The Subset Sum Problem (SSP) is one of those combinatorial problems that are very easy to understand (take a bunch of integer numbers and verify whether there exists a subset of these numbers which sums up to a given target integer), but it can be very difficult to solve.The SSP is actually an NPcomplete problem, but it is "weakly" NP-hard, implying that there are instances of SSP that can be solved in polynomial time.For this particular problem, the instance hardness can be measured by evaluating the so-called "density" index, which basically compares the number of involved integer numbers to the number of bits we need for their binary representation.In our preliminary study on the use of adiabatic quantum computing for the SSP, we investigate the actual feasibility in solving hard instances of the problem.In fact, hard SSP instances are those requiring a large number of bits for the representation of the integers, while the analog nature of the quantum computer does not allow us to ensure highly accurate integer representations.Some preliminary computational experiments performed on D-Wave quantum annealer are presented and compared to standard solvers for classical computers.
The Subset Sum Problem (SSP) is one of those combinatorial problems that are very easy to understand (take a bunch of integer numbers and verify whether there exists a subset of these numbers which sums up to a given target integer), but it can be very difficult to solve.The SSP is actually an NPcomplete problem, but it is "weakly" NP-hard, implying that there are instances of SSP that can be solved in polynomial time.For this particular problem, the instance hardness can be measured by evaluating the so-called "density" index, which basically compares the number of involved integer numbers to the number of bits we need for their binary representation.In our preliminary study on the use of adiabatic quantum computing for the SSP, we investigate the actual feasibility in solving hard instances of the problem.In fact, hard SSP instances are those requiring a large number of bits for the representation of the integers, while the analog nature of the quantum computer does not allow us to ensure highly accurate integer representations.Some preliminary computational experiments performed on D-Wave quantum annealer are presented and compared to standard solvers for classical computers.
Many applications in automated auditing and the analysis and consistency check of financial documents can be formulated in part as the subset sum problem: Given a set of numbers and a target sum, find the subset of numbers that sums up to the target. The problem is NP-hard and classical solving algorithms are therefore not practical to use in many real applications.We tackle the problem as a QUBO (quadratic unconstrained binary optimization) problem and show how gradient descent on Hopfield Networks reliably finds solutions for both artificial and real data. We outline how this algorithm can be applied by adiabatic quantum computers (quantum annealers) and specialized hardware (field programmable gate arrays) for digital annealing and run experiments on quantum annealing hardware.
The subset sum problem (SSP) is a typical nondeterministic-polynomial-time (NP)-complete problem that is hard to solve efficiently in time with conventional computers. Photons have the unique features of high propagation speed, strong robustness, and low detectable energy level and therefore can be promising candidates to meet the challenge. Here, we present a scalable chip built-in photonic computer to efficiently solve the SSP. We map the problem into a three-dimensional waveguide network through a femtosecond laser direct writing technique. We show that the photons sufficiently dissipate into the networks and search for solutions in parallel. In the case of successive primes, our approach exhibits a dominant superiority in time consumption even compared with supercomputers. Our results confirm the ability of light to realize computations intractable for conventional computers, and suggest the SSP as a good benchmarking platform for the race between photonic and conventional computers on the way toward "photonic supremacy."
We illustrate the adiabatic quantum computing solution of the knapsack problem with both integer profits and weights. For problems with n objects (or items) and integer capacity c, we give specific examples using both an Ising class problem Hamiltonian requiring n+c qubits and a much more efficient one using n+[log2c]+1 qubits. The discussion includes a brief mention of classical algorithms for knapsack, applications of this commonly occurring problem, and the relevance of further studies both theoretically and numerically of the behavior of the energy gap. Included too is a demonstration and commentary on a version of quantum search using a certain Ising model. Furthermore, an Appendix presents analytic results concerning the boundary for the easy-versus-hard problem-instance phase transition for the special case subset sum problem.
The Subset Sum problem asks whether, given n integers and a target, some subset of the integers sums to the target. Its best known worst-case running time is O∗(2n/2) (Horowitz and Sahni, 1974), whereas the best quantum upper bound is O∗(2n/3) (Bernstein, Jeffery, Lange, and Meurer, 2013). The k-SUM problem is a parameterized version of Subset Sum asking whether there are k integers that sum to the target. The best classical upper bound for it is O(n⌈k/2⌉), whereas the best quantum running time is O(nk/3) (Tani, 2009). For random instances, a quantum algorithm with running time O(nΦk) is known, where Φk=62k−⌊k/7⌋−⌊(k+3)/7⌋ (Schrottenloher, 2021). We present a new quantum algorithm solving worst-case k-SUM in time O(nΨk), where Ψk=Φk−9[k≡3mod7]−18[k≡6mod7]. The algorithm is not only faster for all k congruent to 3 or 6 modulo 7, but also gives a worst-case guarantee rather than a guarantee restricted to single-solution random instances. Combining our algorithm for 7-SUM with the standard block reduction technique yields an O∗(22n/7) quantum algorithm for Subset Sum, improving the previously known O∗(2n/3) algorithm.
The Subset Sum Problem is a member of the NP-complete class, so no known polynomial time algorithm exists for it. Although there are polynomial time approximations and heuristics, these are not always acceptable, yet exact-solution algorithms are unfeasible for large input. Quantum computation offers new insights for not only the Subset Sum Problem but also the entire NP-complete class; most notably, Grover's quantum algorithm for an unstructured database search can be tailored to identify solutions to problems within mathematics and computer science. This paper discusses the physical and conceptual feasibility of quantum computation and demonstrates the utility of quantum search by analyzing the time complexities of the classical dynamic programming algorithm and Grover's algorithm in solving the Subset Sum Problem, evincing the implications this has on the NP-complete class in general.
We investigate the implementation of an oracle for the Subset Sum problem for quantum search using Grover's algorithm. Our work concerns reducing the number of qubits, gates, and multi-controlled gates required by the oracle. We describe the compilation of a Subset Sum instance into a quantum oracle, using a Python library we developed for Qiskit and have published in GitHub. We then present techniques to conserve qubits and gates along with experiments showing their effectiveness on random instances of Subset Sum. These techniques include moving from fixed to varying-width arithmetic, using partial sums of a set's integers to determine specific integer widths, and sorting the set to obtain provably the most efficient partial sums. We present a new method for computing bit-string comparisons that avoids arbitrarily large multiple-control gates, and we introduce a simple modification to the oracle that allows for approximate solutions to the Subset Sum problem via Grover search.