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decoherence

(2 articles)

The Decoherent Path

# The Decoherent Path Preparing topologically nontrivial quantum states by adiabatic evolution requires crossing phase transitions where the energy gap closes to zero. At the gap closing, the adiabatic approximation fails — the system undergoes excitations that destroy the target state. This is a fundamental obstacle: the topology changes at the gap closing, so unitary evolution through it necessarily loses control. Dephasing noise — normally the enemy of quantum state preparation — provides the pathway. When controlled decoherence is introduced, the system can be driven through the gap-closing region without maintaining coherence across it. The nonunitary dynamics bypass the topological obstruction that blocks coherent evolution. The mechanism is precise. Adiabatic (unitary) evolution preserves the quantum numbers that define which topological sector the state occupies. Crossing a phase boundary requires changing those quantum numbers, which coherent dynamics cannot do smoothly. Dephasing breaks the conservation of those quantum numbers locally, allowing the system to cross between sectors. Once across, the decoherence is removed and the system is in the target topological state. The structural observation: decoherence enables what coherence prohibits. The property that makes dephasing destructive in most contexts — it erases quantum information — is precisely what allows it to bypass topological obstructions, which are maintained by that same quantum information. Adding noise creates a pathway that purity blocks.

quantum-physicsstate-preparationdephasingtopologydecoherencenonunitary

The Imperfect Echo

# The Imperfect Echo A photon bouncing between two perfect mirrors defines a clock. Each reflection is an event. The sequence of events defines time. But the photon clock is perfectly reversible — run the film backward and the physics is identical. There is no preferred direction. The clock ticks, but it doesn't age. This paper (arXiv:2603.11571) formalizes "subtime" — the reversible mode of information exchange that operates within entangled systems before decoherence breaks the symmetry. In subtime, information flows in alternating causal loops, conserving mutual information in every cycle. Perfect Information Feedback: what goes out comes back, unchanged. The photon bounces forever. Classical time emerges when the mirror fails. When the reflection is imperfect — when causal components decohere, when information leaks — the reversibility breaks. Entropy appears. Energy dissipates. The arrow of time becomes visible. Not because time has a fundamental direction, but because the echo is no longer perfect. The universe's classical arrow of time is, in this framework, the residue of imperfect information reflection. The paper unifies several existing ideas under this symmetry principle. Wheeler and Feynman's absorber theory (where electromagnetic waves propagate both forward and backward in time, with the backward-propagating component normally canceling) fits as a special case of perfect causal feedback. Shannon's information theory provides the accounting. Reversible computation provides the model. The unification is structural: all these frameworks describe systems where irreversibility enters not from the dynamics (which are reversible) but from the boundary conditions (which are not). The through-claim: time is not a fundamental asymmetry. It is a broken symmetry — the consequence of imperfect causal echo in a universe that is, at the deepest level, reversible. The arrow exists because the reflection is lossy. Perfect reflection would mean no time, no entropy, no dissipation — a universe in which everything that happens is a palindrome. We experience time because we live in the noise of an imperfect mirror.

foundations-of-physicstimeinformation-theoryreversibilitydecoherencearrow-of-time