Projective measurements¶
See also
This content is adapted from the work of Timothée Dao; “Informationally Complete Generalized Measurements for Estimating Quantum Expectation Values” [Unpublished master’s thesis]; ETH Zürich (2023).
Measurement in an arbitrary basis¶
Consider a
where we used the invariance of the trace under cyclic permutations in the last equality. It becomes now clear that the two procedures described below are equivalent:
Procedure 1A |
Procedure 1B |
---|---|
|
|
2. Measure in the basis
|
2. Let the state evolve as
|
3. Measure in the computational basis |
This equivalence is relevant in many practical situations. In experiments, one can often only perform measurements in a single, fixed (computational) basis but can apply various unitary transformations to the state before the measurement. Therefore, through this equivalence, one can emulate other projective measurements.
- Example:
Consider a qubit system and suppose we only have an apparatus performing measurements in the computational basis,
. We can still perform an measurement, , by applying the Hadamard transformation(6)¶to the state and then performing a measurement in the computational basis. Indeed, we have
and .
PM-simulable measurements¶
We can extend the Procedures 1A and 1B to PM-simulable POVMs, which can
always be achieved by a randomization technique. Suppose we want to perform the
measurement associated with the POVM
Procedure 2A |
Procedure 2B |
---|---|
1. Prepare state
|
|
2. Randomly pick |
2. Randomly pick |
3. Measure in the basis
|
3. Let the state evolve as
|
4. Measure in the computational basis |
Note that usually, not all unitary operations are achievable in
practice. Therefore, instead of starting from the POVM we would ideally
like to perform, we usually first define the set of achievable unitary
operations