量子纠缠技术问询：一对多/多对多纠缠及宇宙纠缠存续性问题

1. Can quantum entanglement exist in one-to-many or many-to-many relationships? #

Absolutely—entanglement isn’t limited to pairs of particles. Multi-particle entanglement is a well-studied, experimentally verified area of quantum mechanics.

• One-to-many: You can have a single particle entangled with a group of others. For example, using parametric down-conversion (a standard lab technique), we can split a photon into an entangled state with two other photons. Measuring the first photon’s polarization will instantaneously fix the combined quantum state of the other two, even if they’re separated by large distances.
• Many-to-many: States like the Greenberger-Horne-Zeilinger (GHZ) state involve three or more particles where the entire system shares a single, indivisible quantum correlation. For a 3-photon GHZ state, measuring any one photon’s polarization tells you exactly what the other two will measure—this can’t be explained by just pairwise entanglement. Another example is the W state, where entanglement is distributed such that any subset of particles retains some degree of correlation.

These multi-particle states are foundational for quantum computing (e.g., cluster states) and advanced quantum communication, so they’re far from just theoretical concepts.

2. Are there entangled particles that have persisted since the Big Bang? #

Extremely unlikely, and we have no direct observational evidence of such particles.

In the early universe, particles were packed so tightly that nearly every particle was entangled with its neighbors. But two processes erased almost all of this initial entanglement over time:

• As the universe expanded, particles moved apart rapidly, reducing the chance of sustained interaction.
• Decoherence became dominant. Any entangled particle that interacted with its environment—whether cosmic microwave background photons, other particles, or even spacetime fluctuations—would have its local entanglement spread to the environment instead. Over 13.8 billion years, this process would have wiped out nearly all initial localized entanglement.

For a particle pair to retain Big Bang-era entanglement, they’d need to have been completely isolated from the rest of the universe since the first moments of expansion. That’s practically impossible in our interconnected cosmos; even hypothetical "isolated" particles would have interacted with CMB photons long ago, breaking their original quantum correlation.

3. Does entanglement decay with entropy increase and become a local phenomenon? #

Let’s break this down with thermodynamics and quantum theory:

Entanglement itself doesn’t "decay" or get destroyed—it spreads. When a system of entangled particles interacts with its environment, the entanglement between the original particles transfers to the environment. The total amount of entanglement across the entire universe (system + environment) remains the same, but the local entanglement we can observe between the original particles disappears.

This process is directly tied to entropy increase: decoherence (the spread of entanglement) causes the system to behave more classically, and its entropy rises as it loses quantum coherence. From our perspective as local observers, the entangled state seems to fade, and the particles act like independent, classical objects. So yes, for any localized system, entanglement will appear to decay as entropy increases—because the quantum correlations get absorbed into the larger, more disordered environment.

Globally, though, entanglement is still present; it’s just no longer concentrated in the small, manageable system we care about. Think of it like dropping a drop of ink into a pool: the ink doesn’t disappear, it spreads out until you can’t see it locally.

内容的提问来源于stack exchange，提问作者Joe