In this paper, we investigate quantum algorithms for graph colouring problems, in particular for 2- and 3-colouring of graphs. Our main goal is to establish a set of quantum representations and operations suitable for the problem at hand. We propose unitary- as well as measurement-based quantum computations, also taking inspiration from answer set programming, a form of declarative programming close to traditional logic programming. The approach used is one in which we first generate arbitrary solutions to the problem, then constraining these according to the problem's input. Though we do not achieve fundamental speed-ups, our algorithms show how quantum concepts can be used for programming and moreover exhibit structural differences. For example, we compute all possible colourings at the same time. We compare our algorithms with classical ones, highlighting how the same type of difficulties give rise to NP-complete behaviour, and propose possible improvements.
In this paper, we investigate quantum algorithms for graph colouring problems, in particular for 2- and 3-colouring of graphs. Our main goal is to establish a set of quantum representations and operations suitable for the problem at hand. We propose unitary- as well as measurement-based quantum computations, also taking inspiration from answer set programming, a form of declarative programming close to traditional logic programming. The approach used is one in which we first generate arbitrary solutions to the problem, then constraining these according to the problem's input. Though we do not achieve fundamental speed-ups, our algorithms show how quantum concepts can be used for programming and moreover exhibit structural differences. For example, we compute all possible colourings at the same time. We compare our algorithms with classical ones, highlighting how the same type of difficulties give rise to NP-complete behaviour, and propose possible improvements.
Abstract Graph colouring with minimum colours is a well-known NP-complete problem in computer science and mathematics, having diverse real-life applications such as scheduling, register allocation, and frequency assignment. However, a classical computer requires exponential time to solve the graph colouring problem with the minimum number of colours. Therefore, classical approaches become infeasible for solving this problem on large graphs. In contrast, quantum computing offers the potential for exponential speed up on specific problems, although solutions are typically obtained with a certain probability of correctness. This work proposes a quantum-classical method for solving the graph colouring problem with guaranteed correctness. The proposed method is inspired by Grover’s quantum search algorithm, the Clifford circuit, and the classical binary search algorithm. Here, Grover’s algorithm helps to achieve a quadratic speed up, Clifford circuit helps to mitigate the errors, and binary search helps to find the chromatic number with guaranteed correctness in polynomial time. We also conducted several experiments to evaluate the proposed method's performance and correctness, and the experimental results corroborate our claims regarding the proposed method. Additionally, a mathematical analysis is performed to assess its time and qubit complexities, and to compare its efficiency with existing methods.
The graph colouring problem (GCP) is one of the most interesting, studied and difficult combinatorial optimisation problems. That is why, several approaches were developed for solving this problem, including exact approaches, heuristic approaches, metaheuristics and hybrid approaches. In this paper, we try to solve the graph colouring problem using a new approach based on the quantum inspired cuckoo search algorithm. The first contribution consists in defining an appropriate quantum representation based on qubit representation to represent the graph colouring solutions. The second contribution is the proposition of a novel measure operator based on the adjacency matrix. The third contribution involves the proposition of an adapted hybrid quantum mutation operation. To show the feasibility and the effectiveness of the algorithm, we have used the standard DIMACS benchmark, and the obtained results are very encouraging.
Abstract Graph coloring with minimum colors is a well-known NP-complete problem in computer science and mathematics, having diverse real-life applications such as scheduling, register allocation, and frequency assignment. However, a classical computer requires exponential time to solve the graph coloring problem with the minimum number of colors. Therefore, classical approaches become infeasible for solving this problem on large graphs. In contrast, quantum computing offers the potential for speedup on specific problems, although solutions are typically obtained with a certain probability of correctness. This work proposes a quantum–classical method for solving the graph coloring problem and finding the chromatic number of a graph with guaranteed correctness, and also reduces the time and qubit complexities. The proposed method is inspired by Randomized Grover’s search algorithm, and the classical binary search algorithm. Here, Randomized Grover’s search algorithm provides quadratic speedup and this algorithm is useful in real situations when the number of solutions is not known in advance in contrast to the Grover’s search algorithm. The binary search algorithm helps to achieve logarithmic speedup, which reduces overall complexity. The experimental results strongly validate our claims and demonstrate the superiority of the approach. Furthermore, a comprehensive mathematical analysis is carried out to examine its complexities, as well as to compare its computational efficiency with existing methods.
Quantum computing holds significant promise for solving NP-hard combinatorial problems by leveraging quantum properties such as superposition, entanglement, and interference. In this study, we apply Grover’s algorithm to a constrained scheduling problem modeled as a graph coloring task with an added complexity: ensuring that the total processing time of all jobs assigned to a machine does not exceed its capacity. This additional constraint increases the problem’s difficulty beyond traditional graph coloring. Our proposed quantum framework demonstrates the ability to generate valid schedules that satisfy both coloring and capacity constraint, highlighting the practical potential of quantum computing for addressing complex scheduling challenges.
We consider an inverse problem for a finite graph $ (X, E) $ where we are given a subset of vertices $ B\subset X $ and the distances $ d_{(X, E)}(b_1, b_2) $ between all pairs of vertices $ b_1, b_2\in B $. The distance between vertices $ x_1, x_2\in X $ is defined as the minimal number of edges in paths connecting the vertices. This problem can be regarded as a discrete version of the boundary rigidity problem in Riemannian geometry or the inverse travel time problem in geophysics. We develop quantum computing methods to find a solution to the problem, and show that the solution is unique under certain conditions. We prove the following uniqueness result: when $ (X, E) $ is a tree and $ B $ is the set of leaves of the tree, the graph $ (X, E) $ is uniquely determined in the class of all connected graphs having a fixed number of vertices. We present a quantum algorithm which, under arbitrary conditions, produces the graph $ (X, E) $, or one of those, which has the given number of vertices and the required distances between vertices in $ B $. To this end we develop an algorithm that takes in a qubit representation of a graph and combine it with Grover's search algorithm. The algorithm can be implemented using only $ O(|X|^2) $ qubits, the same order as the number of elements in the adjacency matrix of $ (X, E) $, and it has a quadratic improvement in computational cost compared to standard classical algorithms. Finally, we consider applications in the theory of computation, and show that a slight modification of the inverse problem above is NP-complete: all NP-problems can be reduced to a discrete inverse problem that we consider.
Quantum algorithms can deliver asymptotic speedups over their classical counterparts. However, there are few cases where a substantial quantum speedup has been worked out in detail for reasonably-sized problems, when compared with the best classical algorithms and taking into account realistic hardware parameters and overheads for fault-tolerance. All known examples of such speedups correspond to problems related to simulation of quantum systems and cryptography. Here we apply general-purpose quantum algorithms for solving constraint satisfaction problems to two families of prototypical NP-complete problems: boolean satisfiability and graph colouring. We consider two quantum approaches: Grover's algorithm and a quantum algorithm for accelerating backtracking algorithms. We compare the performance of optimised versions of these algorithms, when applied to random problem instances, against leading classical algorithms. Even when considering only problem instances that can be solved within one day, we find that there are potentially large quantum speedups available. In the most optimistic parameter regime we consider, this could be a factor of over 105 relative to a classical desktop computer; in the least optimistic regime, the speedup is reduced to a factor of over 103 . However, the number of physical qubits used is extremely large, and improved fault-tolerance methods will likely be needed to make these results practical. In particular, the quantum advantage disappears if one includes the cost of the classical processing power required to perform decoding of the surface code using current techniques.