To celebrate 50 years in print, the journal Physical Review A has compiled a list of its 26 most influential papers of the past half-century. Among these is Robert Raussendorf and Hans Briegel’s milestone 2003 paper, Measurement-based quantum computation on cluster states, a publication that challenged perceptions of measurement and entanglement in quantum physics and altered the course of quantum computing research and development.
Raussendorf’s contribution was critical in terms of its influence on applications.
“But more importantly, it gives us a number of conceptual puzzles," said Raussendorf, Associate Professor in UBC's Department of Physics & Astronomy. "The field of quantum computation needs conceptual struggles in order to mature.”
Raussendorf’s celebrated paper proposes a computational model that is different from the standard “gate” model of quantum computation. The gate model approach to quantum computation is based on the evolution of quantum states according to the Schrödinger equation, which describes how quantum states evolve in time when undisturbed. By contrast, Raussendorf’s approach, so-called “one-way quantum computation,” involves manipulating information through the irreversible process of quantum measurement.
A one-way quantum computer uses the principle of entanglement to prepare a resource state (in Raussendorf and Briegel’s original work, an object known as a cluster state). These states have inherent computational power, and a sequential measurement of the probabilistic values of each qubit generates universal computations to solve problems. Information is processed as earlier measurements inform later measurements. The correlations that arise in this sequence, also enforced by the entanglement of the original resource, drive the computation; it’s called one-way computation because the cluster state is destroyed by this measurement.
“A lot of early efforts in quantum computing considered the Schrödinger equation,” explained Raussendorf. The Schrödinger equation describes waves or states and their function in a quantum mechanical system. “Everything went back to unitary transformation exchange, and this idea that the Schrödinger equation was an anchor point. But one-way computing is different; we invented an approach that bypasses the Schrödinger equation.”
Raussendorf’s paper, authored during his PhD studies at Ludwig Maximilians University in Germany jointly with Hans Briegel, altered the course of quantum computing research, providing new theoretical avenues and inspiring a generation of researchers to build on his work. The original paper, A One-Way Quantum Computer (Physical Review Letters, 2001, has to date been cited more than 2400 times. The 2003 paper celebrated in the Physical Review A anniversary edition provides a detailed description of this computational scheme, and has been cited 800 times.
Raussendorf continues to drive the field forward, and currently leads the Stewart Blusson Quantum Matter Institute’s Grand Challenge: Pushing the Boundaries of Noisy Intermediate Scale Quantum (NISQ) Computing by Focusing on Quantum Materials Problems. This research program is working to promote the development and improvement of quantum computing technologies via their potential to solve new quantum materials puzzles.