| """ | |
| Common state-preparation circuits, returned as gate-tuple lists ready to | |
| feed straight into `run_circuit` (or to concatenate with more gates first). | |
| The GHZ-state snippet in particular -- `[('h', 0), ('cx', 0, 1), ('cx', 1, | |
| 2), ...]` -- shows up hand-written at the top of practically every | |
| experiment and test script built on this package; `ghz_state` is that | |
| snippet, written once. | |
| """ | |
| from ..circuits.topology import entangling_layer | |
| __all__ = ['ghz_state'] | |
| def ghz_state(n_qubits): | |
| """ | |
| Build the GHZ-state preparation circuit: | |
| (|00...0> + |11...1>) / sqrt(2). | |
| Implementation: H on qubit 0, then a linear CX chain (qubit 0 -> 1, | |
| 1 -> 2, ..., n-2 -> n-1) propagating the superposition outward -- | |
| reuses `entangling_layer(n_qubits, pattern='linear')` for the chain. | |
| Parameters | |
| ---------- | |
| n_qubits : int | |
| Number of qubits, must be >= 2 (a single qubit has no partner to | |
| entangle with, so an n=1 "GHZ state" is undefined here). | |
| Returns | |
| ------- | |
| list[tuple] | |
| e.g. ghz_state(3) == [('h', 0), ('cx', 0, 1), ('cx', 1, 2)] | |
| Examples | |
| -------- | |
| >>> import dense_evolution as de | |
| >>> sim = de.DenseSVSimulator(3) | |
| >>> sim.run_circuit(de.ghz_state(3)) | |
| >>> sim.get_probabilities()[[0, 7]] # |000> and |111>, each 0.5 | |
| """ | |
| if n_qubits < 2: | |
| raise ValueError(f"ghz_state needs at least 2 qubits, got {n_qubits}") | |
| return [('h', 0)] + entangling_layer(n_qubits, pattern='linear', gate='cx') | |