Quantum Chemistry Achieves 0.94 Accuracy, Paving Way for Quantum Advantage
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The pursuit of quantum advantage, where quantum computers outperform classical methods, faces a critical challenge in determining the necessary scale and precision of these machines, particularly for complex molecular simulations. Scott N. Genin from OTI Lumionics Inc., Ohyun Kwon and Seon-Jeong Lim from Samsung Advanced Institute of Technology, along with colleagues, now demonstrate a significant step towards achieving this goal.
The team executed the iterative qubit coupled-cluster (iQCC) algorithm, a method designed for future fault-tolerant quantum hardware, at an unprecedented scale using classical processors to simulate the behaviour of complex organometallic compounds. This work establishes a clear threshold, around 200 logical qubits, at which quantum methods may surpass classical approaches in accuracy and efficiency, and importantly, the simulations achieve the lowest error rates and highest correlation with experimental data for these systems, exceeding the performance of established classical techniques. Resource estimates for ground-state electronic structure span orders of magnitude, and no quantum-native method has been validated at a commercially relevant scale. This uncertainty is addressed by executing the iterative qubit coupled-cluster (iQCC) algorithm, designed for fault-tolerant quantum hardware, at an unprecedented scale.
The team utilises a quantum solver on classical processors, enabling simulations of transition organo-metal compounds. Iterative Coupled-Cluster on Classical Processors The study pioneers a novel approach to computational chemistry by executing the iterative qubit coupled-cluster (iQCC) algorithm at an unprecedented scale using a quantum solver on classical processors, enabling simulations demanding hundreds of logical qubits and millions of entangling gates. This methodology addresses the longstanding challenge of determining the qubit counts and error rates necessary to demonstrate a genuine quantum advantage over classical methods, particularly in accurately predicting molecular properties. Researchers developed iQCC, a variational quantum eigensolver type algorithm, representing a trial wave function as a product of unitary transformations applied to an initial quantum register, typically a one-determinant representation of the ground state. This approach utilizes a qubit coupled cluster (QCC) form, deriving generators directly from a qubit Hamiltonian to guarantee lowering of the total ground-state energy and avoid the barren plateau problem that hinders other implementations. The core of the method involves iteratively refining the trial wave function by optimizing amplitudes using an ansatz and computing the energy of the Hamiltonian, expressed as an expectation value over the trial wave function. At each iteration, a subset of the direct interaction set, the collection of entanglers with non-zero gradients, is included in the QCC ansatz, selected based on efficient classical computation. This allows researchers to approximate the energy contributions of entanglers while maintaining computational feasibility.
The team enhanced the efficiency of the iQCC Quantum Solver, enabling simulations equivalent to approximately 200 logical qubits and 10 million two-qubit gates, significantly exceeding the capabilities of previous emulations and current quantum hardware. To benchmark the method, scientists computed the lowest triplet excited state energies of phosphorescent iridium and platinum complexes, materials crucial for organic light-emitting diodes. These complexes were chosen for their industrial relevance and experimentally accessible properties, providing a robust basis for assessing the accuracy of the iQCC algorithm.
Results demonstrate systematic improvements over conventional classical methods, achieving a mean absolute error of 0. 05 eV and an R2 value of 0. 94 relative to experimental data, and establishing a framework for evaluating quantum advantage in chemistry, materials science, and drug discovery. Multireference Character via Iterative Quantum Chemistry Researchers investigated the electronic structure of a series of molecules, employing advanced quantum chemical methods to assess their suitability for quantum computation. The study combined the iterative qubit coupled-cluster (iQCC) algorithm with traditional techniques like coupled-cluster singles and doubles (CCSD) and full configuration interaction (FCI) to determine the impact of multireference character on computational accuracy.
The team explored different Complete Active Space (CAS) selections, varying the number of active orbitals to capture the complex electronic interactions within the molecules. Analysis of T1 and T2 amplitudes, indicators of multireference behaviour, revealed that the ground states of the molecules exhibit predominantly single-reference character, while excited states show a greater degree of multireference influence. The iQCC method demonstrated significant speedup compared to traditional methods, offering a scalable approach to solving the electronic structure problem. Researchers found that the method’s performance plateaus after a certain number of CPUs, highlighting the limitations of parallel computing for this type of calculation. The study also investigated the decomposition of iQCC-generated quantum circuits into elementary quantum gates, assessing their complexity and potential for implementation on future quantum hardware. The results establish that accurate calculations require careful selection of the CAS size, balancing accuracy and computational cost. This work demonstrates the potential of iQCC for tackling complex chemical systems and paves the way for more efficient electronic structure calculations on both classical and quantum computers.
Iridium Compounds Modelled, Quantum Advantage Demonstrated Researchers have successfully executed the iterative qubit coupled-cluster (iQCC) algorithm at an unprecedented scale using classical processors, simulating complex organo-metallic compounds that require hundreds of logical qubits and millions of entangling gates. This achievement represents a significant step towards demonstrating a quantum advantage in chemistry, by accurately modelling systems previously challenging for classical methods.
The team computed the lowest triplet excited state energies of iridium and platinum compounds, commonly used in organic light-emitting diodes, and demonstrated that iQCC outperforms leading classical techniques, achieving the lowest mean absolute error and highest correlation with experimental data. This work establishes a crucial threshold, identifying that systems remain classically tractable up to around 200 logical qubits, thereby clarifying the resource requirements needed to realize a genuine quantum advantage in chemical simulations. The researchers developed a highly scalable classical solver, leveraging a bit-partitioning scheme and parallel computing to achieve linear memory scaling with the number of qubits and Pauli words. While the current simulations were performed classically, the methodology provides a robust framework for evaluating and benchmarking quantum algorithms on future quantum hardware. The authors acknowledge that the current implementation relies on approximations and that further improvements in both algorithms and hardware are necessary to fully unlock the potential of quantum computation for complex chemical systems. Future research will focus on extending the method to even larger systems and incorporating more sophisticated quantum error mitigation techniques to improve accuracy and reliability. 👉 More information 🗞 Towards Quantum Advantage in Chemistry 🧠 ArXiv: https://arxiv.org/abs/2512.13657 Tags: