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fix(em): rebuild virtual distillation as Hadamard test; release 2.0.0-beta.2 / 0.1.0-beta.2 - #51
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The copy-swap circuit inserted uncontrolled SWAPs between copies. Since S rho^⊗M S† = rho^⊗M, a bare permutation is a no-op on the product state, so neither Tr(O rho^M) nor Tr(rho^M) was ever measured and the mitigated estimate degenerated to <H>^copies (e.g. VD of <Z> on |1> returned +1 instead of -1). Replace it with the Hadamard-test circuit of Huggins et al. (PRX 11, 041036, App. C): copies preparations, one ancillary qubit prepared in |+>, and a ladder of ancilla-controlled SWAPs between adjacent copies realizing the derangement. The numerator observable is the original Hamiltonian on the first copy tensored with X on the ancilla; the denominator observable is X on the ancilla alone. BREAKING CHANGE: estimators now receive a Hamiltonian on the denominator call too (a single X on the ancilla) instead of None; distinguish numerator and denominator by observable content. The copy-swap circuit width grows from copies * base_width to copies * base_width + 1. Add exact density-matrix tests: |1> recovers <Z> = -1, pure states Ry(theta) are invariant for copies 2 and 3, and depolarizing noise matches the analytic Tr(Z rho^2)/Tr(rho^2). Update C/Python bindings and the CN/EN docs to the new contract.
Device::line, bidirectional_line, ring, star, grid, from_edges, and line_from_qubits previously accepted zero qubits and returned an empty device. Add DeviceError::EmptyDevice and validate the two construction entry points (line_from_qubits and from_u32_edges) so all factories reject empty input.
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Summary
Fixes and release preparation for
2.0.0-beta.2(Python) /0.1.0-beta.2(Rust/C).Changes
<H>^copies(e.g. VD of<Z>on|1⟩returned+1instead of-1). Replaced with the Hadamard-test circuit of Huggins et al. (PRX 11, 041036, App. C), with exact density-matrix tests and updated C/Python bindings and docs. BREAKING CHANGE: estimators now receive an observable (ancillaX) on the denominator call instead ofNone; circuit width grows by one qubit.Device::line,ring,star,grid,from_edges, etc. previously accepted zero qubits and returned an empty device; all factories now returnDeviceError::EmptyDevice.science::physicscategory slug from both manifests.timeoutandpoll_interval.Test plan
cargo test --workspace --lockedpasses