Quantum Measurements with Photons

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

Measuring the length of an Olympic swimming pool doesn't affect how much water it has in it! We normally don't expect measuring things to change them. In the quantum world, things are very different.Quantum mechanics tells us how the world works at its most fundamental level. It predicts very strange behaviour that can typically only be observed when things are very cold and very small. It has an inbuilt element of chance, allows superpositions of two different states, and includes super-strong correlations between objects that would be nonsensical in our everyday world - entanglement . Despite this strange behaviour, quantum mechanics is the most successful theory that we have ever had - it predicts what will happen almost perfectly! However, it is not completely understood, and some of its implications are still being discovered.One of the great mysteries of quantum mechanics - The Measurement Problem - seeks to answer the question Why don't we see superpositions in the everyday world? ( alive and dead for example). Measurements play a special role in quantum mechanics and have been the subject of intense debate since the theory's development early last century. Recently quantum measurements have emerged to become an important practical issue. This is the result of the advent of quantum information science , which seeks to answer the question What advantage can be gained by specifically harnessing quantum mechanical effects in the storing, transmitting and processing of information? Anticipated future technologies include quantum computers with tremendous computational power, quantum metrology which promises the most precise measurements possible, and quantum cryptography which is already being used in commercial communication systems, and offers perfect security.Unlike measuring the length of a pool, measuring a quantum system necessarily disturbs the system. For example a standard measurement of a system in a superposition of two states finds the system in one of those states with some probability. After the measurement, the system is no longer in a superposition, but is in the state it was measured to be in with certainty. The original superposition state can never be recovered, and that information is lost.More general quantum measurements involve a payoff between the information gained and the disturbance of the system. Quantum mechanics also allows entangling measurements on two or more systems, that leave them in an entangle state. Finally, we can intentionally manipulate the system being measured depending on what the measurement tells us - feedback.These general quantum measurements could play an important role in future quantum technologies: the security of quantum cryptography relies on detecting an eavesdropper by the disturbance their measurements must cause; quantum metrology requires entangled measurements; and some schemes for quantum computation proceed via measurements alone.Single particles of light - photons - are excellent system for developing new quantum measurements, because they suffer from almost no noise. They also have great potential for application in future quantum technologies: schemes for all optical quantum computers are leading contenders, and photons are the obvious choice for both quantum communication and for quantum metrology schemes for measuring optical path lengths. This project will realise new quantum measurements which are entangled, tuneable in the amount of disturbance, and include feedback. It will use an optical crystal to produce up to six photons, optical circuits to realise controlled interactions between them (with feedback), and standard avalanche photodiodes to detect them. A particular focus will be on developing practical schemes for efficiently extracting information from quantum measurements. Finally, the project will design and implement techniques for distinguishing between quantum processes on up to 4 photons.

Publications

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Ahonen O (2008) Entanglement-enhanced quantum key distribution in Physical Review A

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Bonneau D (2010) Integrated quantum photonics in Optics InfoBase Conference Papers

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Bonneau D (2010) Integrated quantum photonics

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Clark A (2009) All-optical-fiber polarization-based quantum logic gate in Physical Review A

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Fulconis J (2009) Fibre source of intrinsically time bandwidth limited photon pairs in Optics InfoBase Conference Papers

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Fulconis J (2008) Quantum information processing with optical fibres in Optics InfoBase Conference Papers

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Fulconis J. (2008) Fibre implementation of a controlled-NOT gate in Conference on Quantum Electronics and Laser Science (QELS) - Technical Digest Series

 
Description This project realised new quantum measurements which are entangled, tuneable in the amount of disturbance, and include feedback.

A particular focus was on developing practical schemes for efficiently extracting information from quantum measurements.
Exploitation Route This work has been taken forward by Dr Jonathan Matthews at the University of Bristol. Dr Matthews has become a leader in the field and Quantum Metrology one of CQPs main research areas.
Sectors Digital/Communication/Information Technologies (including Software)

URL http://www.bristol.ac.uk/physics/research/quantum/
 
Description The initial results from this work have led to the appointment of an academic specialising in this area of research. Dr Jonathan Matthews, who worked on this project has been appointed as a Lecturer to the school of physics and will be developing the technology to take these initial results into a commercial environment.
First Year Of Impact 2010
Sector Digital/Communication/Information Technologies (including Software)
 
Description Imperial College London 
Organisation Imperial College London
Country United Kingdom 
Sector Academic/University 
Start Year 2007
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation Federal University of Rio de Janeiro
Country Brazil 
Sector Academic/University 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation ICFO - The Institute of Photonic Sciences
Country Spain 
Sector Charity/Non Profit 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation Leiden University
Country Netherlands 
Sector Academic/University 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation Sapienza University of Rome
Country Italy 
Sector Academic/University 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation University of Glasgow
Country United Kingdom 
Sector Academic/University 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description PHORBITECH - A Toolbox For Photon Orbital Angular Momentum Technology 
Organisation University of Naples
Country Italy 
Sector Academic/University 
PI Contribution Demonstrating and realization of OAM optical integration.
Collaborator Contribution The investigation of fundamental issues associated with OAM propagation and with OAM effects arising in optical components.
Impact Numerous publications and presentations at various conference and public lectures.
Start Year 2010
 
Description University of Hokkaido 
Organisation Hokkaido University
Country Japan 
Sector Academic/University 
Start Year 2007
 
Description University of Sydney 
Organisation University of Sydney
Country Australia 
Sector Academic/University 
Start Year 2007
 
Description Article in Financial Times 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact A front page article in the Financial Times was published regarding the use of the integrated approach to realise a quantum walk of two correlated photons in a 21 waveguide array.

Article published on the use of the integrated approach to realise a quantum walk of two correlated photons in a 21 waveguide array.

- Increased awareness of physics and its impact to society by the general public.
- An increase in the awareness of the Centre for Quantum Photonics
Year(s) Of Engagement Activity 2012
URL http://www.bristol.ac.uk/physics/news/2012/92.html
 
Description British Science Association Media Fellowship 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I used the skills I learnt on his Media Fellowship at the Irish Times to get the difficult subject of quantum photonics onto the front
pages of the national newspapers. I obtained skills in how to translate research into news and pitch science stories to editors.

Talks generated a lot of interest from the public and I formed a number of working relationships with science journalists as a result of this fellowship. The stories I wrote brought physics to millions of people, making the front page of the Financial Times in 2009. This was also covered in the Daily Mail, Telegraph, Nature as well as in the US and Australia.
Year(s) Of Engagement Activity 2009
URL http://www.britishscienceassociation.org/science-society/impacts