Manipulating weakly bound helium dimers with ultrafast laser pulses reveals their quantum behaviour. This method opens a route towards studying the low-energy dynamics of other exotic and fragile quantum states.
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Her Majesty’s most famous secret agent (whose latest movie, like everything else, has been delayed by the COVID-19 pandemic) will only accept his martini shaken, not stirred. But some physicists prefer to have their helium dimers gently swirled, not shaken — and the result affords them exquisite experimental control worthy of our movie superhero. Writing in Nature Physics, Maksim Kunitski and co-workers report that they have used short laser pulses to gently manipulate a fragile helium dimer, bound by only 150 neV (corresponding to a temperature of 1.7 mK), in such a way that allows them to control, observe and study its delicate quantum behaviour by wave packet interferometry1.
The helium dimer, consisting of two helium atoms held together by only weak van der Waals forces, is quite a popular target for studying quantum effects in simple matter. Because of its shallow binding potential, which can only support a single vibrational and rotational state, the helium dimer has the longest bond length of all diatomic molecules in their ground state, with a separation of 5,200 pm (or 52 Å) between the two atoms — 70 times larger than the bond length of a hydrogen molecule. The wave function of a helium dimer extends far beyond the bound range of the potential well and deep into the classically forbidden tunnelling region, creating a situation known as a ‘quantum halo’2. Quantum halos are studied across many areas from nuclear physics to quantum optics and are characterized by an exponentially decreasing tunnelling wave function with non-zero probability density that reaches out to infinity. In the case of the helium dimer, 80% of the probability distribution resides in this quantum halo.
In their experiment, Kunitski and colleagues applied a short (310 fs) near-infrared laser pulse to an ensemble of helium dimers prepared as a cold molecular beam, dissociating the dimers without ionizing them. The laser pulse imparted an angular momentum onto the initially spatially confined dissociative wave packet, essentially setting each dissociating dimer in rotation (Fig. 1a). A second, more intense laser pulse was used to doubly ionize and ‘Coulomb explode’ the dissociating dimer. That is, two electrons were removed, causing the two resulting singly charged helium ions to rapidly accelerate because of the internal Coulomb repulsion, as illustrated in Fig. 1b.
The two ions were then detected in coincidence, meaning that by working at very low count rates and making use of the momentum conservation that governs such a half-collision process, the measurement was able to unambiguously identify ion pairs that resulted from the same dimer. Both the orientation of the internuclear axis and the internuclear separation at the time of arrival of the second laser pulse was retrieved from the ions’ momentum vectors using a method called ‘Coulomb explosion imaging’3. This method takes advantage of the fact that the kinetic energy of the detected ions is almost exclusively due to their Coulomb energy at the moment they were created, and thus proportional to the inverse of their internuclear distance at the moment the second laser pulse arrived.
The angular momentum transferred to the dimer by the first laser pulse resulted in a partial rotation alignment, determined by the angle between the internuclear axis and the polarization direction of the first laser pulse4. This alignment only affected the dissociating part of the wave function, leaving the rest to maintain its spherical symmetry. As a function of the time delay between two laser pulses, the experiment thus resulted in a movie of an ‘alignment wave’ that spread out to larger and larger distances, while the inner part returned to its initial spherical symmetry. Furthermore, the interference between the dissociative wave packet and the highly delocalized ‘quantum halo’ wave function of the helium dimer in its ground state were used to determine the phase and density of the dissociating wave packet, which, at large internuclear distances, describes the time evolution of a propagating (quasi-)free particle.
The study combines concepts of low-energy quantum physics and strong-field and ultrafast laser physics, and builds a bridge between these two traditionally separate areas of atomic, molecular and optical physics. In doing so, it provides a new tool to image and characterize ultrafast field-induced wave packet dynamics in weakly bound, low-energy, few-body quantum systems by wave packet interferometry and tomography, and to experimentally observe, for example, the quantum phase of a propagating free-particle wave packet. These capabilities open the door to the investigation and manipulation of other low-energy systems, such as the helium trimer. They also afford the possibility of observing and characterizing the dynamics of exotic quantum systems, such as the birth of an Efimov state5,6, that have so far eluded detailed experimental realization.
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The author declares no competing interests.
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Rolles, D. Gently stirred not shaken. Nat. Phys. 17, 165–166 (2021). https://doi.org/10.1038/s41567-020-01119-6