Implementation of a quantum information processor with limited resources

Lead Research Organisation: Queen's University of Belfast
Department Name: Sch of Mathematics and Physics

Abstract

Recent advances in various branches of science have led to a situation where the manipulation of properties at the atomic level - that is, quantum engineering /has become feasible. This achievement has produced such developments as nanotechnology and quantum information processing (QIP) science, and it is widely recognised that these will form the basis of new technologies in the 21st century. QIP aims to exploit quantum mechanics to improve the acquisition, transmission and processing of information. This field has seen explosive growth in recent years, stimulated by the applications such as quantum cryptography, quantum communication, quantum computation and precision measurement, all of which have the potential to surpass their classical counterparts. This new technology will also provide deep insight into many other areas of science, including the understanding of optical, electronic and solid-state devices, and in turn enhancing their performance at the nanometer scale. This will undoubtedly lead to many new applications. Our objective is to undertake a research to find an optimum strategy to implement QIP devices with all currently available techniques possible. Information technology has improved the quality of life in many ways and extremely small circuits designed on microchips can be found everywhere. All these circuits are based on the fact that a computational task is digitised into 0 and 1, then processed by a series of gate operations according to a specific pre-designed algorithm. However, research in QIP has so far been much more fundamental than this. For example, theoretical physicists have successfully answered the questions: Is there a universal quantum gate operation? How can a system be quantum-mechanically manipulated? How can a system be made talk to another system in a quantum mechanical way? How is an error corrected in a quantum processor? However, the realisation of a usable quantum information processor is still far away. This is a proposal to look for new ways in order to realise a quantum information processor efficiently and effectively. The list of our tasks is as follows:1) To identify what the obstacles to implement a quantum device really are.2) To study how to protect a quantum information device from unwanted noise.3) To find how to minimise control and manipulation so that gate operations can be achieved in a system efficiently. This will thereby increase our ability to perform larger tasks within the time limit imposed by the loss of quality of the quantum system. As quantum devices are so small, it is normally extremely difficult to address each device at will, so we will work on the possibility of controlling the system without having to address single devices.4) In the long run, we will investigate the connections between a type of QIP known as one-way quantum computation and different formulations of quantum mechanics. One-way computation may offer us a way of viewing quantum dynamics as only generated by measurements. This could potentially present us with another way of resolving the measurement paradox.

Publications

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Description This joint project was investigated between Vlatko Vedral (VV: Leeds) and Myungshik Kim (MSK: Queen's). During the period, Vlatko Vedral moved from Leeds to Oxford to take up a professorship in quantum

information science and Myungshik Kim was appointed to a chair of theoretical physics at Queen's. Dr. Elinor Irish was hired after her PhD at University of Rochester (USA). After a successful first postdoctoral work under this project, she is now at Durham University as a teaching fellow. There were two PhD students working on the project. After his Laurea at Palermo (Italy), Carlo Di Franco came to Queen's to work on quantum information processing (QIP) on spin chains in the real world situation. Upon finishing his PhD, he has been awarded an Irish Research Council postdoctoral fellowship to continue his research at the National University of Ireland at Cork. Michal Hajdusek was hired as a PhD student at Leeds. He is finishing up his PhD thesis.



About 60 regular papers were published including 18 in Phys. Rev. Lett. and 1 in Science. Additionally, there were 4 review papers in renowned journals such as Rev. Mod. Phys (2), Nature (1) and Science (1).



One of the most important resources for quantum information processing is believed to be quantum correlation which is a stronger form of correlation than conventional correlation. First of all, we investigated what this quantum correlation really means in comparison to conventional correlation. We also tried to answer the questions 'Do we really need quantum correlation in QIP?' and 'Can we observe quantum correlation in the macroscopic world?' In collaboration with experimentalists, we showed a first step toward decoherence-free quantum computation and the realisation of quantum algorithm based on measurement rather than unitary gate operations. It was believed that a quantum state should be prepared in its ground state before any information processing is done. We have shown that the ground state preparation is not a necessary condition for the transfer of quantum information through spin chains. Even when the spins are prepared in

maximally mixed states, we can transfer quantum information perfectly efficiently. We also investigated how to use semiconductor devices and nanostructured devices for quantum information processing.



For objective 1, noise assessment for multipartite entanglement and investigation of entanglement, VV considered semiconductor systems [PRB 78, 2204513 (2008), PRL 99, 170502 (2007)], MSK considered

nanostructured systems [PRL 101, 190504 (2008), PRA 79, 053845 (2009)]. For objective 2, protection of quantum information processors from decoherence, MSK and his experimental coworkers in Vienna realised

simple quantum algorithm in a one-way quantum computer and decoherence-free one-way information transfer [PRL 98, 140501 (2007), PRL 99, 250503 (2007)]. Di Franco had a new way to investigate the

dynamics of a quantum system, which is now called as the information flux. Using the information flux method, together with MSK he found a way to transfer quantum information through a spin chain which is

prepared in a very noisy environment [PRL 101, 230502 (2008)]. For objective 3, new routes for quantum information processing with limited resources, VV investigated how to enhance entanglement with

non-conventional routes [PRL 102, 100503 (2009), Eur. Phys. Lett. 81, 40006 (2008), Int. J. Theor. Phys. 47, 2126 (2008)]. MSK worked on how to extract a singlet state from non-interacting spins [PRL 100, 150501

(2008)]. VV and MSK investigated how to generate and keep entanglement in a macroscopic system such as an optomechanical mirror [PRL 98, 030405 (2007), PRL 99, 250401 (2007)].



Besides, VV and MSK were engaged in investigations on the fundamental aspects of quantum mechanics and QIP [RMP 80, 517 (2008), Nature 453, 1004 (2008), Science 317, 1890 (2007)].
Exploitation Route We strongly hope that our research will be of use to the indutry where the level of control goes to the quantum level. We were engaged in public lectures and discussion sessions to disseminate the research findings. This immediate beneficiaries are academic researchers working on quantum information processing. Some parts of our research will be interface between solid-state device and optics so both the communities will get benefit from our research.



We would like to emphasise the interdisciplinary nature within physics which already covers wide range of natural phenomena.
Sectors Digital/Communication/Information Technologies (including Software)

 
Description Our work has been published in high impact journals. We also presented the results in international conferences.
Sector Digital/Communication/Information Technologies (including Software),Education