Matteo Fadel from the University of Basel will be visiting ENS from 11/11 - 20/11.

Criteria for k-separability, k-producibility and k-strechability of multipartite continuous variable entanglement

Understanding the multipartite entanglement structure of quantum states is an important and complicated issue in quantum information. Typically, the depth of genuine multipartite entanglement is described by the number of particles that are entangled with each other, i.e., the k-producibility. An alternative measure is the k-separability that expresses into how many separable subsets the system can be divided. Here we consider a description of multipartite entanglement in terms of partitions that reveals the relationship between the two concepts. Moreover, a deeper understanding of the entanglement properties is provided by the recently introduced concept of k-stretchability. We introduce experimentally feasible uncertainty-based criteria for all of the three quantities and compare them for the example of quadripartite quantum optical entangled states generated by four-wave-mixing (FWM).

We are happy to welcome Shao Hen Chiew from Ecole des Mines de Paris (Mines ParisTech) and the National University of Singapore for a six-month research internship Sept 2019 - Feb 2020.

Visiting PhD student Zhihong Ren from Shanxi University is joining us May 2019 - Nov 2019

Youcef Baamara from Sorbonne Université is joining us for an internship May 2019 - July 2020

The well-known spin squeezing coefficient efficiently quantifies the sensitivity and entanglement of Gaussian states. However, this coefficient is insufficient to characterize the much wider class of highly sensitive non-Gaussian quantum states. In this talk, we present an extension of spin squeezing based on reduced variances of nonlinear observables. An optimization of the measurement observable under experimental constraints further allows us to identify those observables that will yield the highest achievable sensitivity and the strongest criterion for entanglement. Our results can be used to identify optimal quantum-enhanced phase estimation protocols and entanglement witnesses for increasingly complex quantum states.

Can we exploit the spin ground state of a Bose-Einstein condensate (BEC) for quantum-enhanced metrology? Metrologically useful entanglement is witnessed by the Fisher information (FI), which is upper bounded by the Heisenberg limit (HL). First, we analyze the ground state of ferromagnetic spin-1 BECs at various quadratic Zeeman shifts q. At the central broken axisymmetry (CBA) state at q = 0 we find Heisenberg scaling of the FI. Optimal phase imprinting corresponds to a radiofrequency pulse, and the optimal measurement procedure is counting particles in different magnetic modes. Second, we study the preparation of the CBA state by changing q across a quantum phase transition. We account for particle loss by using the Monte Carlo wave-function method. For 100 particles we find that 25% of the ideal FI survive under realistic conditions. Third, we observe that measuring the particle number in one mode with high probability prepares a macroscopic superposition state (MSS) in the two remaining modes. The FI of the ideal MSSs reaches the HL. The MSS structure is preserved under realistic conditions. In conclusion, we propose a robust preparation of a state which hosts MSSs and can be used for quantum-enhanced metrology close to the HL.

Polina Feldmann from the Leibniz Universität Hannover will be visiting us at ENS from 01/03 - 29/03. She is located in GH223.

Frank Schlawin from the University of Oxford will be visiting us at ENS from 27/02 - 01/03.

Quantum Enhanced High-Precision Measurements of Multiple Parameters

The field of quantum metrology strives to use quantum properties, such as entanglement, to enhance the precision of measurements. In the context of applications such as quantum imaging and field sensing, not just one but multiple parameters must be estimated. This uncovers a series of questions from most fundamental concerns to practical considerations along the path towards the realization of quantum technologies.

In the framework of this internship, we shall theoretically study the generation and characterization of highly sensitive quantum states for multiparameter measurements, as well as suitable observables for precision measurements. We will apply our results in the context of atomic spin systems or photonic multimode continuous-variable systems.

The project will provide the opportunity to get familiar with the topics of

• Quantum information theory (entanglement, quantum measurement process, metrology, ...)

• Quantum optics (Phase-space formalism, continuous variables, ...)

• Bose-Einstein condensates (Spinor quantum gases, discrete variables, ...)

An extension towards a PhD thesis is in principle possible. Collaborations with local experimental groups at LKB on the same topic can be established.

Any potentially interested candidates are encouraged to contact me (come see me in my office GH216 or write an email), also in case of any questions or requests for further details.