The Qubit Phase

Introduction

Do you remember our introductory example of the quantum coin? Coins have two states, heads or tails. In the air, our quantum coin is in a state of superposition of both states.

Let’s have a closer look at this coin. It continuously flips between heads and tails. Once the coin lands, this rotation determines whether we see heads or tails.

Look even closer. This coin also spins along its edge.

At first sight, the spin doesn’t matter for the direction of heads and tails, but it affects the coin’s trajectory through the air. This matters for the orientation once the coin lands, doesn’t it?

In quantum mechanics, particles have such a spin, too. It’s the phase. In physical terms, the quantum mechanical phase originates from the concept that every quantum entity may be described as a particle and as a wave.

The debate over whether light is particles or waves dates back over three hundred years. In the seventeenth century, Isaac Newton proclaimed that light consists of a stream of particles. About two hundred years later, in the nineteenth century, Thomas Young countered that light consists of waves. He devised the double-slit experiment to prove his claim.

In this experiment, a beam of light is aimed at a barrier with two vertical slits. The light passes through the slits, and the resulting pattern is recorded on a photographic plate.

If one slit is covered, there’s a single line of light aligned with whichever slit is open. Intuitively, we would expect to see two lines aligned with the slits if both slits are open, but something else happens. The photographic plate is entirely separated into multiple lines of lightness and darkness, in varying degrees.

The implications of Young’s experiment are counter-intuitive if you regard light as a stream of particles, but it makes perfect sense once you start thinking of light as a wave.

The characteristic of a wave

The main characteristic of a wave is that it goes up and down as it moves. This entails a variety of other characteristics.

• The wavelength is the distance over which the wave’s shape repeats.
• The amplitude of a wave is the distance between its center and its crest.

The following figure depicts these characteristics.

Another distinct property of waves is that they interfere with each other. Simply put, ...