Adjusting UI for measurement on several modes

[antalszava]

Issue description

Strawberry Fields allows having multi-mode measurements in a Program by having measurement operators targetting several registers that reference the modes to be measured.

Within the definition of a program, this means having MeasurementOperator | (q[0], q[1], q[2], ...) , where q[0], q[1], ... refer to the registers of the modes to be measured.

Intuitively, one could want to define the same measurement by having consecutive measurement operators targetting a single register in the following fashion:

    MeasurementOperator | q[0]
    MeasurementOperator | q[1]
    MeasurementOperator | q[2]
    (...)

Specific example
Let's have a look at a specific example, where it is expected to see correlation between the outcomes for each mode

1. Running the following example:

prog = sf.Program(2)
eng = sf.Engine("gaussian")
with prog.context as q:
    S2gate(1) | (q[0], q[1])
    MeasureFock() | (q[0], q[1])
results = eng.run(prog, shots=5)
results.samples

can result in an output like:

array([[1, 1],
       [4, 4],
       [1, 1],
       [0, 0],
       [1, 1]])

2. This is in contrast with the following Program definition:

prog = sf.Program(2)
eng = sf.Engine("gaussian")
with prog.context as q:
    S2gate(1) | (q[0], q[1])
    MeasureFock() | q[0]
    MeasureFock() | q[1]
results = eng.run(prog, shots=5)
results.samples

which can result in an output like:

array([[1, 1],
       [0, 1],
       [1, 2],
       [3, 0],
       [2, 4]])

With the second example, mode q[0] gets reinitialized to the vacuum state after being measured.

• Expected behavior:
  The previous two code snippets both output samples where the samples for each run of the program are correlated.

• Actual behavior:
  There is a correlation between samples of the two modes for each run of the program only for the first code example.

• Reproduces how often:
  Each time.

• System information:

Strawberry Fields: a Python library for continuous-variable quantum circuits.
Copyright 2018-2020 Xanadu Quantum Technologies Inc.

Python version:            3.7.6
Platform info:             Linux-4.19.11-041911-generic-x86_64-with-debian-buster-sid
Installation path:         /anaconda3/lib/python3.7/site-packages/strawberryfields
Strawberry Fields version: 0.14.0
Numpy version:             1.18.4
Scipy version:             1.4.1
SymPy version:             1.5.1
NetworkX version:          2.3
The Walrus version:        0.12.0
Blackbird version:         0.2.3
TensorFlow version:        2.0.0

Additional information

This would most likely include a change in how compilers handle measurement operators and introduce logic so that compilers will automatically group measurements that are terminal.

[josh146]

Also worth mentioning that while this is unintuitive, it is also expected behaviour; the backend API was designed such that after a measurement operation, the mode is always traced out and replaced with the vacuum state. This allows the state to be re-used without needing to add additional modes to the system, which might be costly.

I propose that we simply modify the compilers such that terminal measurements of a program (e.g, those that are 'dangling' nodes of the DAG with no other operations depending on them) are compiled into a single measurement result.

That is,

Sgate(0.5) | q[0]
MeasureX  | q[0]
S2gate(q[0].par) | (q[0], q[1])
MeasureFock() | q[0]
Xgate(0.5) | q[2]
MeasureFock() | q[1]
MeasureX | q[2]

would be compiled to give

Sgate(0.5) | q[0]
MeasureX  | q[0]
S2gate(q[0].par) | (q[0], q[1])
Xgate(0.5) | q[2]
MeasureFock() | (q[0], q[1])
MeasureX | q[2]

where the two terminal fock measurements are combined, but the non-terminal MeasureX in the middle of the circuit is not.

[felipeoyarce]

Hi @antalszava and @josh146 ! I would like to work on this issue 🤗

[josh146]

Hi @felipeoyarce, nice! Feel free to open a WIP PR, and ask any questions as they arise :)