A Quantum Double-slit Experiment Run with Molecules for the First time
Richard Feynman once stated that the double-slit experiment reveals the central challenges of quantum mechanics, putting us ”up against the and peculiarities of nature and paradoxes and mysteries”.
Nandini Mukherjee, Richard Zare, and their co-workers at Stanford University, United States, have currently revealed that when helium (He) atoms collide with deuterium molecules (D2) in a quantum superposition of states, the spreading can take two distinct courses that interfere with one another. The researchers reveal the disturbance by looking at its results on the spread of D2 molecules, which lose rotational energy in the collision.
Zare and colleagues developed an ultracold molecular beam of a mixture of D2 and helium in which collisions occur at an effective temperature of 1K (272 ° C). They coaxed the D2 molecules into a particular rotational and vibrational energy state but in two distinct orientations relative to the laboratory frame of reference, at right angles to one another, using two sets of polarised laser pulses. These act as both ‘slits’ that spread the helium atoms.
Crucially, the researchers can likewise prepare the D2 molecules in a consistent superposition of both orientations, meaning with the wavefunctions of the two superposed states remaining in synchrony with one another. When helium atoms spread off the superposed molecules, the atoms ‘feel’ both orientations at once.
The quantum particles in the classic double-slit experiment each travel through both slits in a superposition of trajectories. In this instance, in contrast, it is as if there is only a single slit that is itself in a superposition of positions.
The collisions cause the D2 molecules to fall back to the rotational ground state for this vibrational level, which Zare and colleagues, afterward selectively ionize and evaluate. The experimental measurements matched this prediction approximately.
Physical chemist David Clary of the University of Oxford, UK, states that the work develops the understanding of how molecular spreading can switch molecules between different quantized rotational states.
It has long been a goal to develop an experiment that can measure such transitions in all the first and final quantum states,’ he states. Progress in this direction has been made by the Stanford team has made by utilizing quantum interference to reveal the distinct rotational states, he adds.
Before, quantum interference effects in molecular spreading have been seen. In one previous experiment, interference was observed for photoelectrons produced from an oxygen molecule due to the fact that each electron could communicate with either of the two atomic nuclei. However, what makes their experiment different, states Mukherjee is that ”we have full control of the “slits”.
As in a diatomic molecule, they are not two atoms in a fixed relationship; however, they are developed by superposing the molecular orientations and so can be adjusted at will, instead of modifying the slit width or separation or blocking one of them off.
Clary hopes this method might finally cause the ‘holy grail’ of quantum control with an experiment where all the first and final quantum states of the spread molecules can be picked. Mukherjee states that the method will also work for bimolecular gas-phase chemical reactions. In that case, she states: ”you could control the product of reactive chemical collisions” with quantum precision.
The researchers think their results are also probing fundamental aspects of quantum behavior. ”We depict the preparation of a new kind of matter: a molecule prepared in a coherent superposition of states with a known and controllable phase relating the superposed states,’ states Zare. They hope their method might be utilized to research decoherence, by which quantum phenomena turn into classical outcomes through environmental interactions.
Reference:
H Zhou et al, Quantum mechanical double slit for molecular scattering, Science, 2021, DOI: 10.1126/science.abl4143
