Reviewerly/exHarmony huggingface dataset:
Multipartite Entanglement in Quantum Networks Using Subgraph Complementations

Quantum networks are important for quantum communication and consist of entangled states that are essential for many tasks such as quantum teleportation, quantum key distribution, quantum sensing and quantum error correction. Graph states are a specific class of multipartite entangled states that can be represented by graphs. We propose a novel approach for distributing graph states across a quantum network. We show that the distribution of graph states can be characterised by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{a}$</tex> system of subgraph complementations, which we also relate to the minimum rank of the underlying graph and the degree of entanglement quantified by the Schmidt-rank of the quantum state. We analyse resource usage for our algorithm and show it to match or be improved in the number of qubits, bits for classical communication and EPR pairs utilised, as compared to prior work. The number of local operations is efficient, and the resource consumption for our approach scales linearly in the number of vertices. This presents a quadratic improvement in completion time for several classes of graph states represented by dense graphs, and implies a potential for improved fidelity in the presence of noise. Common classes of graph states are classified along with the optimal time for their distribution using subgraph complementations. We also provide a framework to similarly find the optimal sequence of operations to distribute an arbitrary graph state, and prove upper bounds along with providing approximate greedy algorithms.
Asymptotic Analysis of Problem Formulations for Quantum Annealers

Quantum annealing is a promising technique to solve NP-hard problems more efficiently than state-of-the-art classical computing algorithms. Current generation quantum annealers require users to formulate problems following specific models: either as an Ising Hamiltonian, or as a quadratic unconstrained binary optimization (QUBO) polynomial. In this paper, we show that a problem may be formulated in multiple, fundamentally different ways using these models, and that the choice of problem formulation drastically impacts the scope of problem instances that the quantum annealer is able to solve. Using Boolean satisfiability (SAT) solving as an example, we investigate which characteristics of problem formulation are determinant in solution viability. Drawing inspiration from classical algorithmic asymptotic analysis, we establish a framework for measuring the complexity of any given formulation using the QUBO or Ising models, which we argue can be useful for practitioners to evaluate and compare the different ways one may tackle a problem using quantum annealers.
Offline Quantum Circuit Pruning for Quantum Chemical Calculations

A variational quantum eigensolver (VQE) designed for noisy intermediate-scale quantum (NISQ) computers is a promising hybrid quantum-classical algorithm for quantum chemical calculations (QCC). Its accuracy and computational cost strongly depend on an ansatz, and unitary coupled-cluster (UCC) ansatze are well-known chemistry-inspired ansatze that can achieve a high accuracy for QCC. However, UCC-based quantum circuits have a high depth and a lot of 2-qubit controlled NOT (CNOT) gates which are sensitive to noises, leading to the low accuracy of VQE running on NISQ computers. To address this issue, we propose a novel offline quantum circuit pruning technique that reduces the number of CNOT gates and depth of UCC-based quantum circuits by reducing the number of excitation operators per parameter to a user-specified value. It is applied only once before VQE is executed (i.e., offline), which is the advantage over state-of-the-art quantum circuit pruning techniques. The evaluation using a quantum computer simulator with a noise model based on a real NISQ device shows that our offline pruning technique improves the accuracy of UCCSD-VQE while sustaining the potential accuracy of the UCCSD ansatz and has a synergy with a well-known noise mitigation method called zero noise extrapolation (ZNE).
Application of the Variational Quantum Eigensolver to the Ultimate Pit Problem

Determining the ultimate pit profile that maximizes the profit while minimizing the cost of mine extraction is a fundamental problem in open-pit mining. The development of quantum computing hardware and advances in heuristic quantum algorithms make it possible to explore quantum computing as a solution for the ultimate pit problem. Here, we cast the ultimate pit problem as a Hamiltonian ground-state search problem, which we approximately solve using the variational quantum eigensolver algorithm. Moreover, we use a domain decomposition approach to extend the reach of today's small-scale quantum hardware. We demonstrate the procedure on IBMQ devices using four qubits. To the best of our knowledge, this is the first attempt to use heuristic quantum algorithms to map small-scale instances of the ultimate pit problem on quantum hardware.
Evaluating Quantum Machine Learning Approaches for Histopathological Cancer Detection: Classical, Hybrid Simulation, and IBM Quantum Computing

This study presents an effective analysis of a hybrid transfer learning algorithm in medical image processing, specifically focusing on histopathological cancer detection. The research involves a two-part approach. First, a hybrid model was created, combining classical and quantum units for classifying images into cancerous and non-cancerous cells. Second, the performance of this hybrid model was compared with traditional models, quantum simulators like Pennylane, and IBM's real quantum computers (Lima, Quito, Belem, and Jakarta), considering factors such as the number of qubits, Variational Quantum Classifier (VQC), and number of images. The objective was to aid users in selecting the optimal combination of classical and quantum units for accurate classification. The study found that the hybrid transfer learning model, when used with the Pennylane quantum simulator, yielded slightly higher accuracy than hybrid models implemented with IBM's quantum computers.
Maximizing Key Distribution Capability: An Application in Quantum Cryptography

Quantum key distribution (QKD) integrated wavelength-division multiplexing (WDM) offers an information-theoretically secure solution to the key exchange problem. In this paper, we investigate the joint problem of how to efficiently schedule QKD to provision sufficient secret keys over WDM networks. Specifically, we propose and formulate the problem of maximizing key distribution capability (MKDC) by employing a mixed integer linear programming (MILP) model for the first time. We then suggest two near-optimal algorithms adopted to address larger-scale problems in polynomial time. One employs the linear programming relaxation technique combined with a rounding algorithm (LPR-RA) and the other is inspired by the fact that the application with a higher risk of disruption is prioritized to recharge secret keys, dubbed progressive serving algorithm (PSA). Simulation results show that both the LPRRA and PSA can approach the best feasible solution or the upper-bound of the problem. In addition, by means of using NetSquid, an open simulator for quantum networks, we conduct experiments to get insight into the BB84, a protocol applied popularly in QKD networks nowadays.
Quantum Distributed Algorithms for Approximate Steiner Trees and Directed Minimum Spanning Trees

We present two algorithms in the Quantum CONGEST-CLIQUE model of distributed computation that succeed with high probability; one for producing an approximately optimal Steiner Tree, and one for producing an exact directed minimum spanning tree, each of which uses <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\overline{O}(n^{1/4})$</tex> rounds of communication and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tilde{O}(n^{9/4})$</tex> messages, achieving a lower asymptotic round and message complexity than any known algorithms in the classical CONGEST-CLIQUE model. At a high level, we achieve these results by combining classical algorithmic frameworks with quantum subroutines. Additionally, we characterize the constants and logarithmic factors involved in our algorithms as well as related classical algorithms, otherwise obscured by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\overline{O}$</tex> notation, revealing that advances are needed to render both the quantum and classical algorithms practical.
Extremum Seeking Control of Quantum Gates

To be useful for quantum computation, gate operations must be maintained at high fidelities over long periods of time. In addition to decoherence, slow drifts in control hardware leads to inaccurate gates, causing the quality of operation of as-built quantum computers to vary over time. Here, we demonstrate a data-driven approach to stabilized control, combining extremum-seeking control (ESC) with direct randomized benchmarking (DRB) to stabilize two-qubit gates under unknown control parameter fluctuations. As a case study, we consider these control strategies in the context of a trapped ion quantum computer using physically-realistic simulation. We then experimentally demonstrate this control strategy on a state-of-the-art, commercial trapped-ion quantum computer.
QFactor: A Domain-Specific Optimizer for Quantum Circuit Instantiation

We introduce a domain-specific algorithm for numerical optimization operations used by quantum circuit instantiation, synthesis, and compilation methods. QFactor uses a tensor network formulation together with analytic methods and an iterative local optimization algorithm to reduce the number of problem parameters. Besides tailoring the optimization process, the formulation is amenable to portable parallelization across CPU and GPU architectures, which is usually challenging in general purpose optimizers (GPO). Compared with several GPOs, our algorithm achieves exponential memory and performance savings with similar optimization success rates. While GPOs can handle directly circuits of up to six qubits, QFactor can process circuits with more than 12 qubits. Within the BQSKit optimization framework, we enable optimizations of 100+ qubit circuits using gate deletion algorithms to scale out linearly with the hardware resources allocated for compilation in GPU environments.
Learning to Optimize Quantum Neural Networks Without Gradients

Quantum Machine Learning is an emerging sub-field in machine learning where one of the goals is to perform pattern recognition tasks by encoding data into quantum states. This extension from classical to quantum domain has been made possible due to the development of hybrid quantum-classical algorithms that allow a parameterized quantum circuit to be optimized using gradient based algorithms that run on a classical computer. The similarities in training of these hybrid algorithms and classical neural networks has further led to the development of Quantum Neural Networks (QNNs). However, in the current training regime for QNNs, the gradients w.r. t objective function have to be computed on the quantum device. This computation is highly non-scalable and is affected by hardware and sampling noise present in the current generation of quantum hardware. In this paper, we propose a training algorithm that does not rely on gradient information. Specifically, we introduce a novel meta-optimization algorithm that trains a meta-optimizer network to output parameters for the quantum circuit such that the objective function is minimized. We empirically and theoretically show that we achieve a better quality minima in fewer circuit evaluations than existing gradient based algorithms on different datasets.
QNet: A Quantum-Native Sequence Encoder Architecture

This work proposes QNet, a novel sequence encoder model that entirely inferences on the quantum computer using a minimum number of qubits. Let n and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d$</tex> represent the length of the sequence and the embedding size, respectively. The dot-product attention mechanism requires a time complexity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$O(n^{2}\cdot d)$</tex> , while QNet has merely <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$O(n+d)$</tex> quantum circuit depth. In addition, we introduce ResQNet, a quantum-classical hybrid model composed of several QNet blocks linked by residual connections, as an isomorph Transformer Encoder. We evaluated our work on various natural language processing tasks, including text classification, rating score prediction, and named entity recognition. Our models exhibit compelling performance over classical state-of-the-art models with a thousand times fewer parameters. In summary, this work investigates the advantage of machine learning on near-term quantum computers in sequential data by experimenting with natural language processing tasks.
Efficient Control Pulses for Continuous Quantum Gate Families Through Coordinated Re-Optimization

We present a general method to quickly generate high-fidelity control pulses for any continuously-parameterized set of quantum gates after calibrating a small number of reference pulses. We find that interpolating between optimized control pulses for different quantum operations does not immediately yield a high-fidelity intermediate operation. To solve this problem, we propose a method to optimize control pulses specifically to provide good interpolations. We pick several reference operations in the gate family of interest and optimize pulses that implement these operations, then iteratively re-optimize the pulses to guide their shapes to be similar for operations that are closely related. Once this set of reference pulses is calibrated, we can use a straightforward linear interpolation method to instantly obtain high-fidelity pulses for arbitrary gates in the continuous operation space. We demonstrate this procedure on the three-parameter Cartan decomposition of two-qubit gates to obtain control pulses for any arbitrary two-qubit gate (up to single-qubit operations) with consistently high fidelity. Compared to previous neural network approaches, the method is 7.7x more computationally efficient to calibrate the pulse space for the set of all single-qubit gates. Our technique generalizes to any number of gate parameters and could easily be used with advanced pulse optimization algorithms to allow for better translation from simulation to experiment.
Reducing the Compilation Time of Quantum Circuits Using Pre-Compilation on the Gate Level

In order to implement a quantum computing application, problem instances must be encoded into a quantum circuit and then compiled for a specific platform. The lengthy compilation process is a key bottleneck in this workflow, especially for problems that arise repeatedly with a similar yet distinct structure (each of which requires a new compilation run thus far). In this paper, we aim to overcome this bottleneck by proposing a comprehensive pre-compilation technique that tries to minimize the time spent for compiling recurring problems while preserving the solution quality as much as possible. The following concepts underpin the proposed approach: Beginning with a problem class and a corresponding quantum algorithm, a predictive encoding scheme is applied to encode a representative problem instance into a general-purpose quantum circuit for that problem class. Once the real problem instance is known, the previously constructed circuit only needs to be adjusted-with (nearly) no compilation necessary. Experimental evaluations on QAOA for the MaxCut problem as well as a case study involving a satellite mission planning problem show that the proposed approach significantly reduces the compilation time by several orders of magnitude compared to Qiskit's compilation schemes while maintaining comparable compiled circuit quality. All implementations are available on GitHub (https://github.com!cda-tum!mqt-problemsolver) as part of the Munich Quantum Toolkit (MQT).
Minimizing Deployment Cost of Hybrid Applications

Within Quantum Software Engineering, the development of hybrid classical-quantum applications is an active research topic. Concretely, there is a focus on the usage of microservices architectures to offer quantum functionalities through classical service gateways. However, while there are methodologies and tools to automatically determine the best deployment for classical applications, this is not the case for hybrid applications, which are especially complicated to deploy due to their specific quantum-requirements. In this paper, we propose Cost Minimizer, a tool to determine the most cost-effective classical and quantum machines to deploy hybrid microservice-based applications. We believe Cost Minimizer will be a step towards achieving a better interaction between classical and quantum computing systems.
CryptoQFL: Quantum Federated Learning on Encrypted Data

Recent advancements in Quantum Neural Networks (QNNs) have demonstrated theoretical and experimental performance superior to their classical counterparts in a wide range of applications. However, existing centralized QNNs cannot solve many real-world problems because collecting large amounts of training data to a common public site is time-consuming and, more importantly, violates data privacy. Federated Learning (FL) is an emerging distributed machine learning framework that allows collaborative model training on decentralized data residing on multiple devices without breaching data privacy. Some initial attempts at Quantum Federated Learning (QFL) either only focus on improving the QFL performance or rely on a trusted quantum server that fails to preserve data privacy. In this work, we propose CryptoQFL, a QFL framework that allows distributed QNN training on encrypted data. CryptoQFL is (1) secure, because it allows each edge to train a QNN with local private data, and encrypt its updates using quantum homomorphic encryption before sending them to the central quantum server; (2) communication-efficient, as CryptoQFL quantize local gradient updates to ternary values, and only communicate non-zero values to the server for aggregation; and (3) computation-efficient, as CryptoQFL presents an efficient quantum aggregation circuit with significantly reduced latency compared to state-of-the-art approaches.
Parameter Transferability in QAOA Under Noisy Conditions

The quantum approximate optimization algorithm (QAOA) is widely recognized for its potential to deliver quantum advantage in combinatorial optimization tasks. As a hybrid classical-quantum algorithm, it achieves an approximate solution to the combinatorial optimization problem by preparing a specific quantum state via a classical optimization of the quantum circuit parameters. A key property of QAOA is the observed concentration and transferability of optimal parameters across distinct problem instances, linked to local graph characteristics such as subgraph types (lightcones) and node degrees (parity) [1]. However, quantum noise poses a critical challenge in the current NISQ era, hindering the performance of QAOA, particularly for large system sizes. In this follow-up study, we investigate the resilience of parameter transferability under noisy conditions. Leveraging zero-noise extrapolation (ZNE), we evaluate the transferability's robustness across various random graphs under different noise levels. Our analysis reveals that the transferability of optimal parameters from smaller ‘donor’ graphs to larger ‘acceptor’ graphs retains its effectiveness, even under substantial noise when combined with ZNE. Importantly, by illustrating that QAOA parameter transferability can still be exploited effectively among NISQ devices, we offer a pragmatic approach to enhance the algorithm's performance. Our research reinforces the promise of QAOA for large-scale applications under the practical challenges of quantum noise.
Investigating the Characteristics of Ising Machines

Recently, Ising machines have garnered attention as accelerators for resolving combinatorial optimization problems by converting them into Quadratic Unconstrained Binary Optimization (QUBO) problems. Ising machines have been rapidly developed with various implementations. For instance, there are different Ising machines like quantum annealing machines that use analog quantum circuits, simulated annealing machines that use digital processors like CPU, GPU, and VE, and digital annealing machines that use digital circuits such as CMOS and FPGA. However, the characteristics of Ising machines have not been fully comprehended, because a standardized evaluation method and a commonly used benchmark program for Ising machines have not been established yet. This research focuses on combinatorial clustering as a benchmark program for the evaluation of Ising machines, aiming to investigate the characteristics of Ising machines.
Grover's Implementation of Quantum Binary Neural Networks

Binary Neural Networks (BNNs) are the result of a simplification of network parameters in Artificial Neural Networks (ANNs). The computational complexity of training ANNs increases significantly as the size of the network increases. This complexity can be greatly reduced if the parameters of the network are binarized. Binarization, which is a one bit quantization, can also come with complications including error and information loss. The implementation of BNNs on quantum hardware could potentially provide a computational advantage over its classical counterpart. This is due to the fact that binarized parameters fit nicely to the nature of quantum hardware. Quantum superposition allows the network to be trained more efficiently, without using back propagation techniques, with the application of Grover's Algorithm for the training process. This paper looks into two BNN designs that utilize only quantum hardware, as opposed to hybrid quantum-classical implementations. It also provides practical implementations for both of them. Looking into their scalability, improvements on the design are proposed to reduce complexity even further.
General Algorithms for SPAM Noise Characterization

We present a general denoising algorithm for characterizing state preparation and measurement (SPAM) errors. This algorithm works by constructing a set of canonical super-operators using a set of efficient unitary circuits. This allows us to completely estimate the state and noise in the measurement apparatus up to the unavoidable “blame” gauge. We also show how this blame gauge can be fixed given prior information about the system, like the purity of the original state and independence properties of detector noise.
Superstaq: Deep Optimization of Quantum Programs

We describe Superstaq, a quantum software platform that optimizes the execution of quantum programs by tailoring to underlying hardware primitives. For benchmarks such as the Bernstein-Vazirani algorithm and the Qubit Coupled Cluster chemistry method, we find that deep optimization can improve program execution performance by at least 10x compared to prevailing state-of-the-art compilers. To highlight the versatility of our approach, we present results from several hardware platforms: superconducting qubits (AQT @ LBNL, IBM Quantum, Rigetti), trapped ions (QSCOUT), and neutral atoms (Infleqtion). Across all platforms, we demonstrate new levels of performance and new capabilities that are enabled by deeper integration between quantum programs and the device physics of hardware.