A First 3D-chip-based Analog Photonic Quantum Computer Demonstrates Quantum Fast Hitting

November 7th, 2018
Analog quantum computing has been an appealing tool with potential real-life applications on various optimization and simulation tasks, and much less stringent requirement on error corrections as universal quantum computing. Quantum walks, a key protocol for analog quantum computing, has theoretically shown many quantum advantages in those tasks, for instance, the speedup in fast hitting tasks in glued tree structures. However, if one would like to bring the advantages into reality, there exist many prerequisites on the physics quantum system. A primary thing is to make it scalable so that it can cope with a real problem of certain complexity.

The demonstration of analog quantum computing has normally been in a very small scale for proof-of-principle studies, and the number of photons is the main resource for expanding the quantum system. Until very recently, as Nature Photonics reported [https://www.nature.com/articles/s41566-018-0282-5], a scalable analog quantum computing device was realized on an integrated quantum photonic chip, which opened a new roadmap of using the dimension and scale of the quantum evolution system as the new resource for analog quantum computing.

In the research conducted by Prof. Xian-Min Jin and his team in Shanghai Jiao Tong University, collaborating with Prof. Myungshik Kim in Imperial College London and Dr. Carlo Di Franco in Nanyang Technological University, the hexagonal graphs as an alternative structure was proposed for the fast hitting tasks instead of the traditional binary glued trees. The hexagonal graphs resemble the gluing of two tree-like structures, and it is highly scalable to be mapped in the three-dimensional integrated quantum photonic chips, where the longitudinal direction represents the evolution time, and the cross-section view shows the hexagonal structure formed by waveguide arrays. The group led by Xian-Min Jin have formed very strong experimental expertise and used it to report a largest-scale integrated photonic chip to demonstrate the first quantum walk experiment in real two-dimensional space [Science Advances 4, eaat3174 (2018)].

For the research on quantum fast hitting, they injected photons from the "Entry" waveguide and measure the light intensity at the "Exit" waveguide as the "hitting efficiency", and found that quantum walks perform quadratically faster than classical random walks, showing the first experimental demonstration of quantum advantages in fast hitting tasks.

The experimental implementation for fast hitting on glued trees is an exciting progress, as this realizes one of the most representative examples that quantum theoreticians have raised to showcase the speedup by quantum walks. Besides, considering the protocol's essence as an optimization process and the similarity between binary trees and decision trees in computer science, the protocol of fast hitting on glued trees may further trigger many useful applications that utilize quantum speed-up for tasks such as logistics, finance and information searching. "Our demonstration can also be seen as a first step towards the realization of scalable quantum fast search. By adding more photons we can also build a large network of nonclassical states which can be used for applications as well as fundamental studies." said Prof. Myungshik Kim.

The scalable quantum device enabled by the integrated quantum photonic chip also provides an excellent platform for quantum simulation of other physical systems, and may potentially stimulate the research of many multidisciplinary topics and emerging scientific questions, including astronomy simulation, quantum machine learning [Physical Review Letters 120, 240501 (2018)], quantum topological photonics [arXiv: 1810.01435 (2018)], quantum imaging for biological and medical applications, and so on.

In October this year, the team led by Prof. Xian-Min Jin launched a first software for photonic analog quantum computing named FeynmanPAQS [arXiv: 1810.02289 (2018)], with an aim of encouraging wider participation and brainstorming for various simulation proposals and potential applications connecting real problems. It would be delightful in the near future to see more quantum benefits to be brought into reality.

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Provided by Shanghai Jiao Tong University

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