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Quantum technology is a general term for new technologies where quantum properties have been harnessed for technological applications. Quantum sensors and quantum random number generators are already commercial products, whereas quantum computers, for instance, are still at a stage of development where we are waiting to see their real potential. 

A central phenomenon pertaining to quantum technologies is the superposition of quantum states, which is, unfortunately, associated with unnecessary mysticism. I was once asked what the most common misunderstanding related to quantum physics is. I said that in my opinion it is that “an object is in several places at the same time”, or the closely related notion that “in superposition a system is simultaneously in multiple states”. After that, many people have asked me what I actually meant. Isn’t that exactly how superposition is explained in YouTube videos?

 I will try to explain the matter by means of an illustrative story.

The superposition state is like Singapore in between the poles

Antti and Bertta are two friends who like to travel. Antti knows only two possible destinations: the North Pole and the South Pole. His favourite activity is to stare at stars in the sky, and the polar regions are well suited for such an activity. At the North Pole, he admires the Big Dipper in particular. The Big Dipper cannot be seen from the South Pole at all, and for Antti, seeing it in the sky is evidence of being at the North Pole. In turn, Antti goes to the South Pole to admire the Southern Cross, which cannot be seen from the North Pole. 

Antti is not only fascinated by these two destinations, but he is not even aware of the possibility of travelling to any other place. Bertta is more open-minded and decides to travel to Singapore near the Equator. One night, Bertta sees both the Big Dipper and the Southern Cross in the sky at the same time. 

Bertta gives Antti a call and tells him that she is admiring these two constellations at the same time while resting on her deck chair. 

Antti finds this impossible – it would mean that Bertta would be simultaneously at the North and South Poles! Bertta’s statement is in conflict with what Antti knows about the position of the stars and the movement of the Earth.

The story illustrates the difference between a quantum-physical system and an everyday system as follows:

In the information world a bit has exactly two states, 0 (North Pole) and 1 (South Pole). A quantum-physical bit or qubit has in addition to these states also all superpositions of these. The superpositions can be compared to the fact that there are plenty of other places than just the two Poles. Just as Singapore is not the North and South Pole at the same time, nor is the superposition state simultaneously 0 and 1. Superposition states are a new kind of state that we don’t encounter in the world of bits. 

In a sense, though, superposition states are between the bit states of 0 and 1. 

This state of being in between is manifested so that observations made about the states through measurements are something in between the observations we would get in similar measurements respectively for the states 0 and 1.

Also in this sense, it is comparable to the notion that the observations in Singapore (with the Big Dipper and the Southern Cross both visible in the sky) have certain similarity with the observations made both at the North Pole and at the South Pole, but also some difference (the constellations are in different places in the sky).

A superposition state is not a vague one by any means, and neither is Bertta’s resting position in Singapore. Each superposition state is exactly as valid as 0 and 1. In fact, for qubits these two latter states have no special significance. We could pick any two states and present all other states as their superposition states. 

Let’s make it even clearer: a quantum system is only in one state at a time!

Quantum mechanics says nothing about the location of a particle

Perceiving superposition as simultaneous states is closely linked with the misconception that before measurement, a quantum particle can be everywhere simultaneously. Though it would also be problematic to argue that a particle is always in one place, whether we measure it or not. 

If we must say something about it, the closest to the truth would be that a particle has no specific location until it is measured. But we need not say even this. The point is that quantum mechanics is indifferent in this regard. Quantum mechanics gives the probability of a given measurement result when the measurement is conducted. 

If we determine the location of a particle by measuring it, we can find it in exactly one place.

“Well”, you might say, “now you've taken everything interesting out of the subject by nitpicking about the meanings of words. Being everywhere at the same time was the coolest thing about quantum mechanics.”

While writing this column, I may have slipped into the role of quantum pedant. In my opinion, the idea that a particle can exist everywhere at once is so mystical that it hinders any effort to gain insight into quantum theory. We should take a cue from Bertta in the story and broaden our perspectives, which can be accomplished by studying quantum physics.