A curious observer’s guide to quantum mechanics, pt. 2: The particle melting pot – My programming school


One of the quietest revolutions of our current century is the entry of quantum mechanics into our everyday technology. It is used that quantum effects were limited to physics laboratories and delicate experiments. But modern technology increasingly relies on quantum mechanics for its basic operation, and the importance of quantum effects will only increase in the coming decades. Thus, physicist Miguel F. Morales has done a great job of explaining quantum mechanics for the rest of us in this seven-part series (the rest of mathematics, we promise). Below is the second story in the series, but you can always find the initial story here.

Welcome to our second guided walk in the Quantum Mechanical Jungle! Last week, we saw how particles move like waves and collide like particles and a particle takes many paths. Surprisingly, it is a well-explored field of quantum mechanics – it is on a paved nature path around the visitor’s center.

This week makes it clear how to mix melted and growing particles so that they can close the trail and go a little deeper into the forest. This is a subject that is usually reserved for physics majors; It is rarely discussed in popular articles. But the payoff is understanding how LIDAR works and a great invention to make it an optical comb in the laboratory. So let’s get our (quantum) hiking boots a little dirty – it will be worth it.

Two particles

Let’s start with a question: If the particles move like waves, what happens when I overlap the path of two particles? Or is there another way, do particle waves only interact among themselves, or do they interconnect?

On the left is the interferometer from last week, where a single particle is split by the first mirror and takes two very different paths. On the right is our new setup where we start with particles from two different lasers and combine them.
Enlarge / On the left is the interferometer from last week, where a single particle is split by the first mirror and takes two very different paths. On the right is our new setup where we start with particles from two different lasers and combine them.

Miguel Morales

We can test this in the lab by modifying the setup used last week. Instead of dividing the light from a laser into two paths, we can use two different lasers to create the light coming in the rear half-silver mirror.

We need to be careful about the laser we use, and the quality of your laser pointer is no longer up to the task. If you carefully measure light from a normal laser, the color of the light and the phase of the wave (when wave peaks occur) revolve around it. This color swirl is not harmful to our eyes – the laser still looks red – but it turns out that the exact shade of red changes. This is a problem. Money and modern technology can fix – if we get enough cash, we can buy accurate mode-locked. Because of these, we can make two laser-emitting photons of the same color with time-aligned waves.


When we combine light from two high-quality lasers, we see exactly the stripe pattern we saw earlier. The waves of particles formed by two different lasers are interacting!

So what happens if we go back to the single photon limit? We can reduce the intensity of two lasers so much that we see that the photons appear on the screen at once, like small paintballs. If the rate is sufficiently low, only one photon will exist between the lasers and the screen at a time. When we do this experiment, we will see that photons come on one screen at a time; But when we look at the accumulated pointillism painting, we will see the same streaks that we saw last week. Once again, we are seeing single particle interference.

It turns out that all the experiments we have done before have given exactly the same answer. Nature does not matter whether a particle is mating with itself or two particles interacting among themselves – a wave is a wave, and the particle’s wave acts like any other wave.

But now that we have two precision lasers, we have many new experiments that we can try.

Two colors

First, let’s try to interfere with photons of different colors. Let’s take the color of a lasers and make it a bit more blue (less wavelength). When we look at the screen, we see the stripes again, but now the stripes move slowly. The presence of stripes and their speed are both interesting.

First, the fact that we see stripes, particles of different energies still interact.

The second observation is that the striped pattern is now time dependent; Stripes run sideways. As we separate the color between the lasers, the speed of the stripes increases. Musicians in the audience will already recognize the beating pattern we are seeing, but before we go to clarification, improve your experimental setup.

If we are content to use narrow laser beams, we can use a prism to combine light currents. A prism is usually used to split a single light beam and send each color in a different direction, but we can use it backwards and the light from two lasers is the same, with careful alignment. Use a prism to connect into the beam.


Increasing light from two lasers with different colors combined with a prism /. After the prism the light beats in intensity.

Miguel Morales

If we look at the intensity of the combined laser beam, we will see the intensity of light. Dim. Musicians will recognize this by tuning their instruments. When the sound from the tuning fork is combined with the sound of a slightly out-of-string string, one can hear the beat as if the sound oscillates between loud and soft. The speed of the beats is the difference in frequencies, and the beat is tuned by adjusting the beat speed to zero (zero difference in frequency). Here we are seeing the same thing with light – the green frequency is the color difference between the lasers.

While this makes sense when thinking about musical instruments, one wonders when thinking about photons. We started off with two steady streams of light, but now it ticks on being bright and bunchy in the light. As the difference between the colors of the lasers widens (they are de-tuned), there is rapid pulsing.

Paintball in time

So what if we actually reduce the lasers again? Again we see that photons hit our detector like small paintballs at once. But if we look at the arrival time of photons carefully, we see that it is not random – they arrive on time with beats. No matter how low we tilt the lasers – photons can be so rare that they only show 100 beats each – but they will always arrive in time with the beats.

This pattern is even more interesting if we compare the time of arrival of photons in this experiment with the laser pointer we saw last week. One way to understand what is happening in a two-slit experiment is to direct the wave nature of quantum mechanics, where photons can be moved from side to side: paintballs can hit bright areas and not dark areas. We paint in two-color beam


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