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`pow(k) @` modifier rejects non-integer exponents

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Issue 类型
功能
描述清晰度
基本清楚
活跃度
冷清
技术栈
python
领域
quantum-computing

调研方向

Start with the power-modifier check near src/pyqasm/visitor.py:1627, where the integer-only validation and the existing negative-integer inversion live. Then look at src/pyqasm/linalg.py and src/pyqasm/decomposer.py to see how a single-qubit matrix is turned into U(θ, φ, λ). Done means pow(0.5) @ x matches the sx matrix, pow(0.25) @ x and pow(1.5) @ z work, and unsupported multi-qubit or runtime-valued exponents raise a named ValidationError, with tests in tests/qasm3/test_gate.py.

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描述

enhancement llm-assisted qasm3 qasm3-coverage

Limitation

The power modifier is defined as: "The modifier
pow(k) @ replaces its gate argument U by its kth power U^k for some positive integer
or floating point number k (not necessarily constant)."

pyqasm requires k to be an integer and rejects everything else.

Example QASM failure

OPENQASM 3.0;
include "stdgates.inc";
qubit[1] q;
pow(0.5) @ x q[0];

ValidationError: Power modifier argument must be an integer in gate operation QuantumGate(...)

Integer exponents, including negative ones, work today:

pow(2) @ x q[0];    // OK
pow(-1) @ x q[0];   // OK

so the restriction is specifically on the fractional case — which is the interesting one, since
pow(0.5) @ x is the square root of X.

Change Requested

  1. Accept a floating-point k in pow(k) @.
  2. For a fractional k, the repeat-the-gate expansion used for integers does not apply. The
    unrolled result must be computed from the gate's matrix as U^k via matrix power — the
    principal branch, matching the spec's definition.
  3. Where a fractional power of a gate has a known closed form in the standard library, prefer
    emitting that named gate over a synthesised U(θ, φ, λ). For example pow(0.5) @ x is
    sx.
  4. If a fractional power cannot be realised for a given gate, raise a ValidationError naming
    the gate and the exponent — not the current blanket "must be an integer".

Implementation Details

  • The rejection is at src/pyqasm/visitor.py:1627. The surrounding block already handles
    negative integers by inverting and repeating, so the integer path is a useful reference for
    where the fractional branch belongs.
  • src/pyqasm/linalg.py and the Cython src/pyqasm/accelerate/linalg.pyx already provide the
    matrix machinery used by the decomposer; a matrix power via eigendecomposition fits there
    rather than in the visitor.
  • The single-qubit result can be routed through the existing ZYZ / U(θ, φ, λ) decomposition
    in src/pyqasm/decomposer.py, so the fractional case reduces to "compute the matrix, then
    decompose" and reuses tested code.
  • Decide the multi-qubit policy explicitly. A fractional power of a two-qubit gate is well
    defined mathematically but expensive to synthesise; rejecting it with a clear message is a
    reasonable first increment, provided the message says so.
  • The spec's "not necessarily constant" clause implies a runtime-valued exponent. That cannot
    be resolved during unrolling and is out of scope here — reject it with a specific message.
  • Tests: tests/qasm3/test_gate.py — pow(0.5) @ x against the sx matrix, pow(0.25) @ x,
    pow(1.5) @ z, a fractional power under ctrl @, and a clear error for the unsupported
    multi-qubit case.
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