The iconic quantum double slit experiment, which shows how matter can behave like waves that exhibit interference and superposition, was demonstrated for the first time with single molecules as a slit.
Richard Feynman once said that the double-slit experiment reveals the central riddle of quantum mechanics and confronts us with “the paradoxes and mysteries and peculiarities of nature”.
Richard Zare, Nandini Mukherjee and their colleagues from Stanford University, USA, have now shown that when helium atoms collide with deuterium molecules (D2) In the case of the quantum superposition of states, the scattering can take two different paths, which mutually interfere. The researchers revealed the interference by looking at its effects on the scattered D. regard2 Molecules that lose rotational energy when they collide.
Zare and colleagues generated an ultra-cold molecular beam from a mixture of D2 and helium, in which collisions occur at an effective temperature of 1K (-272 ° C). With two sets of polarized laser pulses, they elicited the D2 Molecules in a certain rotational and vibrational energy state, but in two different orientations in relation to the laboratory frame of reference, at right angles to each other. These act as the two “slits” that the helium atoms scatter.
It is crucial that the researchers also do the D2 Molecules in a coherent superposition of both orientations – that is, the wave functions of the two superimposed states remain synchronous with each other. When helium atoms scatter on the superimposed molecules, the atoms “feel” both orientations at the same time.
In the classic double slit experiment, the quantum particles cross both slits in a superposition of orbits. In this case, on the other hand, it is as if there were only a single gap, which itself lies in a superposition of positions.
The collisions cause the D2 Molecules for this vibration level fall back into the rotational ground state, which Zare and colleagues then selectively ionize and analyze. The experimental measurements agreed very well with this prediction.
Physical chemist David Clary of the University of Oxford, UK, says the work will advance the understanding of how molecular scattering can switch molecules between different quantized rotational states. “It has long been a goal to set up an experiment that can measure such transitions in all initial and final states of the quanta,” he says. The Stanford team has “made progress in this direction” by using quantum interference to reveal the various rotational states, he adds.
Quantum interference effects in molecular scattering have been observed before. In an earlier experiment, interference for photoelectrons emitted by an oxygen molecule was observed because each electron could interact with one of the two atomic nuclei. But what sets her experiment apart, says Mukherjee, is that “we have full control over the ‘slots'”. They are not two atoms in a fixed relationship as in a diatomic molecule, but are created by superimposing the molecular orientations and can thus be adjusted as required – similar to changing the gap width or the distance or blocking one of them.
Clary hopes that this approach could ultimately lead to the “Holy Grail” of quantum control with an experiment that can select all the starting and ending quantum states of the scattered molecules. Mukherjee says the approach will also work for bimolecular gas-phase chemical reactions. In this case, she says, “the product of reactive chemical collisions could be controlled with quantum precision”.
The researchers believe their results also examine fundamental aspects of quantum behavior. “We describe the production of a new type of matter: a molecule that is produced in a coherent superposition of states with a known and controllable phase that relates the superimposed states,” says Zare. They hope that their method could be used to study decoherence, where quantum phenomena become classical results through interactions with the environment.