Squeezed spin states currently represent the leading strategy for reducing quantum noise in precision measurements of atomic sensors. These Gaussian states, however, have a limited limited ability to improve the measurement precision. The properties of more sensitive non-Gaussian states are harder to determine due to their increased complexity.

In our recent study, we identify the scaling of the quantum enhancement of non-Gaussian spin states, well beyond the reach of spin squeezing. Our analytical results provide feasible and optimal strategies to implement these ideas in atomic systems under realistic conditions. Congratulations to Youcef, our PhD student, for these results!

• Y. Baamara, A. Sinatra, M. Gessner, Scaling laws for the sensitivity enhancement of non-Gaussian spin states, Phys. Rev. Lett. 127, 160501(2021).

We are offering research projects as internships and Master's theses to be carried out in the Quantum Information Theory Group at ICFO.

Project topics include:

• Quantum correlations and quantum information theory

• Quantum metrology and quantum-enhanced measurement precision

• Quantum optics and atomic spin systems

Please contact Manuel Gessner by email if you are interested.

Quantum information theory and metrology

We are offering a PostDoc position at ICFO in the field of quantum information theory and metrology. Possible topics of research include

• Development of quantum metrology protocols in atomic or optical systems

• Quantum-inspired superresolution imaging

• Many-body physics of ultracold atoms

• Theory of quantum correlations (multipartite entanglement, steering, etc)

More details on the opening and the application process can be found here.

After three fantastic years in Paris, our research will continue at ICFO, starting November 1st 2021. A PostDoc position will be available.

Quantum hypothesis testing and exoplanet detection

We consider the problem of detecting the faint emission of a secondary source in the proximity of the much brighter source. We frame this problem as (asymmetric) quantum state discrimination and compare with direct imaging. We find that one can significantly reduce the probability of error for detecting the presence of the weak secondary source, even when the two sources have small angular separations. If the weak source has relative intensity ϵ≪1 to the bright source, we find that the error exponent can be improved by a factor of 1/ϵ. We also find the measurements that are optimal in this regime, which are interferometric measurements. We suggest an application for the search of exoplanets using imaging techniques.

Quantum phase transitions describe abrupt changes of the ground state of a many-body system when a control parameter of the Hamiltonian is varied across a critical value. The impact of such a transition may, however, be visible in the entire excitation spectrum beyond the ground state. An excited-state quantum phase transition describes a singularity of the higher-excited eigenstates that can be crossed not only by varying a control parameter but also by varying energy.

In our recent work, we show how such an excited-state quantum phase transition can be observed in a spinor Bose-Einstein condensate, and how the properties of different excited-state phases can be distinguished using a dynamical, topological order parameter.

• P. Feldmann, C. Klempt, A. Smerzi, L. Santos and M. Gessner, Interferometric order parameter for excited-state quantum phase transitions in Bose-Einstein condensates, Phys. Rev. Lett. 126, 230602 (2021).

The Einstein-Podolsky-Rosen (EPR) paradox allows us to measure with high precision non-commuting observables of a remote system using a strong form of quantum correlations, called steering. This is paradoxical because it seems to contradict a local complementarity principle, which manifests, e.g., as the Heisenberg uncertainty relation.

In our recent work, we use tools from quantum metrology to formulate this local complementarity principle in a sharper way. This leads to a metrological formulation of the EPR paradox that reveals the underlying steering correlations more efficiently than methods that are based on the uncertainty relation, especially when dealing with non-Gaussian states.

• B. Yadin, M. Fadel, M. Gessner, Metrological complementarity reveals the Einstein-Podolsky-Rosen paradox, Nat. Commun. 12, 2410 (2021).

See also the press coverage by the University of Basel.

My latest colloquium on "Quantum parameter estimation: from fundamentals to applications" for the Donostia International Physics Center (San Sebastián) was recorded live on youtube. You can watch it again under this link: 

https://youtu.be/XNbl9fXiqZY

Abstract: The experimental advances of the last decades have made quantum correlated states of light and matter available in today's laboratories, but the efficient characterization of their multipartite entanglement still poses a great challenge for theory and experiment. Mastering this challenge is a necessary step towards the large-scale implementation of ideas from quantum information theory with potential applications in the development of quantum technologies. Quantum parameter estimation theory, for instance, identifies strategies to overcome classical precision limits of measurements by identifying highly sensitive quantum states and measurement observables. This talk will provide an overview of our recent progress in this field, highlighting in particular the close connection between metrological sensitivity and multipartite entanglement. We will see how suitable observables that capture delicate features of complex quantum states can be identified under experimental constraints, how entanglement can be detected with tools from metrology, and how collective quantum enhancements can be achieved in the simultaneous estimation of multiple parameters. As applications we will show how this theory can improve the precision of atomic clocks and the optical resolution of imaging systems.

How entangled is my quantum system? This question may be answered in many different ways. Often, we are tempted to compress the complex information about multipartite entanglement into a simple integer number, e.g., the number of entangled parties (entanglement depth) or of separable subsets (k-separability). A more precise answer to this question, however, may be given by a Young diagram that describes the separable partitions of a system. 

In our work, we derive entanglement witnesses that are sensitive to the properties of Young diagrams and thereby generalize some of the most widely used entanglement criteria. Central properties of Young diagrams (width, height, Dyson's rank)  correspond to different entanglement quantifiers that are linked to the metrological quantum gain in precision measurements.

• Z. Ren, W. Li, A. Smerzi, and M. Gessner, Metrological detection of multipartite entanglement from Young diagrams, Phys. Rev. Lett. 126, 080502 (2021).