Snapshot from the quantum world

Scientists have presented a new spectroscopy method for determining the electron spin in molecules. In particular, you can display the temporal course of the spins under the influence of a magnetic field – the “quantum pulse” of the electrons, so to speak. The researchers use two time-shifted laser pulses for this, as reported in the specialist journal “Science”. With the help of this technology, they hope to gain new knowledge about spin oscillation in special chemical reactions, but also in organic light-emitting diodes or quantum computers.

Electrons usually occupy the energetic orbitals in a molecule in pairs. The intrinsic angular momentum of the electrons, their so-called spin, is of crucial importance. According to the Pauli principle of quantum theory, two electrons can only move on the same path if their spin is antiparallel: If one electron rotates to the right, the other must rotate to the left.

Separately, much more flexible

Such an electron pair can, however, be separated by supplying energy, for example in the form of light. One of the two electrons is lifted to a higher energy level and can then jump to a free path of a neighboring molecule. After this so-called photo-induced electron transfer, the spin of the original electron partners is no longer restricted by the Pauli principle, which is why it can change independently.

This behavior can be observed, among other things, with radicals – molecules that each contain an odd number of electrons. If the spins of the individual electrons of two radicals are aligned parallel, one speaks of a triplet state of the radical pair. If they are aligned antiparallel, the electrons are in the singlet state. Due to the free development of the two spins, the radical pair oscillates back and forth between the singlet and triplet state.

Determination via laser pulses

How this course develops under certain influences has so far been difficult to understand, since the state of the electron pair was previously not directly traceable optically and could only be evaluated using the chemical end product. A research team led by Ulrich Steiner from the University of Konstanz has now presented a method with which a temporal course of the singlet / triplet settings of a radical pair can be determined optically.

After the pump pulse, the radical pairs switch between the singlet and triplet state. With the help of the push pulse, the ratio can be read out at a specific point in time. © Christoph Lambert / University of Würzburg

With the pump-push technology used for this, two time-shifted laser pulses are used. The first, so-called pump laser pulse, triggers the transfer of electrons between two molecules. The resulting radical pair is then given some time before the second laser is sent afterwards.

“This second, so-called push laser pulse, transfers the electron back again, forcing the system to immediately make the decision between triplet or singlet product. Normally, the radical pair would allow itself several spin oscillation periods for this, ”explains Steiner. In this way, a snapshot of the spin state of the radical pair can be taken, as the electrons leave behind a certain signature depending on their state. If the process is repeated with a different time offset, a temporal course of the spin development can ultimately also be displayed.

Quantum computers and migratory birds

This course, in which the radical pairs oscillate back and forth between the singlet and the triplet state, is of interest in various scientific fields of application. These interactions occur in photoactive proteins such as plant cytochromes, but also in organic light-emitting diodes excited by a magnetic field. This is due to the fact that the spin development of the electrons can be specifically influenced by a magnetic field.

These quantum spin oscillations could also play a role in future quantum computers. “Our radical pair can serve as a model for qubits as they exist as elements in quantum computers,” says Steiner. Scientists also assume that the sense of direction of migratory birds also functions via the influence of the earth’s magnetic field on the spin state of radical pairs. The pump-push technology could also provide further information here. (Science, 2021; doi: 10.1126/science.abl4254)

Source: University of Konstanz

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