# Measurement bitstrings to detection event bitstrings in stim

Does Stim have exposed functionality for converting a measurement outcome string into a detection event string?

I.e. I feed stim a string of 0,1's corresponding to measurements in a circuit along with the circuit itself, and it spits out a string of 0,1's corresponding to the detection events in that circuit? Of course this functionality exists under the hood, but just wondering where and how it's accessible.

You can use stim.Circuit.compile_m2d_converter to get an object that converts measurements into detection event data (and optionally also observable flip data). The actual conversion is then done with the method stim.CompiledMeasurementsToDetectionEventsConverter.convert for in-memory results or stim.CompiledMeasurementsToDetectionEventsConverter.convert_file for on-disk results.

This functionality is also available from the command line using stim m2d.

Example:

import stim

circuit = stim.Circuit("""
X 0
X_ERROR(0.1) 0 1
M 0 1
DETECTOR rec[-1]
DETECTOR rec[-2]
OBSERVABLE_INCLUDE(0) rec[-1] rec[-2]
""")
measurements = circuit.compile_sampler().sample(shots=100)

#############################################
#### Convert from measurements to detection events and observable flips.
#############################################
converter = circuit.compile_m2d_converter()
dets_and_obs = converter.convert(measurements=measurements, append_observables=True)

mes0 = measurements[:, 0]
mes1 = measurements[:, 1]
det0 = dets_and_obs[:, 0]
det1 = dets_and_obs[:, 1]
obs0 = dets_and_obs[:, 2]
print("mes0: ", "".join("01"[int(e)] for e in mes0))
print("mes1: ", "".join("01"[int(e)] for e in mes1))
print("det0: ", "".join("_!"[int(e)] for e in det0))
print("det1: ", "".join("_!"[int(e)] for e in det1))
print("obs0: ", "".join("_!"[int(e)] for e in obs0))


Example output:

mes0:  1111111111111001011111101010111111111111101101110111111111111111111111111111110111111011111111111011
mes1:  0000000100010000110000000000000000000100000000000000000001000000000000100100000000000000000000001100
det0:  _______!___!____!!___________________!___________________!____________!__!______________________!!__
det1:  _____________!!_!______!_!_!_____________!__!___!_____________________________!______!___________!__
obs0:  _______!___!_!!__!_____!_!_!_________!___!__!___!________!____________!__!____!______!__________!___


Note that creating the converter is as expensive as sampling measurements; it involves performing a stabilizer simulation of the circuit to get a reference sample (because otherwise it's unknown if the measurement set defining a detector is supposed to have even or odd parity).

• This is great! And what are the corresponding commands in the C++ API?
– MGN
Oct 27, 2021 at 21:46
• @MGN The relevant method is stim::measurements_to_detection_events. You can see examples of it being used in src/stim/simulators/measurements_to_detection_events.test.cc. Oct 27, 2021 at 22:09