The Double-Slit Experiment is a cornerstone of quantum physics that demonstrates that light is wave light by producing an interference pattern of alternating bright and dark bands on a screen. There is an odd effect that even when single particles pass through one at a time there is still an interference pattern, but it appear that interference pattern is destroyed when observed.
Philosophically, the experiment literally challenges reality and determinism. It raises questions about the nature of observation, measurement, and existence, suggesting that reality is not fixed until it is observed. Its results provoke reflection on the limits of human intuition and understanding, making it a symbol not just of quantum mechanics but of the deep mysteries of the universe.
How Quantum Noise Fluctuations Influences Single-Photon Detection Events
Fixing the Double-Slit Experiment
This experiment has been consistently discussed for over 200 years as it demonstrates the wave–particle duality of light and matter. If you’re unfamiliar with this experiment, I recommend gaining some background knowledge before reading further.
The most intriguing aspect of the experiment is that, when the light is reduced to its smallest units—individual photons—so that they can’t interfere with one another, dots appear at seemingly random positions on the detector. However, over time, these dots still form an interference pattern. This left scientists puzzled: how did each photon ‘know’ to contribute to the pattern? They concluded that, upon detection, the wave function collapses into a single point of light—a photon. The dot positions would still somehow follow a calculable probability distribution, which reflects the underlying interference pattern. This essentially is what quantum mechanics is about, and how its calculated through different probabilities.
How Is a Single Photon Identified?
In single-photon double-slit experiments, the light source is attenuated to very low intensities, so photons arrive at the detector as isolated events—discrete points of light appearing one by one. Each detection corresponds to the absorption of a point of energy, consistent with the photon concept.
However, it’s important to clarify:
- These experiments do not directly measure photons being emitted; we only detect them when they interact with the a photo-electric detector.
- Sources like highly attenuated lasers reduce the average number of photons per pulse to less than one, but this is a statistical average, not a guarantee.
- More precise single-photon sources use heralded photons from entangled pairs, where detecting one photon signals the presence of its partner. Even then, we infer a photon was emitted based on the detection of its entangled counterpart, not direct measurement of the emission.
Thus, a “single photon” is operationally defined by detecting individual quanta of light one at a time, rather than by directly confirming their emission. ie. We don’t actually know if a “single photon” is actually emitted from the source.
Single-Photon Detection and Background Noise
At the energy scale of a single photon, background noise should be considered.
- Ambient light leaks and thermal noise can produce false detections.
- Detectors have a threshold that must be exceeded for a signal to be detected.
- When light intensity is extremely low—a single dot on the screen—will no longer arise from the photon signal, but will need a combined amplitude of signal and noise as to get a single event to be detectable. In short time frames, single dots would be likely appear random and noisy.
- Additionally, even without external electromagnetic noise, noise from quantum-level fluctuations still exists and would contribute to the randomness observed in detection events.
Emergence of the Interference Pattern Over Time
Despite noise, the interference pattern becomes clear over many detections:
- Noise contributes a roughly uniform, flat background of random dots.
- Genuine photon detections cluster at positions predicted by the interference of the quantum wavefunction.
- Over time, the statistical distribution of dots reveals the characteristic bright and dark fringes of interference.
The wavefunction governs the probability of detection at each point, while the detector’s thresholding and noise add randomness to individual events.
Conclusion
In single-photon double-slit experiments, single photons are operationally defined as individual detection events appearing as points of light on the detector. While the energy is reduced so that only isolated events occur, we cannot directly confirm that a single photon is emitted each time—only that a single photon is detected.
What appears to be happening instead is that the light source is reduced to the point that it no longer has enough energy to trigger the detector. Only when combined with background electromagnetic (EM) noise does the wave sometimes become strong enough to produce a single photoelectric event. The dots appear random in nature because, at low intensities, most of the signal is overwhelmed by noise. Over time, however, the signal-to-noise ratio improves, eventually revealing the original interference pattern coming from the light source.
This is much like how CMOS cameras capture images: there is a significant amount of random noise in low light, and individual pixels may display varying random values largely attributed to various EM background noise. However, with a long exposure, the overall signal accumulates and rises above the noise, resulting in a much clearer and more coherent image.
Even at its lowest energy state, light still travels as a wave. When single points are triggered on a detector, it is the wave—combined with background noise—that provides just enough energy for the detector to register a signal. Since noise is largely flat, the original signal will still appear over time.
While the quantum wavefunction provides a probabilistic range of detection positions, the exact position of each detected dot is determined by the interplay of the signal with noise, including quantum-level fluctuations, which introduces randomness within the predicted probability distribution.
Here’s an attempt of a simulation of this with the added EM background noise.
Acknowledgments
This work is my original contribution, inspired by a breakthrough moment while watching videos from Looking Glass Universe followed by lecture on wave-particle duality by Neil Johnson at The Royal Institution.
Citation
Sebastian Gibbs, 7 July 2025, Fixing the Double-Slit Experiment
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