All information processed by a computer (such as the one you are using now) is processed in terms of bits – elementary units of information that can be in one of two possible states. These states are usually referred to as 0 and 1. All the information on your computer is stored in coded form as long sequences of zeroes and ones. For instance, a sequence of three bits allows for eight different combinations, and can thus represent eight different numbers (or letters, or cities, etc): 000=0, 001=1, 010=2, 011=3, 100=4, 101=5, 110=6, and 111=7. In a quantum computer, the situation is fundamentally richer. To see why, we must first recognize that any computing machine stores information not abstractly, but rather in some concrete physical form: for example, the location of beads on an abacus, the electric current through a transistor in an ordinary computer, or electrical impulses traveling along neurons in your brain. In a conventional computer, the 0 state might be represented by the transistor being "off" (no current), and 1 by "on" (current flowing). Information is thus physical, and as such, the processing of information is subject to the laws of physics. Ordinary computers do rely on the laws of quantum physics. Transistors are quantum devices, but transistors are traditionally too large to be able to harness the full potential of quantum weirdness, which happens at the scale of atoms and subatomic particles. However, technology has now advanced to such a degree that we are able to construct and manipulate devices at the atomic scale. This means we have the potential to create computers that store and process information in a fully quantum way. For example, an electron is a remarkable, purely quantum "device" that behaves somewhat like a tiny, perpetually spinning bar magnet. If it is placed in the field of another magnetic, it has two natural states, either aligned with the field, or opposite to it. We call these two states spin up and spin down. Thus the electron can be used to store one bit of information, say spin down = 0, spin up = 1. So far, this is the same as an ordinary computer, except that the information is stored in an incredibly small space. For example, these may be electrons in atoms, and so each bit of information occupies the space of an atom, which is much smaller than in any conventional storage medium such as your computer's hard disk. Now we come to the quantum "magic." We noted above that one bizarre feature of the quantum world is that it is possible for a single particle to behave as if it is in more than one place at the same time. This is a general property of the quantum world – things can exist simultaneously in more than one state, called the principle of superposition (see quantum foundations). In the case of the electron, it can exist in both spin up and spin down states simultaneously. In other words, instead of just 0 or 1, it can be 0 and 1. How does this help us? If we have, say, three such "quantum bits," or "qubits," then instead of just being in the states 000 or 001 or 010, etc. (the eight possibilities listed above), they can be, in a certain sense, in all these states simultaneously. It is then possible to manipulate these qubits, using the laws of quantum physics, to perform multiple calculations simultaneously: a quantum-parallel computer. This quantum parallelism results in computing power that grows exponentially, doubling with each additional qubit. Adding 1 qubit increases the computing power by a factor of 2. Adding 2 qubits increases it by 4. Adding 3 qubits increases it by 8, and so on. With just a hundred qubits, the raw computing power would far exceed anything we might hope to achieve with a conventional computer. However, the trick is in how these qubits are "manipulated," which is the quantum analogue of a conventional computer program, telling the computer what kind of calculation to perform. So far there are only a few kinds of problems that we know how to ask a quantum computer to solve, although, to be sure, these problems are of great practical importance. One of them, the question of how to write a given, very large number as a product of prime numbers, is at the heart of one of the most commonly used methods of modern encryption. With a working quantum computer, you could easily decipher encrypted messages currently floating around on the Internet between banks, governments, and so forth. (However, even a quantum computer cannot be used to eavesdrop on the quantum key distribution process discussed above!) Theoretical physicists and mathematicians are currently working very hard to expand the types of problems a quantum computer could be asked to solve. There are also a number of experimental challenges. Most importantly, as we saw in our discussion of quantum cryptography, quantum information – stored in superposition and entangled states – is very delicate, and easily destroyed by outside influences. Ideally, a quantum computer must be perfectly isolated from its environment while it is performing its quantum-parallel computation. Of course in practice this is not possible, resulting in random errors in computation. Nevertheless, physicists and computer scientists have built upon the classical theory of fault-tolerant error correction (necessary to stabilize conventional computations running in unstable environments or running for very long periods of time), to develop a set of techniques that allow us to protect quantum information from realistic errors. These techniques require good, but not perfect, control of a quantum system, and experimental physicists and engineers worldwide are working toward developing quantum computers that will be robust in the real world. So far, quantum computers are mostly a theoretical construct, although very simple proof-of-principle versions have been built. Once experimental physicists succeed in constructing large quantum computers, we should be able to harness the weirdness of the quantum world to perform, in seconds, certain types of calculations, which, on a conventional computer, would take thousands of years. Such attempts to harness the quantum world to create powerful and practical new technologies forces physicists to think more deeply about how the universe works – the foundations of quantum theory, which in turn may lead to new insights into the biggest problem of all: combining the quantum and relativity theories into a single, unified theory of quantum gravity. To learn more about quantum information at Perimeter Institute and the researchers, please click here. |
