Dr Xiao-Qi Zhou and colleagues at the University of Bristol's Centre for Quantum Photonics and the University of Queensland, Australia, have shown that controlled operations — ones that are implemented on the condition that a "control bit" is in the state 1 — can be dramatically simplified compared to the standard approach. The researchers believe their technique will find applications across quantum information technologies, including precision measurement, simulation of complex systems, and ultimately a quantum computer — a powerful type of computer that uses quantum bits (qubits) rather than the conventional bits used in today's computers.
Nature Communications - Adding control to arbitrary unknown quantum operations
Although quantum computers promise significant advantages, the complexity of quantum algorithms remains a major technological obstacle. We have developed and demonstrated an architecture-independent technique that simplifies adding control qubits to arbitrary quantum operations—a requirement in many quantum algorithms, simulations and metrology. The technique, which is independent of how the operation is done, does not require knowledge of what the operation is, and largely separates the problems of how to implement a quantum operation in the laboratory and how to add a control. Here, we demonstrate an entanglement-based version in a photonic system, realizing a range of different two-qubit gates with high fidelity.
A major obstacle for realizing a quantum computer is the complexity of the quantum circuits required. As with conventional computers, quantum algorithms are constructed from a small number of elementary logic operations. Controlled operations are at the heart of the majority of important quantum algorithms. The traditional method to realize controlled operations is to decompose them into the elementary logic gate set. However, this decomposition is very complex and prohibits the realization of even small-scale quantum circuits.
The researchers now show a completely new way to approach this problem. "By using an extra degree of freedom of quantum particles, we can realize the control operation in a novel way. We have constructed several controlled operations using this method," said Dr Xiao-Qi Zhou, research fellow working on this project, "This will significantly reduce the complexity of the circuits for quantum computing."
"The new approach we report here could be the most important development in quantum information science over the coming years," said Professor Jeremy O'Brien, director of the Centre for Quantum Photonics, "It provides a dramatic reduction in quantum circuit complexity — the major barrier to the development of more sophisticated quantum algorithms — just at the time that the first quantum algorithms are being demonstrated."
The team now plans to apply this technique to implement some important quantum algorithms, such as the phase estimation algorithm and Shor's factoring algorithm.
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