Quantum computing roadmaps predict the availability of 10,000qubit devices within the next 3-5 years.With projected two-qubit error rates of 0.1%, these systems will enable certain operations under quantum error correction (QEC) using lightweight codes, offering significantly improved fidelities compared to the NISQ era.However, the high qubit cost of QEC codes like the surface code (especially at near-threshold physical error rates) limits the error correction capabilities of these devices.In this emerging era of Early Fault Tolerance (EFT), it will be essential to use QEC resources efficiently and focus on applications that derive the greatest benefit.In this work, we investigate the implementation of Variational Quantum Algorithms in the EFT regime (EFT-VQA).We explore the ideas of partial quantum error correction (pQEC), a strategy that error-corrects Clifford operations while performing 𝑅 𝑧 (𝜃 ) rotations via magic state injection instead of the more expensive T-state distillation, and adapt it to VQAs.Our results show that pQEC can improve VQA fidelities by 9.27x over standard approaches.Furthermore, we propose architectural optimizations that reduce circuit latency by ∼ 2x, and achieve qubit packing efficiency of 66% in the EFT regime.The source code can be accessed here https: //github.com/siddharthdangwal/EFT-VQA.

Variational quantum algorithms (VQAs) show potential for quantum advantage in the near term of quantum computing, but demand a level of accuracy that surpasses the current capabilities of NISQ devices. To systematically mitigate the impact of quantum device error on VQAs, we propose Clapton: Clifford-Assisted Problem Transformation for Error Mitigation in Variational Quantum Algorithms. Clapton leverages classically estimated good quantum states for a given VQA problem, classical simulable models of device noise, and the variational principle for VQAs. It applies transformations on the VQA problem's Hamiltonian to lower the energy estimates of known good VQA states in the presence of the modeled device noise. The Clapton hypothesis is that as long as the known good states of the VQA problem are close to the problem's ideal ground state and the device noise modeling is reasonably accurate (both of which are generally true), then the Clapton transformation substantially decreases the impact of device noise on the ground state of the VQA problem, thereby increasing the accuracy of the VQA solution. Clapton is built as an end-to-end application-to-device framework and achieves mean VQA initialization improvements of 1.7x to 3.7x, and up to a maximum of 13.3x, over the state-of-the-art baseline when evaluated for a variety of scientific applications from physics and chemistry on noise models and real quantum devices.

Quantum computing is in an era of limited resources. Current hardware lacks high fidelity gates, long coherence times, and the number of computational units required to perform meaningful computation. Contemporary quantum devices typically use a binary system, where each qubit exists in a superposition of the 0 and 1 states. However, it is often possible to access the 2 or even 3 states in the same physical unit by manipulating the system in different ways. In this work, we consider automatically encoding two qubits into one four-state ququart via a compression scheme. We use quantum optimal control to design efficient proof-of-concept gates that fully replicate standard qubit computation on these encoded qubits.

Superconducting quantum devices are a leading technology for quantum computation, but they face several challenges. Gate errors, coherence errors and a lack of connectivity all contribute to low fidelity results. In particular, connectivity restrictions enforce a gate set that requires three-qubit gates to be decomposed into one- or two-qubit gates. This substantially increases the number of two-qubit gates that need to be executed. However, many quantum devices have access to higher energy levels. We can expand the qubit abstraction of |0〉 and |1〉 to a ququart which has access to the |2〉 and |3〉 state, but with shorter coherence times. This allows for two qubits to be encoded in one ququart, enabling increased virtual connectivity between physical units from two adjacent qubits to four fully connected qubits. This connectivity scheme allows us to more efficiently execute three-qubit gates natively between two physical devices.