Simon Benjamin's

A quantum error-correcting code is a central piece of fault-tolerant quantum computation that is required to realise large-scale quantum algorithms. Unfortunately, the state-of-the-art codes with the best parameters that require the least overheads do not yet admit experimentally feasible implementations. Therefore, exploring codes that are both high-performing and practical on proposed quantum computing architectures is essential. Recently, modular system architectures – comprised of equivalent units (modules) that are interconnected in some network – have been Read more…

The most promising class of algorithms for near-term quantum computers are variational quantum algorithms (VQAs), where a parametrised circuit of quantum gates calculates some cost function, which is then optimised to produce the solution to a problem, e.g. calculating the ground-state energy of a molecule. Typically, the optimisation methods used in VQAs are based on gradient descent methods similar to those popular for training modern machine learning models, but these can suffer from the existence Read more…

A promising approach to build a scalable quantum computer is to manufacture a large array of qubits on a chip. This is common to several hardware architectures including superconducting qubits and spin-qubits in silicon. The qubits are engineered to perform logical quantum operations and detect local errors at the same time, allowing us to implement scalable fault-tolerant quantum computation. In practice we should expect that some small fraction of the qubits will be permanently broken Read more…

Present and near-term quantum computers could help us to solve important practical problems by preparing increasingly complex quantum states that cannot be simulated classically with realistic levels of resource. However, imperfections in their quantum operations (noise) severely limits their practical applicability. Two recent works [arXiv:2011.05942, arXiv:2011.07064] have introduced an approach that now enables researchers to exponentially suppress errors in near-term quantum computers without the need to implement prohibitively expensive quantum error correcting codes. QuESTlink is an elegant Read more…

QuESTlink is a wonderful and extremely useful tool when it comes to testing, understanding or developing quantum algorithms. It is a Mathematica interface that lets its users to take full advantage of the highly efficient QuEST code without resorting to writing low-level C code: see our blogpost here. We have collected a number of Mathematica notebooks that demonstrate a variety of quantum algorithms; we can interactively simulate these circuits with the use of QuESTlink. Our Read more…

Quantum algorithms have introduced a new paradigm in computing, but they typically estimate expectation values by sampling output probabilities of a quantum computer. In order to reduce the uncertainty of such an expectation value, one needs to repeat measurements in a quantum algorithm many times. We consider the measurement cost of a large class of variational quantum algorithms in our preprint [1], such as imaginary time evolution and quantum natural gradient descent. All these approaches, Read more…

Gradient descent is well-known and widely used in many scientific applications. In many scenarios the aim is to find the minimum of a function; gradient descent does this by always moving in the direction of the steepest descent. This is analogous to how a person would find the way from the top of a mountain to a valley while navigating through heavy fog, which obscures the path ahead. Cost functions, however, in practice depend on Read more…