Quantum Fundamentals – Superposition

By Taha KhanReviewed by Louis CastelJun 17 2025

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What Is Quantum Superposition?

Quantum superposition is one of the fundamental principles of quantum mechanics. It means that a quantum object such as an electron, photon, or qubit can be in a combination of different states at the same time. A classical bit takes on a value of either 0 or 1, whereas a quantum bit (qubit) can exist in a superposition of both states simultaneously. Mathematically, it is represented as a linear combination of the basis states 0 and 1. This property is central to quantum theory and is the basis for technologies like quantum computing and quantum communication.¹

One of the most well-known illustrations of superposition is Schrödinger’s cat, a thought experiment proposed by physicist Erwin Schrödinger in 1935. It imagines a cat sealed inside a box, existing in a strange state of being both alive and dead at the same time, at least until someone opens the box and observes the outcome. ^{1, 2}

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The Origins and Theoretical Basis of Superposition

The concept of quantum superposition was established by the work of early 20th-century physicists. Paul Dirac, Erwin Schrödinger, and Werner Heisenberg contributed key ideas to the development of quantum theory.

For instance, Dirac formalized the principle of superposition in the language of linear algebra, introducing the bra-ket notation (or Dirac notation) that is still used today. He suggested that a quantum state could be represented as a linear combination of basis states. ³ Similarly, Schrödinger provided a dynamic framework with his wave equation, which describes how quantum states evolve over time. His equation showed that particles are not simply point-like objects but can be described by wave functions. ² Heisenberg introduced matrix mechanics, another formulation of quantum mechanics that ultimately proved equivalent to Schrödinger's wave mechanics. ⁴

The superposition principle states that if a quantum system can be in state A and state B, it can also be in any linear combination of those states:

    |ψ⟩ = c₁|A⟩ + c₂|B⟩,

Here, c₁ and c₂ are complex numbers representing probability amplitudes. The probabilities of observing either state are given by the squares of these amplitudes (|c₁|² and |c₂|²), but until a measurement is made, the system exists in this blend of possibilities. ⁵ This contrasts with classical physics, where a system occupies a single, well-defined state at any given time.

Observing Superposition: Is It Real?

The quantum superposition is demonstrated by the double-slit experiment. When particles such as electrons or photons pass through two slits, they create an interference pattern typical of waves, implying that each particle travels through both slits simultaneously in a superposition of paths. This phenomenon cannot be explained by classical particles, and it confirms the wave-particle duality and superposition principle. ⁶

Superposition has been experimentally observed not only with photons and electrons but also with larger molecules like C60 fullerenes. However, superposition is fragile, and the interactions with the environment cause quantum decoherence, which destroys the superposition and forces the system into a definite state. This leads to the measurement problem of why and how observation collapses a superposition into a single outcome. The exact mechanism of collapse is still a subject of interpretation and debate. ⁷

Role in Quantum Technologies

In quantum computing, qubits exploit superposition to encode and process a vast amount of information simultaneously, allowing quantum computers to tackle certain problems exponentially faster than classical computers.

For instance, Google’s Sycamore processor, with its 53 qubits, demonstrated quantum computing supremacy by performing a specific computation in 200 seconds that would take a classical supercomputer thousands of years to complete. ⁸ IBM and other companies are also advancing quantum processors based on superposition and entanglement principles. ⁹

Superposition also enables quantum sensors with high sensitivity and quantum communication protocols like quantum key distribution, which rely on superposed states to guarantee secure information transfer.

Challenges and Misconceptions

Quantum superposition often leads to misconceptions, such as the idea that particles literally exist in two places at once in a classical sense. But in reality, the particle has a probability amplitude for being found in either place, and it behaves like a wave until it is measured.

Another source of confusion is the idea of collapse. When we observe a quantum system, the superposition appears to collapse into one definite outcome. However, interpretations differ on what this really means. Some interpretations, like the Copenhagen interpretation, treat collapse as fundamental. ⁴ Others, like many-worlds, suggest all outcomes happen, each in a separate universe.

Technically, maintaining superposition in large-scale quantum systems is extremely challenging due to decoherence caused by environmental noise. Qubits are very sensitive to noise, and maintaining coherence over time is difficult. Scaling quantum computers to useful sizes requires overcoming these hurdles through error correction and isolation techniques.

Applications and Future Potential

The practical applications of superposition are vast. Major players such as Google, IBM, and Microsoft, as well as governments and industries like defense, aerospace, and healthcare, are investing heavily in quantum R&D, leading to further applications.

In cryptography, protocols like quantum key distribution (QKD) use superposition and entanglement to detect eavesdropping, which could lead to secure global communications. ¹⁰ In metrology, quantum-enhanced measurements could improve standards of time and temperature.

Similarly, in pharmaceuticals and chemistry, quantum computers may eventually simulate complex molecular interactions, which is something that classical computers struggle with due to the exponential growth of possible configurations. This could accelerate drug discovery and materials research.

As experimental techniques continue to improve, superposition is set to remain a core principle driving the development of quantum technologies.

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9. IBM Unveils Breakthrough 127-Qubit Quantum Processor. (2021) IBM Newsroom. https://newsroom.ibm.com/2021-11-16-IBM-Unveils-Breakthrough-127-Qubit-Quantum-Processor?mhsrc=ibmsearch_a&mhq=eagle
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