The weirdness of quantum

Before we explore quantum computing, it would be good to understand the behavior of particles as described by quantum mechanics. Below, I describe an experiment that helps us to understand the counter-intuitive nature of quantum theory.

A scary experiment

The famous Quantum Slit experiment describes the behavior of photons/particles and how they interact with each other and themselves. As we will see, this posed a challenge to physicists attempting to describe their behavior.

In the 19^th century, a British scientist, Thomas Young, postulated that light particles traveled in waves, rather than as particles. He set up a simple experiment where he cut two slits on a piece of metal and placed it as a blocker between a light source and a screen. He knew that if light traveled in the same manner as particles, then the particles that passed through the slits would hit the screen. Those that were blocked by the metal would bounce off the surface and would not reach the screen. Effectively, if the light was made of particles, then the screen should look like a spray of paint on a stencil. Figure 1 shows the experiment and the slit formation.

However, he assumed (before the experiment) that light was formed of waves, and the waves, when they passed through the slit, would interfere with one another and form patterns on the screen. The pattern would be defined based on how the waves passing through the slits interacted.

Where the waves interfered with each other (called constructive interference), the screen would display bright spots, and where peaks interfered with troughs (called destructive interference), they would form dark spots. Hence, the pattern would be slit shapes at the center followed by progressively darker slit shapes to the left and the right. Young successfully proved that light traveled in waves.

Figure 1: Young's double slit experiment

Einstein's photons – weirder now

Albert Einstein once more proved to be of great influence in the field of quantum mechanics. He proposed that light was made of photons – a discrete quantum of light that behaved like a particle. As a result, the experiment was repeated and this time, photons were passed through the slit one by one and the patterns still appeared. This could only happen if:

• Photons travelled in waveforms.
• All possible paths of these waveforms interfered with each other, even though only one of these paths could happen.

This supports the theory that all realities exist until the result is observed, and that subatomic particles can exist in superposition. As detectors were placed to observe photons passing through the slits, the patterns disappeared. This act of observation of particles collapses the realities into one.

We have discussed the three principles of quantum mechanics: superposition, entanglement, and interference. These principles are fundamental to the way in which particles are managed within a quantum computer.

Figure 2: A quantum computing timeline

The history of quantum computing and the key milestones are captured in Figure 2. The key takeaway is the contributions made to the field that have brought this technology to the brink of achieving impact at scale.