The Shadow Existence Of Photons

The double-slit experiment – a cornerstone of quantum mechanics revisited in the particle picture.

On the centennial of quantum mechanics, a group of physicists is challenging one of its central paradigms: wave-particle duality. Among them is Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics in Garching. At the heart of their work lies the famous double-slit experiment, long considered a key demonstration of the idea that quantum objects inherently possess both particle and wave characteristics. But according to a paper published in Physical Review Letters this year, this iconic experiment can be explained using only the particle picture – provided two quantum ingredients are present: entanglement and so-called dark quantum states of photons.

It is one of the most fundamental experiments in physics: the "double-slit experiment" first conducted by the British physicist Thomas Young in 1803. In this setup, light passes through a screen with two narrow slits, forming a pattern of bright and dark fringes on a detection screen placed behind it. This "interference pattern" can be elegantly explained using wave optics: circular wavefronts emanate from each slit, overlapping on the screen. Where wave crests reinforce each other, the pattern appears bright; where crests and troughs cancel out, it becomes dark).

This interference pattern is the hallmark of wave behavior. “Later in the 19th century, when Maxwell formulated his equations for electromagnetism, the wave picture of light became firmly established,” Rempe explains. “But our work shows that nothing in science is ever final.”

The Bohr–Einstein debate
A century ago, just as quantum mechanics was being born, the double-slit experiment took center stage again. This time it served as the basis for a famous debate between Albert Einstein – the master of the thought experiment – and Niels Bohr. Einstein, who had contributed a foundational piece to quantum theory in 1905 with his explanation of the photoelectric effect, had by then become a proponent of the particle picture. Bohr, by contrast, was still defending the wave view in his 1922 Nobel lecture. By the mid-1920s, experiments had been performed that supported both perspectives.

The question at the heart of their debate was: what happens when the intensity of the light source is reduced so drastically that only individual quanta of light – what we now call photons – pass through the slits one at a time? Quantum theory led to a startling conclusion: if a photographic plate replaced the screen behind the slits, each photon would leave a single dot. Over time, as more and more photons passed through, an interference pattern would gradually emerge from what initially appeared to be a random distribution of points. The implication: a single photon behaves both as a localized particle and a wave – it “surfs,” as it were, on its own extended wave, simultaneously passing through both slits and interfering with itself. This was the birth of the wave-particle duality, a core idea of the Copenhagen interpretation of quantum mechanics.

The first experiments with strongly attenuated light sources were already conducted at the beginning of the 20th century, but could only be perfected with the highly sensitive methods of modern physics and the development of true single-photon sources. All experiments seemed to confirm the wave-particle duality – not only for light but also for the waves of more massive quantum objects, such as electrons, atoms, or even molecules. These latter are often called matter waves, to contrast them with light waves made of massless photons.

The which-way question
The debate surrounding the double-slit experiment soon led to a new question: could one, perhaps with some clever trick, determine which of the two slits a photon had passed through? One thing was clear: if one slit is closed, allowing only a single path, the interference pattern on the screen collapses into a diffuse spot in the direction of travel. But could ultra-sensitive detectors be placed at both slits to record the photon’s passage more subtly?

According to the Copenhagen interpretation of quantum mechanics, any such measurement inevitably influences the object being measured. For decades, the prevailing paradigm held that any attempt to determine the photon’s path would destroy the interference pattern: the superposed wave emerging from the two slits would collapse.

Why? Because measuring inevitably entails a transfer of momentum between the detector and the photon. This idea lies at the heart of the so-called Heisenberg microscope, a thought experiment devised by Werner Heisenberg in the late 1920s to illustrate the position–momentum uncertainty principle. It vividly explains why and how the distribution of photons on the screen changes when one tries to measure their path.

Entanglement as the culprit?
In the late 1980s, theorists developed new experimental schemes that cleverly bypassed the momentum-transfer problem. These experiments were designed so that the detectors interacted only extremely weakly with the photons – so weakly that any momentum transfer was excluded. Yet even these minimally invasive measurements, when repeated often enough, yielded statistically significant information about the photon.

Gerhard Rempe’s group at the University of Konstanz managed to perform such an experiment, publishing the results in Nature in 1998. The outcome was striking: even without momentum transfer, the interference information was destroyed. Clearly, some other mechanism must have been responsible for the collapse of the superposed wave.

“I suspected entanglement was the culprit,” Rempe recalls with a smile. He has since returned to the double-slit experiment in his thoughts again and again. This was hardly surprising: for decades, his research has focused on the interaction of single photons and atoms – the very essence of light-matter interaction.

Rempe has also long been at the forefront of developing experimental techniques using optical cavities. One can picture such a cavity as a tiny chamber formed by two nearly perfect mirrors. A single atom is trapped inside this “mirror cabinet” and bombarded with photons, which bounce back and forth thousands of times before interacting with the atom.

This setup allows for experiments that, in principle, closely resemble the double-slit experiment: instead of two slits, two overlapping optical cavities are used, with a single atom positioned at the crossing point as a kind of quantum observer.

Dark States in the Particle Picture
Rempe frequently discussed the double-slit experiment and the nature of photons observed in his ultra-sensitive experiments with his colleague and friend Celso Villas-Boas at the Universidade Federal de São Carlos in Brazil. Villas-Boas, a theoretical physicist, is the first author of the new Physical Review Letters paper, which emerged from years of such discussions.

The team’s conclusion, simplified, is as follows: photons can exist in a “perfectly dark” quantum state – a fact Rempe already knew from his work with optical cavities.

In such a dark state, photons cannot interact with an atom – for instance, one located on the detection screen of a double-slit experiment. This is not a fundamentally new insight in quantum optics. For this reason, Rempe is quick to point out that the discovery does not imply the existence of a new type of “dark photon,” as was sometimes misinterpreted in popular accounts of the study: “We haven’t invented a new property of photons!” he stresses.

Beyond the wave picture
The otherwise rather complex representation of the paper can be distilled to a simple key message in the context of the double-slit experiment:

• The dark fringes in the interference pattern do not arise because waves cancel each other out.
• Instead, photons striking the screen at those positions are in the dark quantum state and thus cannot interact with the atoms in the screen.
• At the bright fringes, photons are in a bright quantum state.

“The photons are present everywhere,” Rempe says. “But at the dark fringes they are in the dark state and there cannot interact with atoms as observers!”

This leads to an essential follow-up question: “What mechanism ensures that photons are in the dark state at some positions and in the bright state at others?” Rempe’s answer is clear: entanglement.

Entanglement is perhaps the strangest feature of the quantum world. When two quantum particles are entangled, they share a joint state with respect to a given property. Measuring one particle instantly fixes the state of the other – regardless of the distance separating them. Einstein famously called this “spooky action at a distance” and saw it as evidence that quantum mechanics was incomplete.

Today, however, entanglement is a cornerstone of quantum engineering, underpinning technologies such as quantum computers and the secure transmission of quantum keys.

The double-slit in the particle picture
When the slits are labeled “1” and “2,” quantum mechanics dictates a superposition of the two possible photon paths. At the position of an atom on the screen, a superposition arises of the two states:

• “Photon took path 1 and not path 2”, and
• “Photon took path 2 and not path 1.”

Without a measurement, there is no way to know which path the photon might have taken; quantum mechanically, both states are equally valid. By the laws of quantum physics, these possibilities must be considered simultaneously and added together. The result is an entangled state – even though there is only a single photon, because it can, in principle, take both paths.

Now, whether a bright or dark entangled state arises at a given point on the screen depends on the relative path lengths from the two slits. The Physical Review Letters paper shows that this can be rigorously calculated using the mathematical machinery of quantum mechanics – without invoking the wave picture.

The upshot is starkly different from the classical wave description: at the dark fringes of the interference pattern, photons are indeed present; they are simply “invisible” to the atoms in the screen because they are in the dark state.

This new perspective also explains why even the most delicate attempts to determine the photon’s path must inevitably fail: the moment detectors are placed at the two slits, no matter how weakly they interact with the photons, they alter the entangled state. As a result, the interference vanishes.

The concept of dark states also resolves another apparent contradiction from Rempe’s cavity experiments. A photon trapped between two mirrors can be described in two ways, just like in the double-slit experiment:

• As a standing wave, or
• As a particle bouncing back and forth.

“But these two pictures don’t seem to fit together,” Rempe explains. “At the nodes of the standing wave it is dark, so the photon shouldn’t be there at all. But in the particle picture, the photon must pass through these regions.” The solution, according to Rempe, is simple: “At those positions, the photon is in the dark state!”

A New Outlook on the Cosmos
“This is a new way of looking at the double-slit experiment,” Rempe emphasizes. “But that doesn’t mean the classical wave picture is wrong – both are ultimately just more or less intuitive pictures.”

In fact, it is the mathematical formalism of quantum mechanics that produces descriptions of nature extending far beyond what we can intuitively grasp. And as Rempe points out, there are already quantum information experiments and applications where the wave picture simply falls short.

Philosophically, the work raises the question of whether the concept of dark states might also be applicable to matter waves. Quantum mechanics certainly suggests as much, since it makes no fundamental distinction between photons and massive particles in the context of interference. Double-slit experiments using matter waves have been performed since the 1990s, even with relatively large molecules instead of photons. In all such experiments, the familiar interference pattern appears as single molecules traverse the slits. Could the notion of dark states also apply there? That remains to be explored in future studies.

Since the paper’s publication, some have even speculated whether the two great unresolved mysteries of astrophysics – dark matter and dark energy – might somehow be explained by photons in dark states. There is even a proposal to adapt one of the experiments originally designed to search for dark matter to hunt for cosmic dark photons. Yet, as Rempe points out, such ideas remain highly speculative:

“It’s not clear how these photons could remain in the dark state for all the atoms in the universe,” he muses. “Just consider the movements of celestial bodies, galaxies, or clouds of dust and gas relative to one another.” In the double-slit experiment, after all, whether a photon is visible or invisible depends on the position of the atoms in the detection screen.

“Still,” Rempe adds, “when Max Planck introduced his quantum of action, he couldn’t foresee the consequences either.” The history of physics indeed is proof that nature is full of surprises.

(This article was created with the support of science journalist Roland Wengenmayr.)