Author

Eran Kopel

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Preprint Aug 2026

Entanglement breaking in self-referential quantum feedback: a record bound and an exactly solvable loop

A feedback loop that returns a message into the process it describes defines a quantum channel on the message. We ask when such a loop preserves entanglement with a reference, that is, when its channel is not entanglement breaking. For any qubit loop, the negativity of the Choi state is at most half the fidelity betwee...

Eran Kopel · 3 citations

Preprint Aug 2026

The quantum lifetime of a future-referential feedback loop: certified index, architecture floor, and thermal bonus

A quantum feedback loop that returns information from a forward simulation to an earlier internal time induces a completely positive trace-preserving map on a message register; iterating that map eventually destroys its ability to carry entanglement. The entanglement-breaking index N = n_EB(Phi), the round at which thi...

Eran Kopel · 2 citations

Preprint Sep 2026

Measuring the entanglement-breaking index of a qubit collision channel on a quantum processor: protocol and worked example

The entanglement-breaking index of a quantum channel is the smallest number of self-compositions after which the channel destroys all entanglement with any reference system. A photonic experiment has fixed an index of 2; to our knowledge no larger index has been located by measuring the n-round channel across rounds. W...

Eran Kopel · 1 citation

Preprint Aug 2026

A sharp norm inequality for entanglement-breaking channels

For a channel $\Phi$ on $M_d$ written in the generalised Bloch parameterisation $r \mapsto Ar+c$, we prove that entanglement breaking implies $\|A\|_*^2 + \frac{d(d-1)}{2}|c|^2 \le (d-1)^2$, where $\|\cdot\|_*$ denotes the nuclear norm. The bound is attained in every dimension by the completely dephasing channel, and a...

Eran Kopel · 2 citations

Preprint Aug 2026

Certified coherent, informative, and non-entanglement-breaking fixed points of future-referential quantum feedback

We study quantum processes in which information extracted from a forward simulation is returned as input to an earlier internal time of the simulated dynamics: externally the protocol is an ordinary causally ordered circuit, but internally it is future-referential. Contracting a process tensor with a leakage instrument...

Eran Kopel · 0 citations