Quantum entanglement is a phenomenon of quantum mechanics that describes an intimate and instantaneous connection between two or more particles. This connection means that the particles share the same overall quantum state, regardless of the physical distance separating them. To understand this counter-intuitive concept, consider two coins flipped simultaneously, where one is sent to a distant location. In the everyday world, if you see your coin land on heads, you still have no knowledge of the other coin’s result.
In the quantum world, the situation is different. If two particles become entangled and one is measured to have a specific property, the corresponding property of the other particle is immediately known. This correlation exists even if the particles are light-years apart, making entanglement a puzzling aspect of nature. The phenomenon defies classical intuition and establishes that the laws governing the subatomic world are fundamentally different from those we experience macroscopically. The implications of this shared fate across vast distances make entanglement a central topic in modern physics and technology.
Shared Fate The Core Concept of Entangled Particles
Entanglement begins with the creation of a single quantum state shared by a pair of particles, such as photons or electrons. Before any measurement is made, each particle exists in a superposition, meaning its properties, like spin or polarization, are not definitively determined but exist as a combination of all possible outcomes. The particles’ individual properties are indefinite, but their combined state is precisely defined, dictating that their outcomes will always be perfectly correlated or anti-correlated.
When a physicist measures a property of the first particle, the act of measurement forces its quantum state to instantaneously collapse from a superposition to a single, definite value. Because the particles share a single, inseparable quantum description, this collapse simultaneously and instantly determines the corresponding property of the distant second particle. The measurement on the first particle reveals the state of the second particle, not by sending a signal, but because the two were never truly separate entities to begin with.
Einstein’s Spooky Action and Bell’s Theorem
The instantaneous connection inherent in entanglement prompted Albert Einstein to famously refer to it as “spooky action at a distance.” He and his colleagues, in the EPR paradox, argued that this effect suggested quantum mechanics was incomplete. They proposed that the particles must contain “hidden variables”—pre-determined, local information that dictated the measurement outcome, eliminating the need for faster-than-light influence.
This debate remained a thought experiment until physicist John Bell developed a mathematical framework in 1964 to test these possibilities. Bell’s theorem established a statistical limit, known as Bell’s inequality, on the strength of correlations produced by any theory relying on local hidden variables. If local hidden variables were real, experimental results would satisfy the inequality.
Experiments, such as those conducted by Alain Aspect and his team in the 1980s, measured the polarization of entangled photons separated by large distances. The results consistently showed correlations stronger than the limit set by Bell’s inequality, meaning the inequality was violated. This outcome provided evidence that the particles’ connection is non-local and that the reality of entanglement, rather than local hidden variables, describes the subatomic world.
Entanglement in Quantum Computing
The ability of particles to share a single quantum state is the resource that gives quantum computers their potential power. Classical computers use bits that are either a 0 or a 1, whereas quantum computers use quantum bits, or qubits, which exploit the principle of superposition to be both 0 and 1 simultaneously. Entanglement takes this capability and exponentially amplifies it by linking multiple qubits together.
When qubits are entangled, their combined state is described by a single wave function, allowing the system to explore many different computational paths simultaneously. For a system of two entangled qubits, four states can be processed at once; with 30 entangled qubits, the system can represent over a billion states simultaneously. This interconnectedness allows a quantum processor to perform a massive number of calculations in parallel, a process known as quantum parallelism.
Entanglement is directly implemented through multi-qubit quantum gates, such as the Controlled-NOT (CNOT) gate, which flips the state of one qubit based on the state of an entangled partner. By weaving together a network of entangled qubits and applying specific quantum algorithms, these machines can tackle certain problems, like factoring large numbers or simulating molecular interactions, that are currently intractable for even the most powerful classical supercomputers.
The Challenge of Decoherence
The primary obstacle to realizing large-scale quantum computation is the fragility of the entangled state, a problem known as decoherence. Decoherence occurs when the organized, coherent quantum state of a system is destroyed by interaction with the outside environment. Even minimal interference, such as stray electromagnetic fields or minute temperature fluctuations, can cause the entangled quantum information to leak out.
This interaction causes the system’s superposition and entanglement properties to collapse, forcing the quantum system to revert to a definite, classical state. The speed at which this happens is extremely fast, often occurring in mere microseconds, which limits the time available for computation. To combat decoherence, quantum systems must be maintained in highly isolated conditions, typically using environments that are near absolute zero temperature or ultra-high vacuums, making the engineering of stable quantum hardware a significant technical challenge.