By Mark Ward A COMPUTER that exploits the bizarre properties of the quantum world may one day be able to crack the codes protecting sensitive commercial data and other confidential information in a matter of seconds. This frightening scenario has moved a step closer to reality now that scientists have made one of the components needed to create such a machine: a quantum logic gate. Conventional computers are built around a central processing unit that contains millions of logic gates. These carry out computations in a binary code. The simplest, a “not” gate, takes a string of binary digits and converts each 0 into a 1, and vice versa. If logic gates were shrunk to the size of individual atoms, however, strange quantum effects could be brought into play. One of the quirks of the quantum world is that particles can exist in a superposition of states – in effect existing in a number of different states at the same time. If these states were made to represent numbers, a logic gate could carry out several computations simultaneously, as if they were being conducted in parallel universes (“A quantum revolution for computing”, New Scientist, 24 September 1994). A processor built from quantum logic gates would be fantastically efficient. Last year, Peter Shor of AT&T Bell Laboratories in New Jersey developed an algorithm which, if run on a quantum computer, could quickly find the factors of huge numbers. This apparently pointless feat has a practical significance: data sent over computer networks are often encrypted in a code that can only be read by someone who knows the factors of a vast number. So if Shor’s algorithm were ever successfully used, cryptographers might also have to switch to quantum methods. So far, quantum computers exist only in the imagination of physicists. But by creating a quantum logic gate, researchers led by Chris Monroe of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have taken an important step towards making one. Their biggest challenge was to overcome the inherent fragility of any quantum system. Theory states that any attempt to observe a particle in a superposition of states will nudge it into one state or another, rendering the gate useless for subsequent quantum computations. “You need a system that limits the interaction with the environment,” says Monroe. At the same time, the gate’s components, or “bits”, must interact strongly with one another in order for them to carry out computations. The NIST researchers’ gate consists of a single beryllium ion trapped in an electric field. The ion rocks back and forth in the trap and this vibration can be made more violent by sweeping it with laser light of a range of frequencies. Altering the ion’s vibration has a knock-on effect: it changes the angular momentum or “spin” of one of the orbiting electrons between two possible states, called “up” and “down”. This link between the ion’s vibrational state and the spin of the orbiting electron means that the ion can work as a two-bit logic gate. The two lowest vibrational states of the nucleus and the electron’s two spin states each represent 0 and 1 in a binary code. The gate can then be switched from an input state to an output state by sweeping it with laser light. The gate revealed its quantum nature when the researchers tinkered with the frequencies in the laser sweeps. They hit upon a range of frequencies that put the gate into a superposition of states where both the two bits in the input and the output were both simultaneously 0 and 1 (Physical Review Letters, vol 75, p 4714). The researchers are now trying to work out how to link these quantum logic gates into a functioning processor. “To make it useful we are going to need hundreds,