The iconic quantum double-slit experiment, which reveals how matter can
behave like waves that displays interference and superposition, has for the
first time been demonstrated with individual molecules as the slits.
Richard Feynman once said that the double-slit experiment reveals the
central puzzles of quantum mechanics, putting us ‘up against the paradoxes
and mysteries and peculiarities of nature’.
Richard Zare, Nandini Mukherjee and their co-workers at Stanford University,
US, have now shown that when helium atoms collide with deuterium molecules
(D2) in quantum superposition of states, the scattering can take
two different paths that interfere with one another. The researchers reveal
the interference by looking at its effects on the scattered
D2 molecules, which lose rotational energy in the collision.
Zare and colleagues created an ultracold molecular beam of a mixture of
D2 and helium in which collisions happen at an effective
temperature of 1K (–272°C). Using two sets of polarised laser pulses, they
coaxed the D2 molecules into a specific rotational and
vibrational energy state but in two different orientations with respect to
the laboratory frame of reference, at right angles to one another. These act
as the two ‘slits’ that scatter the helium atoms.
Crucially, the researchers can also prepare the D2 molecules
in a coherent superposition of both orientations – that is, with the
wavefunctions of the two superposed states remaining in synchrony with one
another. When helium atoms scatter off the superposed molecules, the atoms
‘feel’ both orientations at once.
In the classic double-slit experiment, the quantum particles each pass
through both slits in a superposition of trajectories. In this case, in
contrast, it’s as if there is just 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 then selectively ionise and analyse. The experimental
measurements matched this prediction closely.
Physical chemist David Clary of the University of Oxford, UK, says that the
work advances the understanding of how molecular scattering can switch
molecules between different quantised rotational states. ‘It has long been a
goal to build an experiment that can measure such transitions in all the
initial and final quantum states,’ he says. The Stanford team has ‘made
progress in this direction’ by using quantum interference to reveal the
different rotational states, he adds.
Quantum interference effects in molecular scattering have been seen before.
In one earlier experiment, interference was observed for photoelectrons
emitted from an oxygen molecule because each electron could interact with
either of the two atomic nuclei. But what makes their experiment different,
says Mukherjee is that ‘we have full control of the “slits”’. They are not
two atoms in a fixed relationship, as in a diatomic molecule, but are
created by superposing the molecular orientations, and so can be adjusted at
will – rather like altering the slit width or separation, or blocking one of
them off.
Clary hopes that this approach might ultimately lead to the ‘holy grail’ of
quantum control with an experiment where all the initial and final quantum
states of the scattered molecules can be selected. Mukherjee says that the
approach will also work for bimolecular gas-phase chemical reactions. In
that case, she says, ‘you could control the product of reactive chemical
collisions’ with quantum precision.
The researchers believe their results are also probing fundamental aspects
of quantum behaviour. ‘We describe the preparation of a new type of matter:
a molecule prepared in a coherent superposition of states with a known and
controllable phase relating the superposed states,’ says Zare. They hope
their method might be used to study decoherence, by which quantum phenomena
turn into classical outcomes through interactions with the environment.
Reference:
H Zhou et al, Quantum mechanical double slit for molecular scattering,
Science, 2021,
DOI: 10.1126/science.abl4143
Tags:
Physics