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[FEATURE] Decompose `p`, `phaseshift` and `u1` as `rz` instead of `h; rx; h`

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  • #470 di @tonycoder-hub — integrata

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Funzionalità
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
python
Ambito
compilers

Direzione di ricerca

Start with phaseshift_gate and ONE_QUBIT_ROTATION_MAP in src/pyqasm/maps/gates.py, then read the phaseshift-gate section of docs/gate_decompositions.md and inspect the existing decomposition tests. Verify single-gate unrolling and the CLIFFORD_T rebases for p(pi/4) and p(pi/2). Run the unitary checks for the changed decomposition, including csx and rccx, and confirm they match up to global phase.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

enhancement
Feature Description

unroll decomposes p, phaseshift and u1 into three gates, h; rx(θ); h. A single rz(θ) gives the same gate up to a global phase. The longer form then grows during rebase, because each h expands again:

Input Basis Output on main Output for the same gate written as rz
p(0.3) q[0]; ROTATIONAL_CX 5 gates: ry(π/2); rx(π); rx(0.3); ry(π/2); rx(π) rz(0.3)
p(pi/4) q[0]; CLIFFORD_T 5 gates: h; h; t; h; h t
p(pi/2) q[0]; CLIFFORD_T 5 gates: h; h; s; h; h s

Reproduction on main at a742796, Python 3.12:

import pyqasm
from pyqasm.elements import BasisSet

src = 'OPENQASM 3.0;\ninclude "stdgates.inc";\nqubit[1] q;\np(pi/4) q[0];\n'
m = pyqasm.loads(src)
m.rebase(BasisSet.CLIFFORD_T)
print(pyqasm.dumps(m))  # h q[0]; h q[0]; t q[0]; h q[0]; h q[0];

OpenQASM 2 programs that use qelib1.inc unroll p and u1 the same way.

The decomposition is phaseshift_gate. ONE_QUBIT_ROTATION_MAP maps u1, U1, phaseshift and p to it. The csx and rccx decompositions also call it seven times in total.

A related point: #468 adds a p rule to rebase. Unroll rewrites a p gate written by the user before rebase sees it, so that rule only applies to the p gates that pyqasm generates itself.

Done when:

  • A single p(θ), phaseshift(θ) or u1(θ) unrolls to one gate.
  • Under CLIFFORD_T, p(pi/4) rebases to t and p(pi/2) rebases to s.
  • The unitary of each changed decomposition, including csx and rccx, still matches the original up to a global phase.
Implementation (Optional)

One option is to make phaseshift_gate emit rz(θ). gate_decompositions.md documents the current form and needs the same update. rz(θ) and p(θ) differ only by a global phase. That should be safe here, because ctrl @ p resolves to cp before this decomposition runs. This reasoning is not verified against the full test suite.

Lingua principale
Python
Stelle
29
Fork
33
Merge medio
1g 22h
PR unite (30g)
16

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