QRACK_DISABLE_QUNIT_FIDELITY_GUARD in README.md

README.md

@@ -259,6 +259,8 @@ Environment variable `QRACK_NONCLIFFORD_ROUNDING_THRESHOLD` sets the non-Cliffor
 
 For single-qubit depolarizing noise channels, `QInterfaceNoisy` exposes environment variable `QRACK_GATE_DEPOLARIZATION`, which is a per-gate, single-qubit (applied to all qubits in gate) noise parameter for depolarizing noise, typically taking a floating-point value between `0` and `1`.
 
+When `QUnit` encounters a situation where acting a coupler gate would require exceeding user-specified maximum system memory resources, it will automatically resort to replacing this gate with a "classical shadow" that will not require additional allocation. In the process, `QUnit` will decrease its internal first-principles unitary fidelity estimation under assumption that the classical shadow replacement effect is worst-case. If the fidelity estimate drops below floating-point rounding epsilon, `QUnit` will automatically `throw` to exit simulation early. However, particularly in cases of "anti-concentration," such as with universal random circuit sampling, this first-principles worst-case fidelity estimate might be very overly severe. In these cases, the worst-case fidelity estimate cannot necessarily be trusted not to be overly pessimistic, but the `throw` condition on low fidelity estimate can be disabled by assigning any value to environment variable `QRACK_DISABLE_QUNIT_FIDELITY_GUARD`.
+
 ## Vectorization optimization
 
 ```sh