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Quantum Metrology in Applications

Periodic Reporting for period 1 - Q-METAPP (Quantum Metrology in Applications)

Reporting period: 2015-04-01 to 2017-03-31

"Quantum technologies have potential of changing in not too distant future the way how our everyday-use devices―such as cars, mobiles or laptops―operate. Apart from more renowned and widely announced benefits of quantum communication or computing, a field that is within a closer reach of the current technologies is quantum sensing, more generally termed as quantum metrology. Its main motivation is to design and harness quantum features of light and matter, in order to be able to perform measurements with unprecedented precisions—limited only by the quantum structure of nature, while simultaneously benefiting from its unique properties such as the phenomenon of entanglement. Quantum-enhanced sensing devices have already been successfully demonstrated in many settings, however, the main obstacle that keeps restricting their performance and impedes their commercialisation is the impact of noise that, unfortunately, is much more efficient in destroying the necessary quantum properties. That is why, in recent years, a huge part of both theoretical and experimental research has been devoted to improving quantum sensing protocols by making them noise-robust and, hence, more implementation friendly.

Within this action novel advanced tools of quantum information theory have been used, in order to achieve such goal in proposing a new generation of quantum metrology schemes. In particular, within the project, noise-robust protocols have been identified in atomic magnetometry, atomic spectroscopy and general settings involving fast control and error-correction operations, all of which have been shown to be capable of beating the standard limits imposed on precision by the noise, and fully benefit from the quantum properties of either light or matter. Moreover, within the theoretical part of the action, even more hope has been shed by demonstrating that the phenomenon of quantum-enhancement in sensing is typical in nature―systems prepared in a random manner at the quantum level should typically allow for the classical precision limits to be broken. Finally, by considering the so-called multi-stage architectures in which, e.g. atoms are utilised as sensors of external fields while being constantly measured with the light, it has been demonstrated by an explicit experiment that, thanks to the sophisticated data inference techniques of classical estimation theory, the noisy output signals can be effectively filtered in real-life implementations, in order to recover the underlying features and track in real time signals encoded ""deep inside"" a given device at the quantum level, despite all the noise appearing ""on the top"".
The project Q-METAPP has been a great success reaching all the key goals set two years ago during its commencement. The dominant part of the action has been dedicated to the research of noise-robust quantum metrology protocols and resulted in the following three major achievements:

• A proposal of the first experimental setup in atomic magnetometry capable of beating the standard limits imposed by the uncorrelated noise.
• Finding of an explicit relation between the scaling of precision attainable in atomic spectroscopy to the dynamics of the atoms being employed.
• Proposal of novel metrology protocols that by using fast control and error-correction operations are capable to fully counterbalance the destructive impact of uncorrelated noise.

Each of the above accomplishments has involved extensive international collaboration with different European institutions and has lead to independent well-received scientific publications.

The second subproject of the action has resulted in two important results answering a long-standing question: whether the quantum enhancement in metrology is a typical phenomenon observed in nature. In particular, thanks to collaboration within the theory groups at the host institution, it has been demonstrated that random states of bosons (in particular, photons) not only exhibit the ultimate Heisenberg scaling in quantum sensing but also preserve it under finite particle losses.

The last major part of the action has been devoted to a joint theory-experiment project conducted at the host institution. As a result of the collaboration, a novel sensing experiment has been designed and conducted, in which an two-stage sensor based on rubidium atoms has been used to track time-varying signals of light beyond the state-of-the-art resolutions. The work has potential to revolutionise the technology of atomic sensors as it has proven, for the first time, that such devices are capable to set a new standard in waveform estimation.

Thanks to the mobility allowance of the fellowship, the works have been presented by the main beneficiary at 15 conferences, workshops and meetings (involving 3 invited talks, 6 contributed talks as well as 5 invited seminars) at institutions both within the EU and the US. Moreover, that action has reached an audience beyond the scientific community, as it has included series of lectures held at the host institutions, as well two tutorial sessions given at regional workshops that have gathered scientists, as well as undergraduate students, working at other local institutions. Consequently, a part of the project has been dedicated to supervision of undergraduate student that resulted in a summer-project report as well as a BA thesis defended on the topic directly related to the action.
The impact of the fellowship can be looked at from three important perspectives. Firstly, the action has been a great scientific success, whose key results have already inspired research being conducted not only at the host, but also at other EU, institutions. Moreover, as each of its subproject involved extensive international collaborations, it has highly strengthened bonds between the host and various research centres spread around the European countries. Secondly, it has opened doors for the beneficiary to continue his career as a mature researcher at world-class institutions, while immensely broadening his scientific expertise. Finally, all the know-how behind the results obtained within the action has been efficiently passed not only onto researchers and graduate students at the host institution but also scientists―as well as university undergraduates to some extent―working at other institutes within the Barcelona area, where the action has been carried out.