Note
Click here to download the full example code
Measurements and postselection¶
In this tutorial, we will walk through the use of measurement operators in Strawberry Fields, and how they may be used to perform postselection. Be sure to read through the introductory teleportation tutorial before attempting this tutorial.
Measurement operators¶
The Blackbird programming language supports the following measurement operations:
Measurement 
Operation 
Shortcuts 
Backend Support 

Homodyne detection of quadrature angle \(\phi\) 



Heterodyne detection 



Photoncounting 
None 

Note
While all backends support homodyne detection, the Gaussian backend is the only backend to support heterodyne detection. On the other hand, Fockbasis measurements are supported in all backends, though the Gaussian backend does not update the postmeasurement quantum state, which would be nonGaussian.
The measurement operators are used in the same manner as all other quantum transformation operations in Blackbird:
MeasurementOperator  (q[0], q[1], q[2], ...)
where the lefthand side represents the measurement operator (along with any required or optional arguments), and the righthand side signifies the modes which are to be measured.
To see how this works in practice, consider the following circuit, where two incident Fock states \(\ket{n}\) and \(\ket{m}\) are directed on a beamsplitter, with two photon detectors at the output modes.
Due to the definition of the beamsplitter, we know that it preserves the photon number of the system; thus, the two output states \(\ket{n'}\) and \(\ket{m'}\) must be such that \(n+m=n'+m'\).
Constructing this circuit in Strawberry Fields with \(n=2,~m=3\), let’s perform only the first Fock measurement.
import numpy as np
# set the random seed
np.random.seed(42)
import strawberryfields as sf
from strawberryfields.ops import *
prog = sf.Program(2)
eng = sf.Engine("fock", backend_options={"cutoff_dim": 6})
with prog.context as q:
Fock(2)  q[0]
Fock(3)  q[1]
BSgate()  (q[0], q[1])
MeasureFock()  q[0]
results = eng.run(prog)
Note
If the BSgate
parameters are not specified, by default a 5050
beamsplitter BSgate(pi/4,0)
is applied.
The default action after every measurement is to reset the measured modes to the vacuum state.
However, we can extract the measured value of mode q[0]
via the results
object returned by
the engine after it has finished execution:
print(results.samples[0][0])
Out:
1
Note
Since measurement is a stochastic process, your results might differ when executing this code.
Since no measurement has yet been applied to the second mode, results.samples[0][1]
will return
None
.
print(results.samples)
Out:
[[1 None]]
Therefore, we know that, to preserve the photon number, q[1]
must be in the state
\(\ket{m+nk}\), where \(m\) and \(n\) are the photon numbers of the initial states and
\(k\) is value returned in result.samples
. Executing the backend again, and this time
applying the second Fock measurement:
prog2 = sf.Program(2)
with prog2.context as q:
MeasureFock()  q[1]
results = eng.run(prog2)
We will find that the second measurement yields \(m+nk\). In this case, we get
print(results.samples[0][1])
Out:
4
Postselection¶
In addition, StrawberryFields also allows the specification or postselection of a required measurement
output, and will condition the remaining unmeasured modes based on this postselected value. When
applying the measurement operators, the optional keyword argument select
can be passed to the
operator. The value should be an integer (or list of integers) for
MeasureFock
, a float for
MeasureHomodyne
, and a complex value for
MeasureHeterodyne
.
For example, we can rewrite the example above using postselection:
prog = sf.Program(2)
eng = sf.Engine("fock", backend_options={"cutoff_dim": 6})
with prog.context as q:
Fock(2)  q[0]
Fock(3)  q[1]
BSgate()  (q[0], q[1])
MeasureFock(select=0)  q[0]
MeasureFock()  q[1]
result = eng.run(prog)
Since we are postselecting a measurement of 0 photons in mode q[0]
, we expect
result.samples[0]
to be 0
and result.samples[1]
to be 5
. Indeed,
print(result.samples)
Out:
[[0 5]]
Warning
If we attempt to postselect on Fock measurement results that have zero probability given the
circuit/state of the simulation, the Fock backend returns a ZeroDivisionError
. For example,
in the previous code snippet, if we instead attempt to postselect two values that do not
preserve the photon number,
>>> eng.run("fock", cutoff_dim=6, select=[1,2])
ZeroDivisionError: Measurement has zero probability.
This check is provided for convenience, but the user should always be aware of postselecting on zeroprobability events. The current implementation of homodyne measurements in the Fock backend does not currently perform this check.
Example¶
Consider the following circuit:
Here, we have two vacuum states incident on a twomode squeezed gate. Homodyne detection in the
\(x\) quadrature of the first output mode is then performed; as a result, the output mode
q[1]
is conditionally displaced depending on the measured value.
We can simulate this conditional displacement using postselection. Utilizing the Gaussian backend, the above circuit can be simulated in Strawberry Fields as follows:
prog = sf.Program(2)
eng = sf.Engine("gaussian")
with prog.context as q:
S2gate(1)  (q[0], q[1])
MeasureHomodyne(0, select=1)  q[0]
state = eng.run(prog).state
To check the displacement of the second output mode, we can use the
reduced_gaussian()
state method to extract the
vector of means and the covariance matrix:
mu, cov = state.reduced_gaussian([1])
The vector of means contains the mean quadrature displacements, and for a single mode is of the form
\(\bar{\mathbf{r}} = (\bar{\mathbf{x}}, \bar{\mathbf{p}})\). Therefore, looking at the first
index of the vector of means for q[1]
:
print(mu[0])
Out:
0.9640275698261913
The \(x\) quadrature displacement of the second mode is conditional to the postselected value in the circuit construction above.
Measurement control and processing¶
In addition to the features already explored above, Strawberry Fields also allows the measurement results of qumodes to be used as subsequent gate parameters. This is simple and intuitive as well  simply pass the register referencing the measured mode as the gate argument, for example like
MeasureX  q[0]
Rgate(q[0])  q[1]
and the Strawberry Fields engine will, in the background, ensure that the measured value of that mode is used as the gate parameter during the circuit simulation.
Note that, the return type of the measurement determines the parameter type, potentially restricting the resulting gates which can be measurementcontrolled.
Measurement 
Return type 
Gates with matching parameter type 

Real number 
All 

Complex number 

Integer 
All 
Classical processing¶
Sometimes, additional classical processing needs to be performed on the measured value before using
it as a gate parameter; Strawberry Fields provides the ability to perform simple classical
processing by simply accessing the .par
attribute of the measured parameter.
These only need to be used when passing a measured mode value as a gate parameter. For example, if we wish to perform a Fock measurement on a mode, and then use the measured value to perform a controlled displacement on other modes, we could do the following:
prog = sf.Program(2)
with prog.context as q:
MeasureFock()  q[0]
Dgate(q[0].par ** 2)  q[1]
In addition, the Strawberry Fields maths module can be used to transform measured mode values within gate arguments:
prog = sf.Program(2)
with prog.context as q:
MeasureFock()  q[0]
Dgate(sf.math.cos(q[0].par) + 2)  q[1]
Note
Only use the functions with sf.math
within the Blackbird code/program context.
Outside of the program context, you may use standard Python/NumPy/TensorFlow
math functions.
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