Measurements

When you call measurement functions such as std.quantum.measure in Guppy, the value they return has type Measurement. This is a copyable type that you can pass around your program until you need the measurement as a boolean, at which point you should call read on it. This will block program execution until the value is available.

Separating the point in your program where you request a measurement from the point where you explicitly call read can give the underlying runtime more opportunities for parallelism and can therefore improve performance.

from guppylang import guppy
from guppylang.std.builtins import owned
from guppylang.std.quantum import measure, qubit

@guppy
def deferred_measurement(q: qubit @ owned) -> None:
    m = measure(q) # measurement requested here
    #   ● 
    #   │
    #   │  <- other quantum operations can be parallelised
    #   │     here while this measurement resolves
    #   ▼
    output("q", m.read()) # result needed here at the latest - blocks until ready

deferred_measurement.check()

For convenience, boolean coercion via __bool__ is implemented on the type, so read is called automatically in conditionals.

@guppy
def conditional_output(q: qubit @ owned) -> None:
    if measure(q):
        output("true", 1)
    else:
        output("false", 0)

conditional_output.check()

For reading an array of measurements, use collect_measurements.

from guppylang.std.builtins import array
from guppylang.std.quantum import measure_array, collect_measurements

@guppy
def output_array(qs: array[qubit, 10] @ owned) -> None:
    ms = measure_array(qs)
    # [... more quantum operations ...]
    output("qs", collect_measurements(ms))

output_array.check()