S C

IE N

T IF

IC H

IG H

LI G

H T

S M

A T

T E

R A

T E

X T

R E

M E

S

2 7 I H I G H L I G H T S 2 0 2 1

X-ray double flashes control atomic nuclei

Using nuclear resonance beamline ID18, nuclear excitations have been coherently controlled with X-ray light for the first time, achieving a temporal control stability of a few zeptoseconds (10-21 seconds). This forms the basis for new experimental approaches exploiting the control of nuclear dynamics, which could lead to more precise time standards.

In the field of atomic dynamics, far-reaching control is the key to many applications, such as the control of chemical reactions or the development of new, more precise time standards. Modern experiments on quantum dynamics can control the quantum processes of electrons in atoms to a large extent by means of laser fields. However, the inner life of atomic nuclei usually plays no role because their characteristic energy, time and length scales are so extreme that they are practically unaffected by the laser fields.

Fresh approaches breathe new life into nuclear physics by exploiting this insensitivity to external disturbances and using the extreme scales of the atomic nuclei for particularly precise measurements. Thus, atomic nuclei can respond to X-rays with an extremely well-defined energy by exciting individual nucleons similar to electrons in the atomic shell. These transitions can be used as clockwork for precise nuclear clocks, but this requires the measurement of nuclear properties with the highest precision.

The quantum dynamics of atomic nuclei were measured at the nuclear resonance beamline ID18. Suitably shaped X-ray pulses were used to control them, with a previously unattained temporal stability of a few zeptoseconds a factor of 100 better than anything previously achieved. This opens the toolbox of coherent control, which has been successfully established in optical spectroscopy, to atomic nuclei providing completely new possibilities and perspectives.

Coherent control uses the wave properties of matter to control quantum processes via electromagnetic fields, e.g., laser pulses. In addition to the frequency or wavelength, each wave phenomenon is characterised by the amplitude (wave height) and phase (temporal position of wave crests and troughs). A simple analogy is the control of an oscillating swing by periodic, wave-like pushing. For this, the exact timing (phase) of the push relative to the swing motion has to be controlled. If the oncoming swing is pushed, it is decelerated. If, on the other hand, it is moving away, its deflection is increased by the push. Analogously, the quantum-mechanical properties of matter can be controlled via correspondingly precise steering of the applied laser fields.

Two samples enriched with the iron isotope 57Fe were irradiated with short X-ray pulses (Figure 14). In the first sample, a controllable double X-ray pulse was generated, and then used to control the dynamics of the nuclei in the second sample. The investigated nuclear excitations which de-excited again by X-ray emission were characterised by a very high sharpness in energy: Mössbauer transitions. The first pulse excited a quantum-mechanical dynamic in the nucleus, analogous to the oscillating swing. The second pulse changed this dynamic, depending on the relative phase of the two X-ray pulses. For example, if the wave of the second pulse hit the second sample in phase with the nuclear dynamics, the nuclei were further excited. By varying the relative phase, it was possible to switch between further excitation of the nuclei and de-excitation of the nuclei, and thus control the quantum-mechanical state of the nuclei. This could be reconstructed from the measured interference structures of the X-ray radiation behind the second sample (Figure 15).

In the past decades, there has been great progress and success in the coherent control of atoms and molecules, with a temporal precision of light down to the attosecond range (a billionth of a billionth of a second), which corresponds to the natural timescale of electrons in atoms. In recent years, the availability of novel radiation sources

Fig. 14: Schematic set-up of the experiment. The double pulse generated in the first sample induces quantum dynamics in the atomic nuclei of the second sample, which can be controlled by delaying a part of the double pulse.