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Background:

I have a function $f(s_i, s_f, x)$ where $s_i \in \{0,1,2,3\}; \quad x,s_f \in \{0,1\}$ which is defined as:

$$ f(s_i, s_f, x) = \begin{cases} 1, & \text{if } (s_i, s_f, x) \in\{(0,0,0),(1,0,1),(2,1,1),(3,1,0)\}\\ 0, & \text{otherwise} \end{cases} $$

also represented by this image:

enter image description here

The function can also be represented as a family of matrices by choosing a particular set of arguments to index the matrix. For example, one representation (let's call this Rep. 1) is to index the rows by $s_f$ and the columns by $x$, giving a set of matrices parametrized by $s_i$:

$$ \begin{equation} f(s_i, s_f, x)= \begin{cases}{\left[\begin{array}{ll} 1 & 0 \\ 0 & 0 \end{array}\right], s_i=0 \quad ;\left[\begin{array}{ll} 0 & 1 \\ 0 & 0 \end{array}\right], s_i=1} \\ {\left[\begin{array}{ll} 0 & 0 \\ 0 & 1 \end{array}\right], s_i=2 \quad ;\left[\begin{array}{ll} 0 & 0 \\ 1 & 0 \end{array}\right], s_i=3}\end{cases} \end{equation} $$

A second matrix representation (Rep. 2) is using $s_f$ for rows, $s_i$ for columns and parametrised by $x$ as follows:

$$ f(s_i, s_f, x)=\left\{\begin{array}{lll} \left(\begin{array}{llll} 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 \end{array}\right), & x=0 \\ \left(\begin{array}{llll} 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \end{array}\right), & x=1 \end{array}\right. $$

Query:

I want to encode the (non-unitary) matrices in Rep. 1 in such a way that we obtain a block unitary matrix, which can then be represented a gate acting on a quantum circuit.

Any kind of unitary is allowed, (such as encoding in a larger block to obtain some form of a controlled gate) which implements the function $f$, although it would be good if $x$ and $s_f$ are not grouped together in the indexing (since I require $x$ at the 'input' and $s_f$ at the 'output' of the gate)

Illustration: As an example of what I am looking for, the matrices in Rep. 2 can be denoted as a unitary by indexing the rows with the ordered pair $(x,s_f)$ and the columns by $s_i$ to obtain the following matrix:

Matrix of Rep. 2

However, as I said, this form does not let me have $x$ as an input to the gate.

Summary: In general, is there any way of encoding the non-unitary projector matrices into a unitary matrix. If not, is there an explanation of why the matrices in Rep. 1 cannot be grouped to obtain a unitary? Personally, I feel that it is not possible to have a unitary encoding which implements the function AND satisfies the restriction on $x$ and $s_f$, since any matrix obtained is sparse with very few non-zero elements. Any help on this is appreciated.

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  • $\begingroup$ I'm not quite sure how you envisage $x$ to be an input and $s_f$ an output (and where has $s_i$ gone?) if the function has inputs $x,s_i,s_f$. If I think of this as a function evaluation of a different function (inputs $x$ and $outputs $s_f$) there might be inputs with no output (which doesn't make sense). $\endgroup$
    – DaftWullie
    Commented May 17 at 7:02
  • $\begingroup$ @DaftWullie This is based on the 'normal factor graph' model (ref) or perhaps the more familiar tensor network model . Think of the function $f$ as a graph node and its arguments $s_i,s_f,x$ as the 'wires' connected to it. Each wire take the same values as its corresponding variable. Depending on the value at each wire, the function returns a value. In our example, our function returns $1$ if the values at the wires satisfy the definition and $0$ otherwise. $\endgroup$
    – Enigma
    Commented May 18 at 18:12
  • $\begingroup$ This method can be used to denote a matrix (or a family of matrices). Thus, it can then be used to represent a unitary matrix (quantum gate) with the argument variables as wires connected to it. We have the freedom to choose where to connect each wire (as the input, output, control, etc.) For eg., the matrix of Rep. 2 can be considered a gate which has $s_i$ to the left (as input) and $(x,s_f)$ to the right. I want a similar unitary form for the matrices in Rep. 1 ideally with $x$ and $s_f$ separated. ($s_i$ is there too, I just do not have any specific restriction on it). $\endgroup$
    – Enigma
    Commented May 18 at 18:15
  • $\begingroup$ You can always embed a contraction $C$ ($\|C\|\leq 1$) into to the "upper corner" of a unitary via its Halmos dilation $\begin{pmatrix} C & \sqrt{(1-CC^*)} \\ \sqrt{(1-C^*C)} & -C^*\end{pmatrix}$, not sure if this is what you want though... $\endgroup$
    – Condo
    Commented May 21 at 19:22

1 Answer 1

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For Rep. 1 you can use a Linear Combination of Unitaries (LCU) for block encoding each of your projections, and then use a control statement according to the the $s_i$ variable. If we designate the four projections by $P_0$, $P_1$, $P_2$, and $P_3$ then: $$ \begin{align} P_0 = 1/2(I+Z),\\ P_1 = 1/2(X+iY),\\ P_2 = 1/2(I-Z),\\ P_3 = 1/2(X-iY), \end{align} $$ where $I$, $X$, $Z$, $Y$ is the Pauli basis. Here is how to code your matrix using Classiq python SDK package:

1. Declare a quantum function that block encodes an equal superposition of two unitaries $\frac{1}{2}(U_1+U_2)$

The following function applies $U_1+U_2$ given that the prepare_qbit is at state 0.

from classiq import *
@qfunc
def lcu2_encoding(select_qfuncs: QCallableList, prepare_qbit: QBit):
    within_apply(
        lambda: H(prepare_qbit),
        lambda: repeat(2,
                       lambda i: control(prepare_qbit==i, lambda: select_qfuncs[i]()
                                        )
                      )
    )

2. Define the four projections using the LCU quantum function

from classiq.qmod.symbolic import pi
@qfunc
def si_0(x: QBit, aux: QBit):
    lcu2_encoding( [lambda: IDENTITY(x), lambda: Z(x)], aux)
@qfunc
def si_1(x: QBit, aux: QBit):
    lcu2_encoding( [lambda: X(x), lambda: (U(0,0,0,pi/2,x), Y(x))], aux)
@qfunc
def si_2(x: QBit, aux: QBit):
    lcu2_encoding( [lambda: IDENTITY(x), lambda: (RY(2*pi,x), Z(x))], aux)
@qfunc
def si_3(x: QBit, aux: QBit):
    lcu2_encoding( [lambda: X(x), lambda: (U(0,0,0,-pi/2,x), Y(x))], aux)

3. Define a function for the full matrix encoding

Here we select each of our four LCUs according the the $s_i$ state (note that each LCU has its own auxiliary):

@qfunc
def my_unitary_encoding(auxs: QArray[QBit], x: QBit, si: QNum):
    repeat(2**si.size,
           lambda i: switch(i, [lambda: control(si==i, lambda: si_0(x, auxs[0])),
                                lambda: control(si==i, lambda: si_1(x, auxs[1])),
                                lambda: control(si==i, lambda: si_2(x, auxs[2])),
                                 lambda: control(si==i, lambda: si_3(x, auxs[3]))]
                           )
          )

The functionality can be verified as follows:

We can define a model in which we initialize the $s_i$ variable to an equal superposition of $|i\rangle$ for $i=0,1,2,3$. Then we can check the value of $x$, given that the auxiliary qubits are at state zero.

@qfunc
def main(auxs: Output[QArray[QBit]], x:Output[QBit], si: Output[QNum]):
    allocate(4,auxs)
    allocate(1,x)
    # X(x) # uncomment here to verify s_i=1,3
    allocate_num(2,False, 0, si)
    hadamard_transform(si)
    my_unitary_encoding(auxs,x,si)
    

qmod = create_model(main)
qprog = synthesize(qmod)
show(qprog) # for vizualizing the circuit
results = execute(qprog).result()
for sample in results[0].value.parsed_counts:
    if sample.state["auxs"]==0:
        print(sample)

The output after running the above code is:

state={'auxs': 0.0, 'x': 1.0, 'si': 3.0} shots=544
state={'auxs': 0.0, 'x': 0.0, 'si': 0.0} shots=504

As we can see, we do not see results for $s_i=1,2$ as $P_1$, $P_2$ return zero projection for $x=|0\rangle$.

In the attached figure you can see how the circuit looks like. The multi-controlled operations use extra auxiliaries. You can diminish these by changing the preferences of the synthesis (e.g., optimizing the quantum model over width)

enter image description here

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