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IT鈥橲 a cardinal rule that heat flows from the hot end of an object to the cold, no matter which end you heat. But now researchers have suggested a way to build a device that conducts heat in only one direction. The idea has only been tested in a computer simulation, but if it works in practice it could lead to circuits that run on heat instead of electricity.

Heat makes molecules vibrate, and the jostling of molecules by their hot neighbours is what makes heat flow. This usually happens in any direction, but now Michel Peyrard of 脡cole Normale Sup茅rieure in Lyon and his colleagues have shown how to make it one-way.

Molecules of most materials vibrate at the same characteristic frequencies regardless of temperature. As they get hotter, only the amplitude increases. But in a few 鈥渘on-linear鈥� materials, such as DNA, the frequency decreases as they get hotter.

The researchers suggest building a device with particles that vibrate at high frequency on the left and particles with a low frequency on the right. Sandwiched in the middle are non-linear particles.

If the left end is heated, the particles vibrate rapidly and start to transfer heat towards the centre. As heat passes into particles of the non-linear material, their rate of vibration slows. This sets up a frequency mismatch that makes the transmission of energy inefficient, says Peyrard. But heat the right side, and the particles vibrate slowly, so the neighbouring non-linear particles conduct heat ever more efficiently as they start to heat up. Such a device would do for heat what diodes do for electricity. Peyrard says once you can make a thermal diode, then a transistor or switch won鈥檛 be far off.

鈥淚t鈥檚 a good piece of work,鈥� says David Campbell of Boston University. But he says some 鈥渟erious materials engineering鈥� will be needed before anyone actually builds the device.