The Code Book
Simon Singh

In order to explain the principles of quantum computing, it helps to return to the end of the eighteenth century and the work of Thomas Young, the English polymath who made the first breakthrough in deciphering Egyptian hieroglyphics. A fellow of Emmanuel College, Cambridge, Young would often spend his afternoons relaxing near the college duck pond. On one particular day, so the story goes, he noticed two ducks happily swimming alongside each other. He observed that the two ducks left two trails of ripples behind them, which interacted and formed a peculiar pattern of rough and calm patches. The two sets of ripples fanned out behind the two ducks, and when a peak from one duck met a trough from the other duck, the result was a tiny patch of calm water--the peak and the trough canceled each other out. Alternatively, if two peaks arrived at the same spot simultaneously, then the result was an even higher peak, and if two troughs arrived at the same spot simultaneously, the result was an even deeper trough. He was particularly fascinated, because the ducks reminded him of an experiment concerning the nature of light which he conducted in 1799.

In Young's earlier experiment he had shone light at a partition in which there were two narrow slits, Young expected to see two bright stripes, some distance beyond the slits. Instead he observed that the light and dark stripes on the screen. The striped pattern of light on the screen had puzzled him, but now he believed he could explain it wholly in terms of what he had seen on the duck pond.

Young began by assuming that light was a form of waves. If the light emanating from the two slits behaved like waves, then it was just like the ripples behind the two ducks. Furthermore, the light and dark stripes on the screen were caused by the same interactions that caused the water waves to form high peaks, deep troughs and patches of calm. Young could imagine points on the screen where a trough met a peak, resulting in cancelation and a dark stripe, and points on the screen where two peaks (or two troughs) met, resulting in reinforcement and a bright stripe. The ducks had provided Young with a deeper insight into the true nature of light, and he eventually published 'The Undulatory Theory of Light,' an all-time classic among physics papers.

Nowadays, we know that light does indeed behave like a wave, but we know that it can also behave like a particle. Whether we perceive light as a wave or as a particle depends on the circumstances, and this ambiguity of light is known as wave-particle duality. We do not need to discuss this duality any further except to say that modern physics thinks of a beam of light as consisting of countless individual particles, known as photons, which exhibit wave-like properties. Looked at this way, we can interpret Young's experiment in terms of photons flooding the slits, and then interacting on the other side of the partition.

So far, there is nothing particularly strange about Young's experiment. However, modern technology allows physicists to repeat Young's experiment using a filament that is so dim that it emits single photons of light. Photons are produced individually at a rate of, say, one per minute, and each photon travels alone toward the partition. Sometimes a photon will pass through one of the two slits, and strike the screen. Although our eyes are not sensitive enough to see the individual photons, they can be observed with the help of a special detector, and over a period of hours we could build up an overall picture of where the photons are striking the screen. With only one photon at a time passing through the slits, we would not expect to see the striped pattern observed by Young, because that phenomenon seems to depend on two photons simultaneously traveling through different slits and interacting with each other on the other side. Instead we might expect to see just two light stripes, simply projections of the slits in the partition. However, for some extraordinary reason, even with single photons the result on the screen is still a pattern of light and dark stripes, just as if photons had been interacting.

This weird result defies common sense. There is no way to explain the phenomenon in terms of the classical laws of physics, by which we mean the traditional laws that were developed to explain how everyday objects behave. Classical physics can explain the orbits of planets or the trajectory of a cannonball, but cannot fully describe the world of the truly tiny, such as the trajectory of a photon. In order to explain such photon phenomena, physicists resort to quantum theory, an explanation of how objects behave at the microscopic level. However, even quantum theorists cannot agree an how to interpret this experiment. They tend to split into two opposing camps, each with their own interpretation.

The first camp posits an idea known as superposition. The superpositionists begin by stating that we know only two things for certain about the photon--it leaves the filament and it strikes the screen. Everything else is a complete mystery, including whether the photon passed through the left or right slit. Because the exact path of the photon is unknown, superpositionists take the peculiar view that the photon somehow passes through both slits simultaneously, which would then allow it to interfere with itself and create the striped pattern observed on the screen. But how can one photon pass through both slits?

[Some interesting things omitted, like their argument.]

For readers who feel uncomfortable with superposition, there is the second quantum camp, who favor a different interpretation of Young's experiment. Unfortunately, this alternative view is equally bizarre. The many-worlds interpretation claims that upon leaving the filament the photon has two choices--either it passes through the left slit or the right slit--at which point the universe divides into two universes, and in one universe the photon goes through the left slit, and in the other universe the photon goes through the right slit. These two universes somehow interfere with each other, which accounts for the striped pattern. Followers of the many-worlds interpretation believe that whenever an object has the potential to enter one of several possible states, the universe splits into many universes, so that each potential is fulfilled in a different universe. This proliferation of universes is referred to as the multiverse."

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