Unveiling the Invisible: Scientists Portray the Shape of a Single Photon for the First Time

It can sound odd to suggest that a photon has a “shape.” Usually we think of photons as point particles, the smallest possible distinct, and non-divisible quantity. Which raises the question of whether there even is a shape, or if there is an internal structure to a photon, how that internal structure would manifest itself.

However, in quantum mechanics we describe particles using a wave function, which tells us how likely we are to find a particle in a specific location, or associated with a specific internal state.

The “shape” of a photon, in this way, does not refer to a physical outer outline, rather it is the three-dimensional intensity distribution, which we could call the photon intensity map. Conceptually, this represents the conceivable probabilities where you are likely to be able to find the photon at a given time following emission. Bright areas on this map represent higher probabilities you will detect a photon in this region, which has coherent spatial ordering of intensity distributions that represents the shape of the photon.

This “cloud” of quantum probabilities is ever changing, which is a product of environmental factors and the characteristics of the atom itself. This helps realize the genesis of quantum mechanics- uncertainty, superposition, and statistical behaviour.

How Did They Visualise the Shape of the Photon? THE QUANTUM PUZZLE IS SOLVED

You will first notice that it is pretty stunning. However, that is not what the researchers were intending to achieve with their study. The scientists wanted to examine a much larger question on how the local environment influences the way in which photons are emitted from an atom or a molecule.

Traditionally, the quantum mechanics of photon emission has placed a narrow emphasis on the extreme simplification of a perfect vacuum condition, where a single atom or molecule is assumed to emit light in absolute isolation from everything else. In contrast, in real life, atoms are not found in isolation, surrounded only by vacuum. Instead, they are surrounded by materials, surfaces, and particles that can greatly influence the behaviour of light.

As a way of understanding these interactions, the researchers built a new theoretical model based on quantum field theory, incorporating a silicon nanoparticle into the system. This nanoparticle had considerable effects:

• It significantly enhanced the probability of emission of a photon from the atom by many factors, up to thousands of times!
• It created the possibility of a photon being re-absorbed and emitted by the same atom many times, which is typically prevented in free space.

Using a powerful mathematical approach known as complex analysis, the researchers were able to practicalize these difficult infinite sets of interactions between the nanoparticle, and a continuous spectrum of light, and turn them into a discrete system. They offered many complex modes in place of an infinite set of possible light states – each one contributing to the behaviour of the emitted photon.

The outcome? A high-resolution quantum simulation of the photon’s intensity distribution just after it is emitted. What the researchers produced is essentially a ‘map’ of the quantum wavefunction of a single photon, providing a clear picture of the probabilistic “shape” of a photon.

The Physics of the Wave Function: Mechanics in Action

At the center of this research is one of the deepest and most mysterious ideas in physics, the notion of the wave function. The wave function contains everything that can be known about a quantum system and is not something that we can directly observe, although it helps provide a mathematical basis to predict all possible outcomes to an interaction or measurement. 

For photons, the wave function will tell where we are likely to observe the particle, at least if we measure the particle flat out, it collapses the wave function giving a definitive answer, destroying the rest of the information.

The innovation of this work is in that not only did they observe the photon once and destroy all other information in the measurement process, they measured it many times theoretically, and simulated their work to then reconstruct the full intensity distribution—not the intensity distribution from one measurement, of just where you would see or detect the photon, they essentially reconstructed the full three-dimensional “terrain” of the photon wave function. Given that they have an accurate 3D view of the photon wave function, they have no precedent in viewing the quantum world so effectively.