The world's smallest engine races in quantum tunnels, where the question of where time is going

Do you have a really tiny engine and it consists of more than 16 atoms? You may have a lot of time because you’ve built something unnecessarily into this wasteful construction - you could say if it was a race, and that’s it, and its name is the great adventure of miniaturization. In the meantime, of course, new quantum physical discoveries are being made.


The research team at Empa and EPFL has developed a molecular engine that is made up of just 16 atoms and rotates reliably in one direction. The new structure may allow energy production at the atomic level, and its special feature is that it works exactly on the boundary between the motion controlled by classical physics and the quantum tunnel effect, thus revealing mysterious phenomena to researchers in the quantum world.

“We have approached the final size limit of molecular motors,” explains Oliver Groening, head of the Empa Functional Surfaces Research Group. The motor is less than a nanometer - in other words, about 100,000 times smaller than the diameter of human hair.

In principle, a molecular machine works in a similar way to its counterpart in the macro world: it converts energy into directed motion. Such molecular motors also exist in nature - for example in the form of myosins. Myosins are motor proteins that play an important role in the contraction of muscles and the transport of other molecules between cells in living organisms.

Energy production at the nanoscale

Like a large motor, the 16-atom motor consists of a stator and a rotor, i.e. a fixed part and a moving part. The rotor rotates on the stator surface and can occupy six different positions.

"For the engine to do a really useful job, it is essential that the stator controls the movement of the rotor so that it can only rotate in one direction," Groening explains.

Inverse ratchet

Because the direction of energy input to the motor is random, the direction of rotation is determined by the motor itself using a ratchet system . However, the nuclear engine works just the opposite of what happens with a ratchet familiar from a macroscopic world, with its asymmetrically toothed gears.

While the ratchet pin moves up along the flat edge and engages in the direction of the steep edge, the atomic version requires less energy to move up the steep edge of the gear than at the flat edge. Therefore, movement in the usual "blocking direction" is advantageous and movement in the "running direction" is much less likely. So movement is practically only possible in one direction.

The research team of Empa and EPFL has developed a molecular engine that consists of only 16 atoms and rotates reliably in one direction. The new structure may allow energy production at the atomic level, and its special feature is that it works exactly on the boundary between the motion controlled by classical physics and the quantum tunnel effect, thus revealing mysterious phenomena to researchers in the quantum world. (Image: Empa)

The researchers implemented a minimalist version of the “reverse” ratchet principle, using a triangular stator consisting of only six palladium and six gallium atoms. The trick is that this structure is rotationally symmetrical but not mirror-symmetric.

As a result, a rotor of only four atoms (a symmetric acetylene molecule) can rotate continuously, although counterclockwise and clockwise rotations must be different. “The engine thus has 99 percent directional stability, which sets it apart from other similar molecular engines,” Groening said. In this way, the molecular motor can pave the way for energy production at the atomic level.

Scanning tunnel microscopic image of a PdGa surface with six barbell-shaped acetylene rotor molecules (approximately 50 million magnification) under different rotational conditions. The scale atomic structure of the stator (blue-red) and the acetylene rotor (off-white, slightly to the left in the vertical direction) is shown schematically on the right. (Image: Empa)

Power from two sources

The tiny motor can be powered by both heat and electricity. The thermal energy causes the directional direction of rotation of the motor to change randomly, for example at room temperature, the rotor rotates back and forth completely randomly at millions of revolutions per second. 

In contrast, the electrical energy produced by a scanning electron microscope from the tip of which tiny current flowed into the motors allowed for directed rotation. The energy of a single electron was enough for the rotors to turn one-sixth of a turn. The higher the amount of energy, the higher the frequency of movement, but also the more likely it was that the rotor would move in a random direction because too much energy could turn the pin on the ratchet in the “wrong” direction.

According to the laws of classical physics, there is a minimum amount of energy that is needed to move a rotor. If the electrical or thermal energy supplied is not sufficient for this, the rotor must stop. Surprisingly, the researchers were able to observe an independent and constant rotational frequency in one direction, even below this limit, at temperatures below -256 ° C or at applied voltages below 30 millivolts.

From classical physics to the quantum world

At this point, we are already in transition from the realm of classical physics to a more mysterious field: the world of quantum physics. According to the rules in force here, particles can "tunnel", that is, the rotor can defeat the tap even if its kinetic energy is not sufficient in the classical sense. 

In addition, this tunneling motion takes place without energy loss, so in theory both directions of rotation should be probable. Surprisingly, in the experiments, the engine continued to rotate in the same direction with a 99 percent probability.

"According to the second law of thermodynamics, entropy can never decrease in a closed system. In other words, if we do not lose energy during the tunnel action, the direction of the motor must be purely random. The fact that the motor still rotates almost exclusively in one direction, it indicates that the tunnel loses energy even during movement, ”Groening pointed out.

Where is the time?

If we look at the issue from a little further away, we can see more strange events. When we watch a movie about something, we can usually tell clearly whether time is moving forward or backward in the movie. For example, if we see a tennis ball that jumps a little higher from the ground after each bounce, we intuitively know that the movie will play back in time. This is because experience teaches us that the ball loses energy with each bounce, so it will bounce lower next time.

If we think of an ideal system to which we do not add energy or take it away, it becomes impossible to determine in which direction time is moving.

A model for such a system could be an “ideal” tennis ball that bounces back to exactly the same height after each hit, so if we see a movie about it, it’s impossible to tell if it’s looking forward or backward, both directions will be equally likely. If energy remained within the system, we would no longer be able to determine the direction of time.

Of course, this principle can be reversed: If we observe a process in a system that makes it clear in which direction time is running, then the system must lose energy or spread energy even more accurately, for example through friction.

Fundamental discovery

Physicists generally assume that there is no friction during the tunnel action, but of course the system does not get any extra energy either. So how is it possible that the rotor always rotates in the same direction? The second law of thermodynamics does not allow exceptions, so the only explanation is that

there is still an energy loss during the tunnel effect, even if it is extremely small. So Groening and his team stumbled upon something truly decisive and fundamental as they lingered on the atoms.

“This minimotor may allow us to examine the processes and causes of energy distribution in quantum tunnel processes,” said the Empa researcher.

(Source: ScienceDaily

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