杏吧原创

Not everything is weird at the nanoscale. Wires so small you鈥檇 expect them to obey the strange laws of quantum mechanics have instead displayed the same electrical properties as ordinary electrical interconnects.

The finding bodes well for conventional computers, because these tiny, conductive wires could make chips smaller. It could be bad news, though, for the super-fast quantum computers that are hoped to come next.

So far, conventional computers have followed Moore鈥檚 law: the density of transistors that a conventional integrated-circuit chip can hold doubles approximately every two years, yielding ever-better performance out of ever-smaller devices.

However, it鈥檚 getting harder to build smaller interconnects to wire up the devices on the silicon chip. As the width of metal wires drops to few tens of nanometres, their resistivity soars because electrons start interacting with nearby surfaces, dissipating more heat and lowering efficiency.

Phosphorous infusion

Also, as wires get down to nanometre scales, quantum behaviour usually dominates. For instance, the entire wire can exist in a superposition of states because of a property called quantum coherence. The wave behaviour of electrons in the wire might then cause them to interfere with each other, disrupting the electrical properties you would expect to see at larger scales.

Now, of the University of New South Wales in Sydney, Australia, and colleagues have etched channels in a silicon chip just 1.5 nanometres wide that behave just like larger wires.

The trick was to infuse them with phosphorus atoms, which provide electrons that can move freely and conduct electricity, turning each channel into a wire. Because the entire wire, except for its ends, was enclosed in the silicon, it was isolated from other surfaces that could disrupt its conductivity.

Coolly classical

The team found that these wires conducted electricity nearly as well as state-of-the-art copper interconnects used in modern microprocessors 鈥� despite being much thinner. Moreover, when they built wires of different lengths, the wires followed Ohm鈥檚 law, in which the resistance of a wire increases with length 鈥� a property of non-quantum, or 鈥渃lassical鈥� conductors.

The lack of quantum behaviour surprises of Arizona State University in Tempe 鈥� especially because the experiments were carried out at a mere 4.2聽kelvin. 鈥淯sually when you go to [such] low temperatures, you expect quantum mechanics to dominate the world. Here they have Ohm鈥檚 law, suggesting that it鈥檚 just like classical behaviour at room temperature,鈥� he says.

He reckons the large number of phosphorus atoms in the wire provided a very high density of electrons (10²¹ per cubic centimetre) and that their mutual scattering destroyed any quantum coherence, leading to classical behaviour.

That bodes well for doing the experiment at higher temperatures. 鈥淚f they behave classically at low temperature, then they are also likely to behave classically at room temperature,鈥� says Simmons.

Coherent problem?

Indeed, Simmons says that the new wires are great news for those hoping for ever-tinier computing devices. 鈥淚t shows that you can maintain low resistivity and make very thin conducting wires, which is obviously essential for down-scaling devices towards the atomic scale,鈥� she says.

The implications for quantum computing are less clear. Simmons鈥檚 team had already shown that individual phosphorus atoms can exist in a superposition of spin states, making up the quantum bits, or qubits, needed for quantum computation. She thinks that the nanowires could be used to interconnect qubits and help build quantum circuits.

Ferry thinks otherwise. 鈥淭his lack of quantum coherence is good for Moore鈥檚 law, but it鈥檚 bad for quantum computing, because you need quantum coherence for quantum computing. This may make it less likely to occur.鈥�

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