Continuous-Variable Quantum Detector And Process Tomography

Project Information

CV-QDAPT

Grant agreement ID: 301032

Closed project

Start date 14 January 2013

End date 13 January 2015

Funded under

• Specific programme "People" implementing the Seventh Framework Programme of the European Community for research, technological development and demonstration activities (2007 to 2013)

Total cost € 209 033,40

EU contribution € 209 033,40

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Final Report Summary - CV-QDAPT (Continuous-Variable Quantum Detector And Process Tomography)

CV-QDAPT: Continuous Variable Quantum Detector and Process Tomography

Michal Karpinski and Brian J. Smith
Department of Physics, University of Oxford
e-mails: [e-mail] [e-mail]
project website: www2.physics.ox.ac.uk/research/optical-quantum-technologies/cv-qdapt

Quantum mechanics, a theory that describes the properties of microscopic particles, such as atoms, molecules or of quanta of electromagnetic radiation – photons, predicts existence multiple counter-intuitive phenomena stemming from the quantum mechanical superposition principle. One example of such phenomenon is the possibility of creating objects in an entangled state – such as highly correlated pairs of distant photons in which a measurement of a property of one photon instantaneously affects the properties of the second one. Only recently have developments in technology allowed to observe these fascinating phenomena experimentally. This and other quantum effects lead to multiple applications of the quantum properties of light, from low intensity light sensing, through extremely precise experimental measurement techniques utilising capability to engineer the quantum properties of light, to secure communication links, whose security is guaranteed by the fundamental laws of physics.
Utilizing quantum states of light offers new exciting possibilities, yet also, due to fragility of their quantum properties, requires special care when preparing, manipulating and detecting them. Whereas some aspects of the properties of low intensity quantum states of light, like polarization, have been well understood and are well controlled experimentally, other still require development of tools and methods to deal with them. One of them are the amplitude and phase properties of light, called the optical field quadratures. They describe the relations between different photon number contributions forming the quantum state of light and are intricately related to the noise and uncertainty properties of low intensity light. Study of these aspects of light forms the field of continuous-variable (CV) quantum optics. Apart from broadening our understanding of quantum properties of light, it yields applications in areas such as ultraprecise low-light level metrology or efficient encoding of information for use in secure communication links.
Description and manipulation of quadrature states of light is an experimentally and theoretically involved task and is currently a rapidly developing field of research. Whereas sources of light with quantum mechanical properties are relatively well developed, this can neither be said about the processes used to manipulate those quantum states of light, nor about the detectors used to detect the light quadratures, called balanced homodyne detectors (BHDs). BHDs are unique measurement devices, in which the measured quantum state of light is combined with a high intensity laser beam, which serves as a means to amplify and detect the quantum properties of light.
In this project we developed experimental and theoretical tools needed to perform full quantum mechanical characterization of a BHD, by recording its response to a set of coherent states – well-known quantum states of light, produced by a stabilized laser, whose properties were verified by independent means. In parallel with experimental work we worked on mathematical and numerical means of reconstructing the mathematical object describing the full quantum mechanical action of a detector. We theoretically analysed several approaches to extracting the detector characterization from experimental data, and found the so-called operational tomography approach to be the most appropriate for experimental implementation. We performed full characterization of the detector response to few-photon level pulses of light by means of operational tomography, and subsequently used it to measure the quantum state of several non-classical states of light. Our work constituted the first independent characterization of a BHD and first experimental implementation of the operational approach to quantum detector and state tomography.
Having verified the action of the BHD we worked on performing faithful characterization of quantum processes. We experimentally implemented the only recently developed method of coherent state quantum process tomography (csQPT) to fully characterize a complex quantum process of Fock state filtration (FSF). This process is an important non-trivial example of so-called measurement induced nonlinearity. It probabilistically enables creation of effective interaction between photons, which under normal conditions do not interact with each other. We performed full characterization of the process by estimating its process tensor from experimental data obtained by probing it with well characterized coherent states. We also developed a faithful numerical model of the process, which provided an excellent match to experimental results. Our results enabled us to identify critical aspects of experimental implementation of the process that need to be met for its optimal operation. They will form an important basis for work on design and implementation of building blocks for further development of application of quadrature states of light in communication and precise measurement techniques, as well as also in the field of low intensity light sensing.
In the experiment described above characterization of the detector by means of operational tomography was only used in an indirect manner, to enable direct application of the csQPT method. Within the project we theoretically developed a novel approach to quantum process tomography that enables direct use of the operational characterization of a detector. Our work complements the existing operational approach to quantum state tomography, thus completing the toolbox for quantum tomography with an independently characterized detector.
Summarizing, the objectives of the project – performing tomography of a balanced homodyne detector and performing tomography of the Fock state filtration quantum process – were successfully realized. Additionally a new approach to quantum process tomography with an unknown detector has been proposed and numerically tested.
Let us now turn to discussing the impact of the project in the following areas: advancement of research and scientific knowledge, impact on the career of the project fellow, and impact on the wider society, including European research society.
The above mentioned results of the project form an important step in advancement of the techniques of continuous variable quantum optics. Some of them – such as the operational approach to tomography – will also have applications to quantum systems other than light. The work described above yielded two published manuscripts in high profile journals, with a further manuscript currently in final stages of preparation, and has been presented at 5 international conferences, including two invited talks. This confirms the high quality of the scientific output of the project.
Work on the above listed tasks has enabled the project fellow, Dr Michal Karpinski, to significantly advance his research career, skill set and interests towards a position of professional maturity. Not only did he make ground-breaking research progress, but also supervised students, participated in research paper and grant proposal writing, multiple conferences and research visits, co-organized workshops etc. The project enabled him to make multiple new connections within the research community. We are convinced the project had a significant positive impact on the fellow’s research career.
The societal impact of the project is two-fold. Dr Karpinski is planning to continue his research career in his native Poland, where he will bring both the research skills and research connections acquired through this project. This will have a positive impact on links between EU countries, advancing the research culture in Poland and optimal use of equipment that has been acquired by Polish research institutions through EU structural funds. Thus we expect the contribution of the project to the European Research Area to be high.
The impact of the project on general society is expected to be less direct, yet potentially of very high value. The nature of the research carried out within the project is mostly fundamental. Yet, through furthering excellence in CV quantum optics the project danced work toward practical implementations of quantum technologies in areas such as metrology, sensing, efficient simulation and communication. These technologies, after reaching the maturity level needed for practical implementation, will have a significant transformative impact on the society.