# Coherent interference at a low photon flux rate

• tistemfnp
tistemfnp
Homework Statement
A laser with a power of 2mW and a sufficiently large coherence length is split into two paths using a 50/50 splitter.

Path 1 directly enters a 2:2 50/50 coupler. Path B includes a 10m delay line, followed by an attenuator, and then connects to the same 50/50 coupler.

In a preparatory step, path 1 is disconnected from the coupler. Photon counters are connected to both outputs of the coupler. The sample rate is set at 100 MHz. The attenuator is adjusted so that, on average, the photon counter detects 10 photons per 10,000 samples per photon counter.

Question 1: Are the statistics governing these detection events consistent with a Poisson distribution? (Let's assume that the dark count rate is nearly zero.)

After the adjustment, both photon counters are disconnected. The outputs of the 2:2 coupler are now linked to a balanced detector. Path 1 is reconnected. Since both paths originate from a single source, they interfere.

Question 2: Does interference only occur in time bins where the photon counters (if they were still connected) would detect a photon? Or does interference occur in all time bins? Please elaborate.

(The primary concern I'd like to address is that when a low photon flux rate results in interference, and fractional portions of photon energy are detected within each time bin, this description no longer aligns with the concept of photons as indivisible entities according to Quantum Mechanics. In practice, the indivisibility of photons appears to be compromised.)
Relevant Equations
detection ~ N1 + sqrt(N1xN2) + N2

Question 1: Yes, the detection events follow a Poisson distribution.

Question 2: Yes, interference phenomena are clearly observed across all time bins even in the regime of much less than one photon per bin (N2 per bin << 1), which implies that the detection of supposedly "quantized" energy is distributed over time intervals, refuting the notion of strict quantization. As quantization stands synonym for the existence of photons -> there are no photons.

Last edited:
berkeman
Does interference only occur in time bins where the photon counters (if they were still connected) would detect a photon?
These time bins are not physical constants that would be unchanged independent of the setup. Trying to "identify them" in the other setup is meaningless.

You get interference in every time bin although it will be negligible (won't change your photon count) in most.
tistemfnp said:
which implies that the detection of supposedly "quantized" energy is distributed over time intervals, refuting the notion of strict quantization.
It does not. The photon detection is always quantized - you'll never measure 5.6 photons.
there are no photons
This is obviously wrong.

berkeman
When I refer to time bins I mean it statistically. Statistically it is of course valid to ask, if interference occurs at all time bins or if it only occurs (statistically) at time bins where (or better when) a photon counter would indicate a detection.

The detection is quantized, but the field is not:

So when it comes to interference, the fields of both inputs of the coupler add at each output (half of them) before a detection takes place. As the sum is detected and the energy per time bin is known, it can be shown, that fractions of energy of a photon are detected in the path where the photon flux is attenuated (a part of the overall detection of the interference). It then doesn't make sense to talk of quantization here.

tistemfnp said:
The detection is quantized, but the field is not:
The overall energy in the field is quantized. The amplitude at a given point is not (and no one claimed so).
tistemfnp said:
it can be shown, that fractions of energy of a photon are detected in the path where the photon flux is attenuated
No, you never detect fractions of a photon. That's not a thing. The detection is a number of photons, which is always an integer. That (and only that) is what it means for the electromagnetic field to be quantized.

PeterDonis

## What is coherent interference at a low photon flux rate?

Coherent interference at a low photon flux rate refers to the phenomenon where photons, even when emitted at a very low rate, can still exhibit interference patterns due to their wave-like properties. This is typically observed in experiments involving single photons or low-intensity light sources, where the quantum nature of light becomes significant.

## How is coherent interference observed at low photon flux rates?

Coherent interference at low photon flux rates can be observed using setups like the double-slit experiment, where individual photons pass through two slits and create an interference pattern on a detection screen. Advanced detectors capable of registering single photons, such as photomultiplier tubes or single-photon avalanche diodes, are used to measure the resulting interference pattern over time.

## Why is studying coherent interference at low photon flux rates important?

Studying coherent interference at low photon flux rates is crucial for understanding the fundamental principles of quantum mechanics, particularly the wave-particle duality of light. It also has practical implications in fields like quantum computing, quantum cryptography, and other technologies that rely on the manipulation of single photons or low-intensity light.

## What challenges are associated with experiments involving low photon flux rates?

Experiments with low photon flux rates face several challenges, including the need for highly sensitive detectors to accurately measure single photons and the requirement for extremely stable and controlled experimental conditions to prevent noise and other disturbances from affecting the results. Additionally, generating and maintaining a consistent low photon flux can be technically demanding.

## What applications can benefit from the study of coherent interference at low photon flux rates?

Applications that can benefit from the study of coherent interference at low photon flux rates include quantum communication, where secure information transfer relies on single-photon signals, and quantum computing, which uses quantum bits (qubits) that can be implemented using single photons. Other areas include precision measurement techniques in metrology and the development of new imaging technologies that exploit quantum properties of light.

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