Weird quantum effects are so delicate it seems they could only happen in a lab. How on Earth can life depend on them?

The point of the most famous thought-experiment in quantum physics is that the quantum world is different from our familiar one. Imagine, suggested the Austrian physicist Erwin Schrödinger, that we seal a cat inside a box. The cat’s fate is linked to the quantum world through a poison that will be released only if a single radioactive atom decays. Quantum mechanics says that the atom must exist in a peculiar state called ‘superposition’ until it is observed, a state in which it has both decayed and not decayed. Furthermore, because the cat’s survival depends on what the atom does, it would appear that the cat must also exist as a superposition of a live and a dead cat until somebody opens the box and observes it. After all, the cat’s life depends on the state of the atom, and the state of the atom has not yet been decided.

Yet nobody really believes that a cat can be simultaneously dead and alive. There is a profound difference between fundamental particles, such as atoms, which do weird quantum stuff (existing in two states at once, occupying two positions at once, tunnelling through impenetrable barriers etc) and familiar classical objects, such as cats, that apparently do none of these things. Why don’t they? Simply put, because the weird quantum stuff is very fragile.

Scientists have observed how quantum superpositions build up in a helium atom within femtoseconds. Just like in the famous double-slit experiment, there are two ways to reach the final outcome.

What would happen if an electric current no longer flowed, but trickled instead? This was the question investigated by researchers working with Christian Ast at the Max Planck Institute for Solid State Research. Their investigation involved cooling their scanning tunnelling microscope down to a fifteen thousandth of a degree above absolute zero. At these extremely low temperatures, the electrons reveal their quantum nature. The electric current is therefore a granular medium, consisting of individual particles. The electrons trickle through a conductor like grains of sand in an hourglass, a phenomenon that can be explained with the aid of quantum electrodynamics.

Flowing water from a tap feels like a homogeneous medium – it is impossible to distinguish between the individual water molecules. Exactly the same thing is true about electric current. So many electrons flow in a conventional cable that the current appears to be homogeneous. Although it is not possible to distinguish individual electrons, quantum mechanics says they should exist. So how do they behave? Under which conditions does the current not flow like water through a tap, but rather trickles like sand in an hourglass?

For the first time ever, laser physicists have recorded an internal atomic event with an accuracy of a trillionth of a billionth of a second.

When light strikes electrons in atoms, their states can change unimaginably quickly. Laser physicists at LMU Munich and the Max Planck Institute of Quantum Optics (MPQ) have now measured the duration of such a phenomenon – namely that of photoionization, in which an electron exits a helium atom after excitation by light – for the first time with zeptosecond precision. A zeptosecond is a trillionth of a billionth of a a second (10−21 s). This is the first absolute determination of the timescale of photoionization, and the degree of precision achieved is unprecedented for a direct measurement of the interaction of light and matter.

When a light particle (photon) interacts with the two electrons in a helium atom, the changes take place not only on an ultra-short timescale, but quantum mechanics also comes into play. Its rules dictate that either the entire energy of the photon is absorbed by one of the electrons, or the energy is distributed between them. Regardless of the mode of energy transfer, one electron is ejected from the helium atom. This process is called photoemission, or the photoelectric effect, and was discovered by Albert Einstein at the beginning of the last century. In order to observe what occurs, you need a camera with an incredibly fast shutter speed: The whole process, from the point at which the photon interacts with the electrons to the instant when one of the electrons leaves the atom, takes between 5 and 15 attoseconds (1 as is 10–18 seconds) as physicists have worked out in recent years.

What would happen if an electric current no longer flowed, but trickled instead? This was the question investigated by researchers working with Christian Ast at the Max Planck Institute for Solid State Research. Their investigation involved cooling their scanning tunnelling microscope down to a fifteen thousandth of a degree above absolute zero. At these extremely low temperatures, the electrons reveal their quantum nature. The electric current is therefore a granular medium, consisting of individual particles. The electrons trickle through a conductor like grains of sand in an hourglass, a phenomenon that can be explained with the aid of quantum electrodynamics.

Flowing water from a tap feels like a homogeneous medium — it is impossible to distinguish between the individual water molecules. Exactly the same thing is true about electric current. So many electrons flow in a conventional cable that the current appears to be homogeneous. Although it is not possible to distinguish individual electrons, quantum mechanics says they should exist. So how do they behave? Under which conditions does the current not flow like water through a tap, but rather trickles like sand in an hourglass?

The hourglass analogy is very appropriate for the scanning tunnelling microscope, where a thin, pointed tip scans across the surface of a sample without actually touching it. A tiny current flows nevertheless, as there is a slight probability that electrons “tunnel” from the pointed tip into the sample. This tunnelling current is an exponential function of the separation, which is why the pointed tip is located only a few Ångström (a ten millionth of a millimetre) above the sample.