THE era of quantum computing has begun in earnest, scientists say. For the first time, they have made a quantum computer that can carry out a task in a way that is impossible with supercomputers.
The bits of a conventional computer can only exist in two states, 0 or 1. In quantum computers, the bits (or 鈥渜ubits鈥�) can be the spin states of a proton, for instance, which exist as a 鈥渟uperposition鈥� of both 0 and 1 until a measurement is made (鈥淲ake up to quantum coffee鈥�, 麻豆传媒, 15 March 1997, p 28).
This allows quantum computers to explore different routes through a mathematical problem simultaneously. In theory, they can quickly perform some tasks, such as factoring huge numbers and cracking ingenious cryptographic codes, that would take a conventional supercomputer years.
Advertisement
Lov Grover, a physicist at AT&T Bell Labs in New Jersey, showed last year how a quantum computer could 鈥済uess鈥� a chosen number in a certain range. The task is similar to a game of 鈥渉igher/lower鈥�-homing in on a number by repeatedly asking if the one you guess is too high or too low.
Repeated questioning would be all a classical computer could do. But Grover showed that a quantum computer could divine the number in one attempt, just like packing all the questions into the states of a qubit. 鈥淚t鈥檚 interesting and kind of surprising that you can somehow do it when you get only one bit out of the computer,鈥� says Norm Margolus, a physicist at the Massachusetts Institute of Technology in Cambridge.
Now Isaac Chuang of IBM鈥檚 Almaden Research Center in San Jose and Neil Gershenfeld of MIT have made a quantum computer that works through another of Grover鈥檚 algorithms, answering two questions about one of four numbers. The problem is similar to asking which of the numbers 1, 2, 3 and 4 is odd and greater than 2.
In the current issue of Physical Review Letters (vol 80, p 3408), the researchers describe how they used the nuclei of a carbon atom and a hydrogen atom in a chloroform molecule as two qubits. Both nuclei had spin 0 and spin 1 states, giving four combinations which existed simultaneously: 00, 01, 11 and 10. Using magnetic fields and radio waves, the researchers manipulated the atoms鈥� spins, making them dance a nuclear jig corresponding to the algorithm鈥檚 logic.
The correct answer to the calculation came when a measurement of the spin states 鈥渟nuffed out鈥� those that did not match the target state. Chuang and his colleagues have since been working on other quantum algorithms, such as the 鈥淒eutsch-Jozsa鈥� algorithm, which spots some properties of a mathematical function far faster than a classical computer.
Although cracking codes is still years away, Grover says the new work proves quantum computers are no longer just an idea. 鈥淚t鈥檚 a remarkable achievement,鈥� he says. 鈥淭hey鈥檝e demonstrated that quantum computing works, not just with pencil and paper, but in the lab.鈥�
