SCIENCE AND ENGINEERING NEWS

Many of today’s electronic transactions are encrypted in part by factoring large numbers into prime numbers–a tough job for even the fastest computers. But suppose factoring were almost as easy as multiplication? Secret codes would be open books to whoever had access to the technique. And such a technique exists, thanks to computers whose workings depend on the weird world of quantum mechanics.

In the February issue of IEEE Spectrum, Contributing Editor Justin Mullins describes the race to build quantum computers. Conventional and quantum processors represent numbers using bits of data. But whereas the conventional kind assign each bit a value of either 0 or 1, the quantum machine exploits a quantum mechanical property known as superposition, that allows bits (qubits) to be a 0 and a 1 at the same time. (Admittedly, the results need disentangling.) With just a few hundred qubits a computer could perform computations on more numbers than there are atoms in the universe, Mullins writes.

Two quantum computing methods have progressed quite far. In one, individual molecules in a liquid serve as computers under the control of nuclear magnetic resonance (the technique used for medical imaging). The qubits are the quantum states of particular atoms within the molecules. These atoms possess a quantum property called spin that is observed in only two states, up or down, when the molecules are in a strong magnetic field. Pulses of radio-frequency energy can manipulate the spin states of atoms, and their interaction with each other in the molecules performs logic operations. Scientists have so far made quantum computers with up to seven qubits using this technique.

Another technique involves lining up a string of very cold ions in a device called a linear ion trap. In this case the ions themselves act as the qubits. Better yet, their interaction with each other can form a qubit that is shared among all the ions, much as different computer companies can share information from a common databus. Because the ions all have the same charge, they repel each other, vibrating as if connected by springs. The vibrations themselves act as the databus.

Neither approach is perfect: ions have proven hard to control, and noise will overwhelm the output signal from liquid-based NMR computers using much more than 10 qubits. So scientists are looking to other technologies, such as a silicon version of the NMR computer and quantum dots. Some believe the key to further advances will be to design a quantum Internet, where quantum computers with only a few qubits perform complex tasks by sharing quantum information among themselves. A three-node quantum Internet may be just 10 years away, writes Mullins.

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