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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
use crate::{states::StateData::Mixed, StateData};
use crate::{Process, ProcessData, C64};
use num_traits::{One, Zero};
use rand::Rng;
use std::iter::Iterator;
use crate::linalg::Trace;
use crate::State;
use serde::{Deserialize, Serialize};
// TODO[design]: Instrument works pretty differently from State and Process; should
// likely refactor for consistency.
#[derive(Serialize, Deserialize, Debug)]
/// Represents a quantum instrument; that is, a process that accepts a quantum
/// state and returns the new state of a system and classical data extracted
/// from that system.
pub enum Instrument {
/// The effects of the instrument, represented as completely positive
/// trace non-increasing (CPTNI) processes.
Effects(Vec<Process>),
/// An instrument that measures a single qubit in the $Z$-basis, up to a
/// readout error (probability of result being flipped).
///
/// Primarily useful when working with stabilizer states or other
/// subtheories.
ZMeasurement {
/// Probability with which a result is flipped.
pr_readout_error: f64,
},
}
impl Instrument {
/// Samples from this instrument, returning the measurement result and
/// the new state of the system conditioned on that measurement result.
pub fn sample(&self, idx_qubits: &[usize], state: &State) -> (usize, State) {
match self {
Instrument::Effects(ref effects) => sample_effects(effects, idx_qubits, state),
Instrument::ZMeasurement { pr_readout_error } => {
if idx_qubits.len() != 1 {
panic!("Z-basis measurement instruments only supported for single qubits.");
}
let idx_target = idx_qubits[0];
match state.data {
StateData::Pure(_) | StateData::Mixed(_) => {
// Get the ideal Z measurement instrument, apply it,
// and then assign a readout error.
// TODO[perf]: Cache this instrument as a lazy static.
let ideal_z_meas = Instrument::Effects(vec![
Process {
n_qubits: 1,
data: ProcessData::KrausDecomposition(array![[
[C64::one(), C64::zero()],
[C64::zero(), C64::zero()]
]]),
},
Process {
n_qubits: 1,
data: ProcessData::KrausDecomposition(array![[
[C64::zero(), C64::zero()],
[C64::zero(), C64::one()]
]]),
},
]);
let (result, new_state) = ideal_z_meas.sample(idx_qubits, state);
let result = (result == 1) ^ rand::thread_rng().gen_bool(*pr_readout_error);
(if result { 1 } else { 0 }, new_state)
}
StateData::Stabilizer(ref tableau) => {
// TODO[perf]: allow instruments to sample in-place,
// reducing copying.
let mut new_tableau = tableau.clone();
let result = new_tableau.meas_mut(idx_target)
^ rand::thread_rng().gen_bool(*pr_readout_error);
(
if result { 1 } else { 0 },
State {
n_qubits: state.n_qubits,
data: StateData::Stabilizer(new_tableau),
},
)
}
}
}
}
}
// TODO: Add more methods for making new instruments in convenient ways.
/// Returns a serialization of this instrument as a JSON object.
pub fn as_json(&self) -> String {
serde_json::to_string(&self).unwrap()
}
}
fn sample_effects(effects: &[Process], idx_qubits: &[usize], state: &State) -> (usize, State) {
let mut possible_outcomes = effects
.iter()
.enumerate()
.map(|(idx, effect)| {
let output_state = effect.apply_to(idx_qubits, state).unwrap();
let tr = (&output_state).trace();
(idx, output_state, tr.norm())
})
.collect::<Vec<_>>();
// TODO[perf]: Downgrade this to a debug_assert!, and configure the CI
// build to enable debug_assertions at full_validation.
assert!(
possible_outcomes.iter().any(|post_state| post_state.1.trace().norm() >= 1e-10),
"Expected output of applying instrument to be nonzero trace.\nInstrument effects:\n{:?}\n\nInput state:\n{}\n\nPostselected states:\n{:?}",
effects, state, possible_outcomes
);
let mut rng = rand::thread_rng();
let random_sample: f64 = rng.gen();
for (idx, cum_pr) in possible_outcomes
.iter()
.scan(0.0f64, |acc, (_idx, _, pr)| {
*acc += *pr;
Some(*acc)
})
.enumerate()
{
if random_sample < cum_pr {
// In order to not have to copy the output state, we need
// to be able to move it out from the vector. To do so,
// we retain only the element of the vector whose index
// is the one we want and then pop it, leaving an empty
// vector (that is, a vector that owns no data).
possible_outcomes.retain(|(i, _, _)| idx == *i);
let (_, mut output_state, tr) = possible_outcomes.pop().unwrap();
if tr.abs() >= 1e-10 {
if let Mixed(ref rho) = output_state.data {
output_state.data = Mixed(rho * (1.0f64 / tr));
} else {
panic!("Couldn't renormalize, expected mixed output from instrument.");
}
}
assert!(
(output_state.trace() - 1.0).norm() <= 1e-10,
"Expected output of instrument to be trace 1."
);
return (idx, output_state);
}
}
let (idx, output_state, _) = possible_outcomes.pop().unwrap();
(idx, output_state)
}