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 single particle takes many paths. Surprisingly, it is a well-explored field of quantum mechanics – it is on the paved nature path around the visitor center.
This week I want to get off the trail and go a little deeper into the forest to see how the particles move and combine in motion. This is a subject usually reserved for physics majors; It is rarely discussed in popular articles. But the payoff is understanding how precision LIDAR works and one great invention to make it an optical comb in the laboratory. So let’s get our (quantum) hiking boots a little dirty – it would be worth it.
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 intermingle?
We can test this in the lab by modifying the setup used last week. Instead of dividing the light from one laser into two paths, we can use two different lasers to create the light coming in the last 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 wander 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 unleash enough cash, we can buy accurate mode-locked lasers. Due to these, we can make two lasers emitting photons of the same color with time-aligned waves.
When we combine light from two high-quality lasers, we see exactly the same strip 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 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 the screen one 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 experiments we have done before have given exactly the same answer. Nature does not matter if a particle is interacting with itself or if two particles are interacting together – a wave is a wave, and the particle wave acts like any other wave.
But now that we have two precision lasers, we have many new experiments that we can try.
First, let’s try to interfere with photons of different colors. Let’s take the color of one of the 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 look at stripes indicates that particles of different energies still interact.
The second observation is that the striped pattern is now time dependent; Stripes run sideways. As we differentiate 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 move on to clarification, let’s improve our experimental setup.
If we are content to use narrow laser beams, we can use a prism to combine light currents. A prism is commonly used to split a single light beam and send each color in a different direction, but we can use it backwards and with careful alignment to light from two lasers in a single Use a prism to connect into the beam.
If we look at the intensity of the combined laser beam, we will see the intensity of the light beats. ‘While the light from each laser was constant when their beams are combined with different colors, the resulting beam oscillates from bright to dim. Musicians will recognize this by tuning their instruments. When the sound from a tuning fork is combined with the sound of a slightly out-of-string string, one can hear ‘beats’ as 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, it is surprising when thinking about photons. We started off with two steady streams of light, but now the light is bright and many times when it faints, it is cut off in time. As the spacing between the colors of the lasers becomes larger (they are de-tuned), there is rapid pulsing.
Paintball in time
So what if we actually reduce the lasers again? Again we see that the photons hit our detector like small paintballs at once. But if we observe the time of arrival of photons carefully, we see that it is not random – they arrive on time with beats. No matter how low we bend the lasers – photons can be so rare that they only show every 100 beats – but they will always arrive on time with the beats.
This pattern is even more interesting if we compare the time of arrival of photons in this experiment with the stripes seen with our laser pointer 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 carried side by side: paintballs can hit bright areas and not dark areas. We see a similar pattern in the arrival of paintballs in two-color beams, but now paintballs are being directed back and forth in time and can only be hit in time with beats. Beats can be thought of as stripes in time.