![\"An](\"http:\/\/images.iop.org\/objects\/ntw\/news\/thumb\/15\/4\/14\/pic1.jpg\" "\\"An")
The heat engine, which converts a difference in temperature to mechanical work, is the archetypal machine of classical thermodynamics. The classical thermodynamic definition of temperature involves the average energy of a large number of particles, and is therefore not directly applicable to a single atom. However, a well-defined, classical thermodynamic temperature can still be obtained for such a particle, using the so-called ergodic theorem, which states that the average energy of a large number of particles in a region of space is equal to the energy of a single particle over a period of time. “That was a really tricky part of the design of the heat engine: how can you make use of the time-averaged definition of temperature?” explains lead researcher Kilian Singer<\/a> of the University of Mainz.<\/p>\nTemperature trap<\/strong><\/p>\n The solution was to confine the particle \u2013 which in this case was a calcium ion (40<\/sup>Ca+<\/sup>) \u2013 in a funnel-shaped trap, allowing it to undergo Brownian motion in a radial direction. The researchers then heated the ion using electrical noise, and as its temperature increased, its oscillations in the radial direction became larger, causing it to sample regions of higher potential, sending the particle towards the end of the trap at which it was less tightly confined. “You can think of it like a balloon in a funnel,” explains team member Johannes Ro\u00dfnagel<\/a>, who is Singer\u2019s PhD student. “When you inflate the balloon, it will move towards the larger end of the funnel.”<\/p>\n When the electrical noise was switched off, however, the ion cooled, causing it to sink back towards the narrower, steeper end of the trap. By turning the noise on and off periodically, the researchers set up axial oscillations of the ion between the two ends of the trap. If left undamped, these oscillations would have become increasingly large until the particle escaped the trap. However, the researchers applied another laser to damp the oscillations, thus maintaining the particle in steady harmonic oscillations.<\/p>\n Auto comparison<\/strong><\/p>\n “We have characterized the damping behaviour of the laser very well and we know exactly how much energy is dissipated by this damping laser,” says Ro\u00dfnagel. “We know that, in a steady state, the energy produced by the engine and the energy damped by the damping laser are equivalent. That’s how we determine the output energy of the engine.” The researchers calculated the output power, finding it to be around 3.5\u00a0\u00d710\u201322<\/sup>\u00a0W. When scaled by the number of particles and difference between the temperatures of the hot and cold reservoirs, the researchers calculate that this output power is comparable to that of a modern car engine.<\/p>\n While Ro\u00dfnagel himself admits that “you will never find a Mercedes driven by our heat engine”, he says that the team’s main goal is to get a better understanding of the thermodynamics of single particles, for the future development of other devices. Specifically, the researchers are interested in turning the idea around to produce refrigerators for heat management in nano-electronics. Their next research goal is to cool the atom further and confine it more tightly, so that it no longer behaves as a classical particle undergoing Brownian motion but as a quantum wavepacket. “The room left at the bottom is in temperature,” says Singer.<\/p>\n