What is Quantum Information

We live in an ‘information age’. Information technology is all around us – computers, cell phones, bar codes, GPS satellites – and it’s constantly ramifying. All of this is a consequence of a specialized branch of mathematics known as ‘information theory’, which is concerned with quantifying, communicating, and manipulating the information encoded into physical systems or ‘states’. In information theory, information is seen as a pattern which distinguishes one physical state from another; the fundamental unit of information is a ‘bit’. A bit can be thought of as a question which is answered either ‘yes’ or ‘no’, that is, ‘one’ or ‘zero’.
Quantum mechanics is the science which describes the behaviour of the extremely small particles that make up reality at the most basic level – protons, neutrons, electrons, quarks. At the quantum scale, reality is not easily understandable by humans used to things being in only one place at once and being able to simultaneously know the speed and position of things. And when information theory gets entangled with quantum theory, things start to get a bit wierd.
The fundamental unit of quantum information is not the bit but the ‘qubit’. If a bit is a question that is answered ‘yes’ or ‘no’, a qubit is a question that could also be answered ‘well, yes and no….’ This is because of the Heisenberg Uncertainty Principle, which states that it is impossible to accurately simultaneously measure the position and the velocity of a subatomic particle without changing them. This means that particles which have not been measured exist not in any discrete state, but as a quantum probability wave called a ‘superposition of states’ which describes merely how likely it is that they are in any given state. Measuring the state of the particle ‘collapses’ the probability wave into a specific physical state, but before it is measured the particle is literally in all possible states at once. This is not just an abstract bit of mathematics; it governs the real behaviour of quantum particles, and is the operating principle behind technology like bar code scanners and digital cameras.
Therefore, a qubit which describes a ‘simple’ quantum system like the spin of an electron in a magnetic field can be in the measured states of one or zero; or it can be in an unmeasured superposition describing the probability that, if measured, it would turn out to be in either of these states. A series of three bits can encode any binary number from 000 (zero) to 111 (seven); a series of three qubits in superposition could encode any or all of these numbers at the same time. Confused? Well, it only gets worse from there.
A quantum system like an electron is in principle indistinguishable from another quantum system in the exact same state. And two electrons can be synchronized into the exact same quantum state (or ‘entangled’), so that measurement of one to determine its state will actually determine the state of both particles, even after they have been seperated (quantum physicists refer to this phenomenon with the incredibly scientific phrase ‘spooky action at a distance’). Therefore, a quantum particle encoding a superpositioned qubit can use an entangled pair of particles to transmit its quantum state instantly across any distance, making it as if the particle itself had ‘teleported’. It’s no wonder even Einstein was confused by quantum physics.
One of the great projects of the new milennium is the quest to built a quantum computer, which can handle quantum information in the same way a classical computer handles classical information, by using it to solve complex mathematical programs. Because quantum systems can encode superpositions, they can theoretically be used to run multiple different programs at the same time. This could not only lead to vast increases in available computing speed and power; quantum computers could also be used to solve programs that a normal computer never could – at least, not before the end of the universe. This is a fact that has serious consequences, as a quantum computer could relatively easily be used to break the ‘secure’ encryption algorithms used for things like military communiques and electronic bank transactions.
Luckily, there is already a form of quantum encryption available which is theoretically unbreakable, even by a quantum computer. Quantum encryption algorithms use information encoded as qubits. Because any attempt by a third party to discover the encryption key is a de facto attempt to measure a quantum system, it would disturb the superposition and thus be immediately detectable to those who were using the encryption. Very simple quantum computers are already being tested in laboratories. They are difficult to construct and run; since any attempt to measure a qubit in a superpositioned state collapses it and thus destroys any quantum information it encodes, the computers can be programmed only through very indirect methods, and as of this date they are not capable of the full range of functions a normal computer can run. However, technology continually advances, and so it seems inevitable that quantum information is soon going to become entangled in our very non-quantum lives.
Sources:Perimiter Institute for Theoretical Physics Quantum Computing Wiki”:http://www.quantiki.orgWikipedia: Qunatum Information