Quantum Backtracking#

class QuantumBacktrackingTree(max_depth, branch_qv, accept, reject, subspace_optimization=False)[source]#

This class describes the central data structure to run backtracking algorithms in a quantum setting. Backtracking algorithms are a very general class of algorithms which cover many problems of combinatorial optimization such as 3-SAT or TSP.

Backtracking algorithms can be put into a very general form. Given is a maximum recursion depth, two functions called accept/reject and the set of possible assignments for an iterable x.

from problem import accept, reject, max_depth, assignments

def backtracking(x):

    if accept(x):
        return x

    if reject(x) or len(x) == max_depth:
        return None

    for j in assigments:
        y = list(x)
        res = backtracking(y)
        if res is not None:
            return res

The power of these algorithms lies in the fact that they can quickly discard large parts of the potential solution space by using the reject function to cancel the recursion. Compared to an unstructured search, where only the accept function is available, this can significantly cut the required resources.

The quantum algorithm for solving these problems has been proposed by Ashley Montanaro and yields a 1 to 1 correspondence between an arbitrary classical backtracking algorithm and it’s quantum equivalent. The quantum version achieves a quadratic speed up over the classical one.

The algorithm is based on performing quantum phase estimation on a quantum walk operator, which traverses the backtracking tree. The core algorithm returns “Node exists” if the 0 component of the quantum phase estimation result has a higher probability then 3/8 = 0.375.

Similar to the classical version, for the Qrisp implementation of this quantum algorithm, a backtracking problem is specified by a maximum recursion depth and two functions, each returning a QuantumBool respectively:

accept: Is the function that returns True, if called on a node, satisfying the specifications.

reject: Is the function that returns True, if called on a node, representing a branch that should no longer be considered.

Furthermore required is a QuantumVariable that specifies the branches that can be taken by the algorithm at each node.

Node encoding

An important aspect of this algorithm is the node encoding. In Montanaros paper a central quantity is the distance from the root \(l(x)\). This however doesn’t generalize well to the specification of subtrees, which is why we encode the height of a node. For example in a tree with maximum depth \(n\) a leaf has height 0 and the root has height \(n\).

This quantity is encoded as a one-hot integer QuantumVariable, which can be found under the attribute h.

To fully identify a node, we also need to specify the path to take starting at the root. This path is encoded in a QuantumArray, which can be found under the attribute branch_qa. To fit into the setting of height encoding, this array contains the reversed path.

We summarize the encoding by giving an example:

In a binary tree with depth 5, the node that has the path from the root [1,1] is encoded by

\[\begin{split}\begin{align} \ket{\text{branch_qa}} &= \ket{0}\ket{0}\ket{0}\ket{1}\ket{1}\\ \ket{\text{h}} &= \ket{3} = \ket{00010}\\ \ket{x} &= \ket{\text{branch_qa}}\ket{\text{h}} \end{align}\end{split}\]

Details on the predicate functions

The predicate functions accept and reject must meet certain conditions for the algorithm to function properly:

  • Both functions have to return a QuantumBool.

  • Both functions must not change the state of the tree.

  • Both functions must delete/uncompute all temporarily created QuantumVariables.

  • accept and reject must never return True on the same node.

The subspace_optimization keyword enables a significant optimization of the quantum_step function. This keyword can be set to True if the reject function is guaranteed to return the value reject(x) also on the non-algorithmic subspace of x. For instance, if x = [0,0,1] in a depth 4 tree, the encoded state is

\[\begin{split}\begin{align} \ket{\text{branch_qa}} &= \ket{0}\ket{1}\ket{0}\ket{0}\\ \ket{\text{h}} &= \ket{1}\\ \ket{x} &= \ket{\text{branch_qa}}\ket{\text{h}} \end{align}\end{split}\]

A state from the non-algorithmic subspace of x is now a state that has non-zero entries in branch_qa at indices less than h ie.

\[\begin{split}\begin{align} \ket{\text{branch_qa}_{NA}} &= \ket{1}\ket{1}\ket{0}\ket{0}\\ \ket{\text{h}} &= \ket{1}\\ \ket{\tilde{x}} &= \ket{\text{branch_qa}_{NA}}\ket{\text{h}} \end{align}\end{split}\]

For the subspace_optimization to return proper results, the reject function must therefore satisfy:

\[\text{reject}(\ket{x}) = \text{reject}(\ket{\tilde{x}})\]


Many implementations of backtracking also include the possibility for deciding which entries of x to assign based on some user provided heuristic. The quantum version also supports this feature, however it is not yet implemented in Qrisp.


The depth of the backtracking tree.


A QuantumVariable representing the possible branches of each node.


A function taking an instance of QuantumBacktrackingTree and returning a QuantumBool, which is True, if called on a satisfying node.


A function taking an instance of QuantumBacktrackingTree and returning a QuantumBool, which is True, if a called on a node where the corresponding branch should no longer be investigated.

subspace_optimizationbool, optional

If set to True, a significant optimization of the quantum_step function will be applied. The reject function has to fullfil a certain property for this to yield the correct results. Please check the “Details on the predicate functions” section for more information. The default is False.


Checking for the existence of a solution

Even though primary purpose of backtracking algorithms is to find a solution, at the core, Montanaros algorithm only determines solution existence. This can however still be leveraged into a solution finding algorithm.

To demonstrate the solution existence functionality, we search the binary tree that consists only of nodes with alternating branching. We accept if we find the node [0,0,1] (doesn’t exist in this tree).

For this we first set up the reject condition.

from qrisp import *

def reject(tree):

    oddity = QuantumBool()
    for i in range(tree.h.size):
        if i%2:
            cx(tree.h[i], oddity)

    parity = QuantumBool()

    for i in range(tree.branch_qa.size):
        cx(tree.branch_qa[i], parity)

    exclude_init = (tree.h < tree.max_depth-1)

    return exclude_init & (oddity != parity)

This function determines first determines the oddity of the height parameter (remember tree.h has one-hot encoding!). Next the parity of the branching path is evaluated. Parity means “is the amount of ones in the path even or odd”.

We will reject the node if the oddity of is unequal to the parity and therefore reject any path that took a 1 after it already took a 1 (same for 0).

On the root and it’s children there will be rejection to allow for two different paths.

We now implement the accept condition:

def accept(tree):

    height_condition = (tree.h == 0)

    path_condition = QuantumBool()
    mcx(tree.branch_qa[::-1], path_condition, ctrl_state = "001")

    return height_condition & path_condition

Subsequently we set up the class instance:

from qrisp.quantum_backtracking import QuantumBacktrackingTree

tree = QuantumBacktrackingTree(max_depth = 3,
                               branch_qv = QuantumFloat(1),
                               accept = accept,
                               reject = reject)


We can evaluate the statevector:

>>> tree.statevector()

The [] indicates that this is the root state. If the tree was in the state of a child of the root (say the one connected to the 1 branch) it would be [1].

Note that the statevector method decodes the QuantumVariables holding the node state for convenient readibility. If you want to see the encoded variables you can take a look at the QuantumSession s statevector method:

>>> tree.qs.statevector()

We can also visualize the statevector of the tree:

>>> import matplotlib.pyplot as plt
>>> tree.visualize_statevector()
>>> plt.show()
Root statevector plot

And finally evaluate the algorithm:

qpe_res = tree.estimate_phase(precision = 4)

Perform a measurement

>>> mes_res = qpe_res.get_measurement()
>>> mes_res[0]

The 0 component has only 2.53% probability of appearing, therefore we can conclude, that in the specified tree no such node exists.

We now perform the same process but with a trivial reject function:

def reject(tree):
    return QuantumBool()

tree = QuantumBacktrackingTree(max_depth = 3,
                               branch_qv = QuantumFloat(1),
                               accept = accept,
                               reject = reject)


qpe_res = tree.estimate_phase(precision = 4)
>>> mes_res = qpe_res.get_measurement()
>>> mes_res[0]

We see a probability of more than 50%, implying a solution exists in this tree.

Finding a solution

Montanaros approach to determine a solution is to classically traverse the tree, by always picking the child node where the quantum algorithm returns “Node exists”. Finding a solution can therefore be considered a hybrid algorithm.

To demonstrate, we search for the node [1,1,1] with a trivial reject function.

def accept(tree):
    height_condition = (tree.h == tree.max_depth - 3)
    path_condition = QuantumBool()
    mcx(tree.branch_qa[-3:], path_condition)

    return height_condition & path_condition

def reject(tree):
    return QuantumBool()

Set up the QuantumBacktrackingTree instance:

>>> max_depth = 4
>>> tree = QuantumBacktrackingTree(max_depth,
                                   branch_qv = QuantumFloat(1),
                                   accept = accept,
                                   reject = reject)

And call the solution finding algorithm:

>>> tree.find_solution(precision = 5)
[1, 1, 1]

Using the subspace_optimization keyword

To demonstrate the usage of this feature, we create two tree instances - one with and one without the optimization.

def accept(tree):
    return QuantumBool()

def reject(tree):
    return QuantumBool()
>>> opt_tree = QuantumBacktrackingTree(3, branch_qv = QuantumFloat(1), accept = accept, reject = reject, subspace_optimization = True)
>>> no_opt_tree = QuantumBacktrackingTree(3, branch_qv = QuantumFloat(1), accept = accept, reject = reject, subspace_optimization = False)

Perform a quantum_step on both of them:

>>> opt_tree.quantum_step()
>>> no_opt_tree.quantum_step()

And evaluate some benchmarks:

>>> no_opt_tree.qs.compile().depth()
>>> no_opt_tree.qs.compile().cnot_count()

With the optimization these values are much better:

>>> opt_tree.qs.compile().depth()
>>> opt_tree.qs.compile().cnot_count()

A one hot encoded integer representing the height of the node. The root has h = max_depth, it’s children have h = max_depth-1 etc.


A QuantumArray representing the path from the root to the current node. The qtype of this QuantumArray is what is been provided as branch_qv. Note that the state of this array is the reversed path, ie. a the node with path [1,1,0,1] in a depth 7 tree has the state: \(\ket{0}\ket{0}\ket{0}\ket{1}\ket{0}\ket{1}\ket{1}\) States that have a non-zero value at entries indexed smaller than h, are considered non-algorithmic and will never be visited (eg. h=3, branch_qa = \(\ket{1}\ket{1}\ket{1}\ket{1}\)).


The QuantumSession of the backtracking tree.


An integer specifying the maximum depth of each node.


Tree quantum operations#


Performs the operators \(R_A\) or \(R_B\).

QuantumBacktrackingTree.quantum_step([ctrl, ...])

Performs the quantum step operator.


Performs quantum phase estimation on the quantum step operator.


Determines a path to a solution.


Initializes the normalized version of the state \(\ket{\phi}\).


Initializes the state of a given node.

Tree evaluation#


Returns a SymPy statevector object representing the state of the tree with decoded node labels.


Visualizes the statevector graph.


Returns a NetworkX Graph representing the quantum state of the backtracking tree.



Returns the subtree of a given node.


Returns a copy of self.

QuantumBacktrackingTree.path_decoder(h, ...)

Returns the path representation for a given constellation of the h and branch_qa variables.