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Physicists Flip A Qubit
By Philip Ball

A quantum computer has had its first thought: using quantum-mechanical rules, physicists have carried out a logic operation.

The process is simpler even than adding one and one, and the apparatus used hardly qualifies as a real computer. But at least Francesco De Martini, of the University of Rome La Sapienza, and colleagues have proved that there is no fundamental reason why quantum computers can't be as logical as conventional ones1.

This was by no means obvious. Quantum computers would manipulate information using the principles of quantum mechanics that describe the behaviour of elementary particles. They could, in principle, be faster and more powerful than those of today. But there's a hitch: quantum rules seemed to prohibit even the simplest logic operation.

In a conventional computer, data is encoded in a string of electrical pulses representing the digits 1 and 0 - rather like Morse code. Each digit is called a bit. Logic operations are performed by devices that receive various input signals (1's and 0's) and combine them to generate certain outputs, encoded in the same binary language.

One of the simplest operations, called NOT, converts an input of 1 into an output of 0 and vice versa - it flips a single bit. This is relatively easy to achieve with standard electronic devices such as transistors.

But in quantum computers, the data aren't necessarily represented by single 1's and 0's. A quantum bit - known as a qubit - can also be in both states at once, like a switch that is simultaneously on and off. This is called a superposition.

Flipping such a qubit - the quantum NOT operation - is then no longer a matter of converting 'on' to 'off', or 1 to 0. Instead, one has to convert the superposition into its opposite.

This is not possible for any arbitrary qubit. But there is a way of doing something very close to the NOT operation that should be good enough, De Martini's group has discovered.

The researchers encode their inputs and outputs in light beams instead of electrical pulses. The fundamental particles of light are photons, which can be prepared in two states of polarization, equivalent to 1's and 0's. By passing a laser beam of photons through a special crystal, the researchers prepared superpositions of the two states.

Light detectors, laser-beam splitters, crystals and mirrors changed the input signal - a photon in a prepared quantum state - to an output photon in the state that corresponds to the best possible approximation of a flipped qubit. In other words, the apparatus carried out a NOT operation, or as good as, on the qubit.

Previous experiments have been concerned with encoding information in quantum states and reading it back out again. Such operations allow information to be sent over long distances in a tamper-proof manner, a procedure called quantum cryptography.

But before now, researchers hadn't managed to do anything much with that information. De Martini's team has shown that quantum information can be transformed without being lost. Putting that knowledge to use in a real quantum computer, however, remains a very distant dream.

# De Martini, F., Name, A.B., Buzek, V., Sciarrino, F. & Sias, C. Experimental realization of the quantum universal NOT gate. Nature, 419, 815 - 818, (2002). |Article|

Nature News Service / Macmillan Magazines Ltd 2002