L–Entanglement, Decoherence, and Spacetime

By U.W. on 30 of June 2025 — as part of our Quantum Hardware Lecture

In a lab, we can count individual photons—emitted one at a time from a single trapped ion—hitting a screen and integrating to bright spots. According to the path‐integral picture, each photon “tests” every possible route from ion to detector, and interference makes those paths that minimize the action stand out as more likely. Unobserved, these photons form smooth interference fringes; the moment we look, each one lands at a single point in what’s sometimes called the collapse of the wave function. This simple setup exposes the measurement problem: quantum mechanics tells us how amplitudes evolve and interfere, but never explains why—or how—a unique outcome appears on our screen.

From the beginning, physicists have wrestled with this gap. In the Copenhagen interpretation, Bohr and Heisenberg drew a “cut” between the quantum system and classical apparatus, treating this collapse as a fundamental postulate. Von Neumann later formalized this with his projection postulate, pushing the mystery to the observer’s mind. Everett’s many-worlds view threw out collapse entirely, positing that every outcome happens in a branching of the so-called multiverse. Objective collapse models instead tweak Schrödinger’s equation to trigger real, physical reductions, at the price of adding new constants. These debates date back nearly a century and were fiercely argued by the founders of quantum theory, yet no single view has won universal acceptance. Moreover, the stunning success of quantum mechanics—in predicting experiments and powering technologies from transistors to lasers—led many practitioners to adopt a “shut up and calculate” mindset, reluctant to revisit what can feel like metaphysical puzzles when the formalism works so well to win prizes.

A more physical account comes from decoherence, where entanglement with countless environmental degrees of freedom—air molecules, stray photons, thermal vibrations—washes out phase information and makes superpositions appear to “disappear.” Decoherence doesn’t force a literal collapse, but it does explain why classical outcomes emerge without changing quantum laws. Some recent, highly speculative ideas even suggest that the same entanglement responsible for decoherence may build space time itself. Van Raamsdonk showed that gluing together many entangled qubits can recreate a smooth spacetime geometry. A conjecture of Maldacena and Susskind envisions each entangled pair as joined by a tiny wormhole, hinting that gravity could be the bookkeeping of quantum links. While this is far from tested experimentally, it suggests a deep, if speculative, connection: the very process that selects definite outcomes might also weave the fabric of space and time, and, thus, bring us into existence. It all leaves me wondering how complex it can be to describe a single photon hitting a detector without running into logic conflicts.

Special thanks to all my past, current, and future environments

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References

[1] W. H. Zurek, “Decoherence, einselection, and the quantum origins of the classical,” Rev. Mod. Phys. 75, 715–775 (2003).

[2] M. Schlosshauer, “Decoherence, the measurement problem, and interpretations of quantum mechanics,” Rev. Mod. Phys. 76, 1267–1305 (2005).

[3] M. Van Raamsdonk, “Building up spacetime with quantum entanglement,” Gen. Relativ. Gravit. 42, 2323–2329 (2010).

[4] J. Maldacena and L. Susskind, “Cool horizons for entangled black holes,” Fortschritte der Physik 61, 781–811 (2013).