Unlike the classic objects we have around us, a quantum object such as a quantum particle can sometimes behave like it does not have a precise configuration at all and is spread on many different ones. By object’s configuration we mean a particular arrangement of its qualities or parts. If we were to measure for example a quantum particles position or its energy we would have different probabilities of finding the particle exactly there.

Being in superposition is a bit like the water on a windy day. The surface of the water is constantly shaped and stirred by waves. Waves on water are usually used when describing superposition because it is the exact way this phenomenon seems to behave when demonstrated in a “double-slit experiment”: A single particle, when shot towards a wall with two separate openings behaves like a wave on water. One particle goes through not one but both of the slits and then interacts with itself and creates the colliding wave ripple pattern when it meets with a solid wall. Something that started as a single particle has demonstrated to have the capability of being something more complex.

Pic 1. A single photon can interact with itself and create a wave pattern when demonstrated in a double-slit experiment.

The principle of quantum superposition states that if a physical system can be measured to be in any of many configurations, then the most general state that describes it is a complex combination of all these possibilities. When we flip a coin and we hear it land on a table, we know without looking at it that it will be heads or tails. In quantum experience the situation is more unsettling: observable properties of things are not defined until they are measured (observed). Until that moment a superposition state is a combination of several different configurations.  

So unlike everything we experience in the macroscopic world, microscopic particles can theoretically be located in a potentially unlimited number of places at once, and to behave in a potentially unlimited number of different ways. In reality, superpositions can never actually be observed – all we can see are the consequences of their existence. Thus, we can never observe a single atom in its indeterminate state, or being in two places at once if we try to measure its position. 

The phenomenon arises from the extreme fragility of quantum states also known as decoherence or “Wave function collapse”. All interactions with their environment have a profound effect, and measurement inevitably requires interaction. Any attempt to measure directly quantum superpositions by the outside world, even with just a single photon, causes them to decohere, effectively destroying the superposition and reducing the state to a single location, and also destroying the ability to interfere with itself.

Fig 1. A strong measurement will disrupt the fragile quantum state with decoherence. A weak measurement however can be used for adaptive measurements of quantum systems.

Quantum decoherence disrupts the “quantumness” of a system and leads to a loss of starting information of the quantum state. In the large-scale world in which we live and see around us it is impossible to isolate anything from interaction with its environment, especially given the countless photons that create light and constantly “measure” the objects. It is the interaction of quantum objects with the environment that produces what we understand as classical objects.

Together with entanglement quantum superposition is a key element in quantum computing. Quantum mechanics allows a qubit, the quantum version of the classical binary bit physically realized with a two-state device, to be in a coherent complex superposition of both states simultaneously. When in classic computing two bits can be either 1 and 0 or 0 and 1 (two outcomes), in quantum computing it’s possible to have 11, 00, 10 and 01 (four outcomes). This is a property which is fundamental to quantum mechanics and quantum computing. The complexity of the quantum state quickly becomes incredibly large: to describe the states of only 100 qubits requires 1,267,650,600,228,229,401,496,703,205,375 different numbers–many trillion times the storage capacity of all computers ever made. Because of the extreme fragility of the superposition quantum computer must maintain a very strict isolation of its constituent qubits in order to function. 

More to read and links to text: 

Video on Youtube by Jonathan Thomas-Palmer 17 September 2019: “Wave Superposition Introduction”  https://www.youtube.com/watch?v=JUCLoczzwPo

The Quantum Atlas Webpage: “Superposition – A fundamental—and not totally unfamiliar—feature of quantum physics”  https://quantumatlas.umd.edu/entry/Superposition

Physics of the Universe, Webpage by Luke Mastin 2009: “Superposition, interference and decoherence” http://www.lukemastin.com/physics/topics_quantum_superposition.html

Nature journal of Science, Online article 09 October 2013 by Eugenie Samuel Reich: “Physicists snatch a peep into quantum paradox”

Scientific American, Online article 21 February 2015, “What makes a quantum computer so different (and so much faster) than a conventional computer?” https://www.scientificamerican.com/article/what-makes-a-quantum-comp/

Text by:
Noora Heiskanen with thanks to Silvia Cotroneo and Jani-Petri Martikainen