Superposition and Entanglement

Superposition and Entanglement in Quantum Mechanics: Comparison

Introduction to quantum mechanics

Quantum mechanics is a branch of physics that deals with the behavior of matter and energy at the subatomic levels. Some of the key concepts in quantum mechanics that depart sharply from classical physics are superposition and entanglement. Understanding the differences and relationships between these two phenomena is essential for grasping quantum mechanics.

Comparison between superposition and entanglement in quantum mechanics

What is superposition?

The concept of superposition

Superposition refers to the ability of quantum particles to exist in multiple possible states simultaneously. For example, an electron can be in a superposition of two locations, two momenta, or two energy levels at the same time. This means the electron literally occupies multiple states concurrently until it is measured.

Mathematical representation

Mathematically, if |0⟩ and |1⟩ are two possible states, then the electron in a superposition state can be represented as:

|ψ⟩ = α|0⟩ + β|1⟩

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Where α and β are complex numbers that specify the probability amplitudes of states |0⟩ and |1⟩ respectively.

Measurements collapse superposition

An important aspect of superposition is that measuring a quantum system “collapses” this superposition into a single state. Prior to measurement, an electron exists in both states 0 and 1, but the act of measurement forces it into one or the other discrete state.

What is quantum entanglement?

Definition of entanglement

Quantum entanglement is a special phenomenon where quantum particles remain connected or correlated even when separated by large distances. So if two particles are entangled, measuring one particle instantaneously affects the state of the other particle.

Nature of entanglement

This seeming “spooky action at a distance” puzzled even Einstein, who called it “spukhafte Fernwirkung” or spooky action at a distance. But experiments have repeatedly demonstrated that entanglement is real. So quantum mechanics allows instantaneous connections between distant particles without any carrier of information.

Representation of entanglement

An entangled state between two particles A and B can be written as:

|ψ⟩ = α |00⟩ + β |11⟩

So the quantum states of the particles are linked rather than independent.

Differences between superposition and entanglement

Number of particles involved

The key difference is that superposition refers to the state of a single quantum particle, while entanglement deals with correlations between two or more particles. So superposition is an intra-particle phenomenon, while entanglement is an inter-particle phenomenon.

Nature of the quantum state

In a superposition, the particle itself exists in a coherent linear combination of multiple states. In entanglement, the particles maintain individual states but exhibit correlations between them.

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Relationships between superposition and entanglement

While superposition and entanglement are distinct concepts, they also have intriguing connections:

Entanglement arises from superpositions

When two quantum systems in superposition states interact, entanglement between the two systems can arise spontaneously. So entanglement often has its origins in superpositions.

Superpositions extend over entangled systems

In some ways, an entangled quantum state can be viewed as a superposition over all particles involved. So quantum states are a superposition over larger systems rather than individual particles.

Both lead to nonclassical effects

Phenomena enabled by superpositions and entanglement like quantum teleportation, quantum cryptography, quantum searches, quantum simulations etc are not possible classically. Both these quintessential quantum effects lead to capabilities beyond classical physics.

Applications of superposition and entanglement

Quantum computing

Superposition and entanglement are key resources for quantum computing and information processing. Qubits in superpositions enable massively parallel computation. While entanglement allows connecting distant qubits and quantum error correction mechanisms.

Quantum cryptography

Quantum key distribution protocols rely on entanglement to securely exchange cryptographic keys between two parties. Any attempt by an eavesdropper to intercept the quantum states alerts the communicating parties.

Quantum sensors

Superposition and entanglement can enhance the precision of quantum sensors like atomic clocks, magnetometers, gravity detectors etc. These quantum devices are more accurate than classical sensors.


In conclusion, superposition and entanglement are related but distinct quintessentially quantum concepts. Superposition deals with coherent linear combinations of states for individual particles, while entanglement concerns nonlocal correlations between particles. Both these phenomena contradict classical physics, but lead to useful quantum technologies like quantum computing, quantum cryptography, and quantum sensing. Understanding the connections as well as differences between superposition and entanglement is key to unlocking the mysteries of quantum mechanics.

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What is superposition in quantum mechanics?

Superposition refers to the ability of quantum particles to exist in multiple states or locations simultaneously. Mathematically it is represented as a linear combination of the possible states.

What is quantum entanglement?

Entanglement occurs when quantum particles interact in such a way that they remain connected across space and time. Measuring one particle collapses the joint state and instantaneously affects the other particle.

Is superposition possible without entanglement?

Yes, superposition is an intra-particle effect dealing with coherence of a single particle state. Superpositions can exist without entanglement between particles.

Can you have entanglement without superposition?

No, you need superpositions to create entangled states when particles interact. Once entanglement is created particles may lose coherence locally but remain entangled.

Are superposed states and entangled states the same?

No, superposition refers to coherence of states for a single particle, while entanglement refers to nonlocal correlation between particles. But both these quantum effects have no classical analog.