Welcome back to our second guided walk in Quantum Mechanical Woods! Last week, we saw how particles move like waves and hit like particles and how a single particle takes many paths. Surprisingly though, this is a well-discovered area of quantum mechanics – it’s on the way to the open nature around the center of the visitor.
This week I want to go a little deeper into the woods to talk about how the particles come together and come together when the trail is paved and in motion. This is a subject that is generally reserved for most people in physics; It is rarely discussed in popular articles. But the payoff is understanding how the precision leader works and seeing a great discovery made of it from the lab, the optical comb. So let’s get our (quantum) hiking boots a little dirty – it’ll be perfect for that.
Two particles
Let’s start with a question: What happens if a particle moves like waves, when I overlap the paths of two particles? Or in other words, do the subtle waves just interact with themselves, or do they merge together?
Miguel Morales
We can test in this lab by modifying the setup we used last week. Instead of splitting the light from one laser into two paths, we can use two different lasers to create the light coming into the final semi-silvered mirror.
We need to be careful about the lasers we use, and the quality of your laser pointer will no longer be at work. If you carefully measure the light with a normal laser, the color of the light and the phase of the wave (when the wave peaks) keeps wandering around. This color wandering is not recognizable to our eyes – the laser still looks red. But it turns out that the shade of red changes. This is a problem money and modern technology can fix – if we take out enough cash we can buy precision mode-locked lasers. Thanks to this, we can take both photographers emitting the same color with time-adjusted wave crests.
When we connect the light with two high-quality lasers, we see exactly the same striped pattern that we saw earlier. Waves of particles produced by two different lasers are interacting!
So what happens if we go to the same photon limit again? We can turn the intensity of two lasers so low that we see a photon appear on the screen one at a time, like small paintbrush paints. If the rate is low enough, only one photon exists between the laser and the screen at a time. When we do this experiment we will see photons reaching the screen one at a time; But when we look at the accumulated pointalism painting, we see the same stripes we saw last week. Once again, we are seeing the interference of a single particle.
It turns out that the experiments we did earlier give exactly the same answer. Nature does not care if one particle interacts with itself or two particles interact with each other – a wave is a wave, and particle waves act like other waves.
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 one of the lasers and make it a little more blue (short wavelength). When we look at the screen we see stripes again, but now the stripes are slowly moving to the side. The look of the stripes and their speed are both interesting.
First, the fact that we see stripes suggests that different particles of energy still interact.
Another observation is that the striped pattern is now time dependent; The stripes run to the side. As we distinguish color between lasers, the speed of the stripes increases. The audience’s musicians will already recognize the way we’re beating the beat, but, before we get an explanation, let’s refine our experimental setup.
If we have the material to use a narrow laser beam, we can use a prism to connect 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 with careful alignment the prism can be used to connect light with two lasers to a single beam.
Miguel Morales
If we look at the intensity of the combined laser beam, we will see the light intensity ‘bit’. When the light from each laser was constant, when their colored beams were combined with slightly different colors, the resulting beams oscillated from bright to dim. Musicians will recognize this by playing their instruments. When the sound from the tuning fork is combined with the sound of a slightly out-of-tune string, one can hear a ‘beat’ like syllabus between loud and soft. The speed of the beat is the difference between the frequencies, and the strings are arranged by adjusting the bit speed to zero (zero difference in frequency). Here we see the same thing with light – the bit frequency is the color difference between lasers.
While this is realized when thinking about instrument strings, it is surprising when thinking of photons. We started with two static streams of light, but now the light is released when it is bright and when it is dizzy. As the difference between the colors of the lasers is enlarged (it is D-tuned), pulsing becomes faster.
Paintball in time
So what if we really lower the lasers again? Again we can see that the photon hits our detector like small paintballs at a time. But if we look carefully at the time when photons arrive, we see that they are not random – they arrive on time with a beat. It doesn’t matter how low we lower the lasers – photons can be so rare that they only show one for every 100 beats – but they will always arrive on time with the beats.
This pattern is even more interesting if we compare the arrival time of photons in this experiment with the stripes we saw with our laser pointer last week. One way to understand what is happening in the two-slit experiment is to illustrate the nature of the wave nature of quantum mechanics pointing to how photons can land simultaneously: Paintballs can hit not in dark regions but in bright regions. With the advent of the paintball ball we see a similar pattern in the two-color beam, but now the paintball times are being directed back and forth in a timely manner and can only hit time with a beat. The beating can be considered as timely stripes.
