchers have built prototypes of quantum computing components.

Imagine a computer that has a set of inputs to process. For each possible input, the computer is in a different quantum state. If you can isolate the system, all those multiple states exist at the same time, and the computer can process every possible input at the same time.

In effect, a quantum computer is multiple computers superposed. One quantum bit, or qubit, lets the machine make two calculations at a time, and each additional qubit doubles the number of inputs, and the number of virtual computers. A quantum computer with just 300 qubits would be like 1090 conventional computers - more than the number of atoms in the known universe.

This speed-up is beyond anything you can get by squeezing more components on a piece of silicon or speeding up the processor's clock. That just adds more bits to the capacity of the system or lets it do more calculations in a given time. A quantum computer could, in theory, do as many calculations as you want - all at the same time.

Some describe this situation as multiple computers in parallel universes. Professor Seth Lloyd of MIT has a better analogy. He describes a quantum computer as being like a musical chord. You can hear several notes at the same time. A quantum system in coherence can have several states, each of which exists at the same time. Operating a quantum computer is like setting off a musical chord, and then tuning in to hear the dominant note - the answer.

It's obvious why this is so useful. A quantum computer as powerful as this could instantly crack any code. Codes rely on the fact that it is very hard to find the prime factors of a large number. For every digit added to a number, the amount of computing time required to crack it goes up exponentially. So, as conventional computers get faster, the computer industry can simply add a few more digits to the encryption key on a WiFi link or a secure webpage.