In this section, we review the concepts of classical and quantum superposition . Quantum superposition is the framework for understanding all quantum phenomena. As we do not observe quantum phenomena in our everyday lives, it may seem confusing at first. However, as unintuitive as the quantum world may appear, there are a vast number of experiments which conclusively show that the universe really does operate according to the law of quantum superposition at the smallest distances accessible today.Footnote 1 Before going into specific details on quantum superposition, it is useful to explain how the term “superposition” is used in different contexts in both classical and quantum physics. At the end of the chapter, we present the related activities and questions. After gaining experience with quantum superposition from working through these problems, it will become more intuitive. The more experience you gain by advancing through this book, the more quantum superposition will make sense.

1.1 Classical Superposition

In classical physics, the concept of superposition is used to describe when two physical quantities are added together to make another third physical quantity that is entirely different from the original two. An example of the “superposition principle” in classical physics is clear when working with waves. Two pulses on a string which pass through each other will interfere following the principle of superposition as shown Fig. 1.1. Noise-canceling headphones use superposition by creating sound waves with the same magnitude as the incoming sound wave but completely out of phase, thereby canceling the sound wave. This destructive interference is illustrated in the second figure of Fig. 1.1.

Fig. 1.1
figure 1

Examples of constructive and destructive interference due to the classical superposition principle

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Another common application of classical superposition is finding the total magnitude and direction of quantities such as force, electric field, magnetic field, etc. For example, to calculate the total electric force \(\vec {F}_{\text{total}}\) on a charge q 2 produced by other charges q 1 and q 3, one would sum the forces produced by each individual charge: \(\vec {F}_{\text{total}} = \vec {F}_{12} + \vec {F}_{32}\). The challenge here is that forces are vectors, so vector addition is needed, as shown in Fig. 1.2.

Fig. 1.2
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A classical superposition is used to calculate the total electric force on a charge q 2 due to charges q 1 and q 3

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1.2 Quantum Superposition

Quantum superposition is a phenomenon associated with quantum systems. Quantum systems include small objects such as nuclei, electrons, elementary particles, and photons, for which the wave-particle duality and other non-classical effects are observed. For example, you would normally expect that an object can have an arbitrary amount of kinetic energy ranging from 0 to infinity (∞) Joules, i.e. a baseball could be at rest or thrown at any speed. However, according to quantum mechanics, the ball’s energy is quantized , meaning it can only have certain values. A specific example of energy quantization is when energies can only have integer values E = 0, 1, 2, 3, …, but not any numbers inbetween. This is counterintuitive, as we cannot observe it with our classical eyes. The gaps in energy are too small to be seen with the human eye and as such can be treated as continuous for classical physics. However, the gaps are more pronounced at smaller sizes, as shown in Fig. 1.3. For example the hydrogen atom is small enough that quantum effects are important, and Bohr needed to quantize the energy levels to successfully model its behavior.

Fig. 1.3
figure 3

Quantum effects associated with energy quantization are important at the atomic and subatomic distances. In this figure, the grey lines represent allowed energies. In quantum systems, the energies are quantized. As we zoom out of the quantum system to see it through a classical lens (represented by the downward arrow), the energies become more dense and appear continuous. This is the reason quantization is not noticeable in everyday objects

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One aspect of quantum superposition can be explained using a coin analogy. A coin has a 50∕50 probability of landing as either heads or tails, as shown in Fig. 1.4.

Fig. 1.4
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A tossed coin has a 50% chance of landing on heads or tails

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Question 1

What state is the coin in while it is in the air? Is it heads or tails?

We can say that the coin is in a superposition of both heads and tails. When it lands, it has a definite state, either heads or tails. Generally, the word “state” means any particular way that a system can possibly be described. For example, the coin can be either heads, or tails, or a combination of heads or tails while flipped in the air. All of these cases are called states of the coin system. While the coin is being flipped it is in a state of superposition. When we observe the coin, we are making a measurement which destroys the superposition.

At any given time, a system can be described as being in a particular state. The state is related to its quantized values. For example, a tossed coin is either in a heads state or a tails state. An electron orbiting a hydrogen atom could be in the ground state or an excited state. A quantum system is special because it can be in a superposition of these definite states, i.e., both heads and tails simultaneously. The outcome of a measurement is to observe some definite state with a given probability.

In Schrödinger’s famous thought experiment, Schrödinger’s cat is placed in a closed box with a single atom that has some probability of emitting deadly radiation at any time. Since radioactive nuclear decay is a spontaneous process, it is impossible to predict for certain when the nucleus decays. Therefore, you do not know whether the cat is alive or dead unless you open and look in the box. (Watch this video.)Footnote 2 It can be said that the cat is both alive AND dead with some non-zero probability. That is, the cat is in a quantum superposition state until you open the box and measure its state. Upon measurement, the cat is obviously either alive OR dead and the superposition has collapsed to a definite, non-superposition state.

Quantum systems can exist in a superposition state, and measuring the system will collapse the superposition state into one definite classical state. This might be hard to understand from a classical point of view, as we usually do not see quantum superposition with our human eyes (i.e in macroscopic objects). Einstein was really bothered by this feature of quantum systems. His friend, Abraham Pais, records: “I recall that during one walk, Einstein suddenly stopped, turned to me, and asked whether I really believed that the moon exists only when I look at it.”Footnote 3

1.3 Big Ideas

  1. 1.

    A particle in a quantum superposition exists as a combination of different states at the same time.

  2. 2.

    Each possible state has a given probability of being observed, but measurement destroys the superposition because only one definite state is seen.

1.4 Activities

Quantum Tic-Tac-Toe in Worksheet 10.3

1.5 Check Your Understanding

  1. 1.

    Discuss whether the following quantities are quantized or continuous:

    1. (a)

      electric charge

    2. (b)

      time

    3. (c)

      length

    4. (d)

      cash

    5. (e)

      paint color

  2. 2.

    An ink is created by mixing together 50% red ink and 50% yellow ink. An artist uses it to stamp a picture of a sun. If the ink behaves like a quantum system in a half-yellow, half-red quantum superposition, what are the different options for what the resulting picture could look like? Some options are shown in Fig. 1.5.

    Fig. 1.5
    figure 5

    Image of the painted suns

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  3. 3.

    If this controversial picture of a dressFootnote 4 is always seen as blue/black by Student A and always seen as white/gold by Student B, is the dress in a quantum superposition?