Double slit interference. Photons arrive
everywhere on the screen, but in the bright/dark regions in a bright/dark
quantum state, respectively, that can/cannot excite the atom which is used to
monitor the interference pattern. Credit: Villas-Boas et al.
Classical
physics theories suggest that when two or more electromagnetic waves interfere
destructively (i.e., with their electric fields canceling each other out), they
cannot interact with matter. In contrast, quantum mechanics theory suggests
that light particles continue interacting with other matter even when their
average electric field is equal to zero.
Researchers from Federal University of
São Carlos, ETH Zurich and the Max Planck Institute of Quantum Optics recently
carried out a study exploring this contrast between classical and quantum
mechanics theories through the lens of quantum optics, the field of study
exploring interactions between light and matter at a quantum level. Their
paper, published in Physical Review Letters, proposes that
classical interference arises from specific two-mode binomial states, which are
collective bright and dark entangled states of light.
"After a long-standing and fruitful
collaboration on cavity QED topics with the first author, Celso J. Villas-Boas,
he and I exchanged many insightful ideas concerning the reported topic over a
period of several years or so," Gerhard Rempe, senior author of the paper,
told Phys.org.
"Inspired by my quantum information
experiments with one atom coupled to two optical cavities and single photons
flying into and out of the cavities, he was wondering what would happen if the
atom were exposed to two light fields which are both in superpositions of zero
and one photon. He described the situation in terms of bright and dark states
of light, a concept which relates to Dicke's bright and dark states of atoms
from the 1950s."
In the scenario considered by
Villas-Boas, the two (or more) atoms described by Dicke in his work are
replaced by two (or more) optical modes containing zero or one photon. Within
this context, the terms "bright" and "dark" entail that a
ground-state atom can be excited or cannot be excited (due to destructively
interfering excitation amplitudes), respectively.
"Responding to this picture, I
compared the situation with that of two coherent states of light that interfere
destructively and therefore cannot excite the atom at the position of a field
node (where the intensity is zero)," explained Rempe. "My comparison
between quantum and classical light fields literally forced us to think about
the relationship between classical interference with intensity maxima/minima
and quantum-mechanical bright/dark states of light."
Building on the ideas discussed with
Rempe, Villas-Boas theoretically analyzed the double-slit experiment using both single-photon (quantum) and coherent
(quasi-classical) states. His analyses showed that experimentally observed
interference patterns, maxima and minima, could in fact be explained in terms
of bright (detectable) and dark (undetectable) states of light.
"Many partly controversial
discussions between the two of us then led to a totally new interference
picture which makes use of particles instead of fields," said Rempe.
"For example, a classical standing-wave light field (composed of two counter-propagating
light waves) with periodic intensity maxima and minima is now described as an
alternating succession of bright and dark states of photons,
respectively."
A notable feature of dark states is that
they contain photons. The new theoretical framework outlined by the researchers
suggests that these photons are present at the nodes of an interference
pattern. As the state they are associated with is dark, however, these photons
were hypothesized to be unobservable using conventional experimental methods.
"This
is a highly counterintuitive picture which initially made us doubt that our
description can be correct," explained Rempe. "Support came from an
experiment that I conducted in my group in the late 1990s, which concerned the
role of a which-path observer in double-slit experiments.
"As had been controversially
discussed at that time, which-path observation (of a particle through the
double slit) can be so gentle as to not exert a momentum kick on the
interfering particle. This raises the so far open question of how the observation
can steer the particle from a bright into a dark region in order to wash out
the interference pattern."
The new theoretical approach outlined by
the researchers provides a quantum optics-based alternative explanation for
classical interference. Specifically, it suggests that which-path detection
changes the state in dark regions to a bright state. Namely, without
necessarily altering the trajectory of a particle, a which-path observer can
alter the state in such a way that the particle becomes detectable.
"In my humble opinion, our
description is meaningful as it provides a quantum picture (with particles) of
classical interference (with waves): maxima and minima result from entangled
bright (that couple) and dark (that do not couple) particle states," said
Rempe. "I would say that our picture resolves aspects (such as which-path
detection) of an old debate involving some of the greatest minds, such as
Newton (particles), Maxwell (waves), Einstein (particles), Millikan (waves),
and many others."
The recent study by Rempe, Villas-Boas
and their colleagues establishes a new view of the classical interference of
waves, which describes the maxima and minima intensity in terms of entangled
bright/dark quantum superpositions of particles. While researchers already knew
that Maxwell's classical equations fail to describe many quantum optics
phenomena, the team's new theory offers a more general description of
interference.
"In some sense, we showed that
Maxwell's equations are a limiting case of quantum mechanics," said Rempe.
"This is achieved by incorporating two effects into the model. First, the
detector monitoring the interference pattern and its coupling to light is
treated fully quantum mechanically. Second, the interference is described as
resulting from entangled particle states.
"These states are found to be
bright or dark (or anything in between), depending on whether they couple to
the detector or not, respectively. A characteristic feature of the dark state
is that it contains particles, but that these particles remain unobservable for
the chosen detector,"
The recent work by this team of
researchers could soon inspire further studies aimed at further developing
their newly introduced theory, which describes interference phenomena in terms
of particles instead of waves. Their proposed framework could advance the
present understanding of classical interference, while also potentially guiding future experimental
efforts.
"In this study, we specifically studied the case of light particles observed by a two-level atom in its ground state," added Rempe. "I think it would also be interesting to explore the case of material particles observed by any other suitable detector such as an ionization device or just deposition on a surface."
by Ingrid Fadelli , Phys.org
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