# Stable Vortex Solitons in Microwave-coupled Atomic Condensates

*By Guangjiong Dong, East China Normal University*

# Quantum Storage of Entanglement in a Cold Atomic Ensemble

*By Bao-Sen Shi, University of Science and Technology of China*

The storage of photonic entanglement is central to the achievement of long-distance quantum communications based on quantum repeaters and scalable linear optical quantum computation. In this talk, I will focus on the review of the experimental progresses achieved on the quantum storage of the quantum entanglement in a cold atomic ensemble in our group recently, including the storage of polarization entanglement, orbital angular momentum entanglement, hyper-entanglement, etc.

# Tunable Parity-time-symmetric Waveguides in a Coherent Atomic Medium

*By Chao Hang, East China Normal University*

# Multiple Period States of the Superfluid Fermi Gas in an Optical Lattice

*By Gentaro Watanabe, Zhejiang University*

# Highly Squeezed 87Rb Atom Condensate through Quantum Phase Transitions

*By Li You, Tsinghua University*

An important direction of atomic quantum gas research concerns the generation of large entangled states. In addition to their applications in quantum information and quantum simulation, entangled states are useful for enhanced metrological precisions beyond the standard quantum limit (SQL) 1/N1/2 of N uncorrelated atoms. Metrologically useful spin squeezed states have been extensively explored and generated in various systems, including in Bose Einstein Condensate (BEC) through collisional interactions and in atomic vapors based on quantum nondemolition measurements.

Twin-Fock state (TFS) with equal number of particles in two orthogonal modes, can provide near Heisenberg limit (HL) precision of 1/N, relying on correlations from particle exchange symmetries. A mixture of TFSs can be created from waiting for spin mixing dynamics to evolve in atomic condensate through elementary spin exchange collisions whereby two atoms in the m_F=0 component oscillate into one atom each in the m_F=±1 components and vice versa. In 2011, three experiments detected such quantum correlations based on spin mixing dynamics in 87Rb spinor BEC [1-3], respectively revealed by a reduced fluctuation in the population difference between the m_F=±1 components [1], two-mode quadrature squeezing through an atomic homodyne detection [2], and the creation of large TFS ensembles of up to 104 atoms and a claimed interferometric sensitivity -1.61dB below the SQL [3].

An important characteristic for the TFS ensemble prepared this way is the broad distribution for the total number of atoms (N) in m_F=±1. To overcome this inherent fluctuation, one post selects experimental data sets whereby a narrow range of N is adopted. Such an approach however significantly reduces the experimental efficiency on top of the already non-optimal conversion efficiency of the TFS. Furthermore, TFS generated from spin mixing dynamics is sensitive to experimental imperfections and external noises during the evolution.

We shall discuss our recent work of deterministic generation of TFS in the F=1 ground hyperfine manifold of 87Rb BEC through quantum phase transitions. By manipulating the net quadratic Zeeman shift, which is the sum of a shift from a bias static magnetic field and the ac-Zeeman shift from a dressing microwave, we drive the ground state condensate initially of all atoms in the m_F=0 component into twin-Fock state with near unity efficiency. The striking narrow distribution of N allows us to beat the quantum shot noise (QSN) limit without post-selection. Specifically, the fluctuations in the population difference between the two modes of the TFS samples are observed to be highly squeezed, at -9.9(2) dB below the QSN. Subtracting detection noise leads to an improved squeezing of -11.8(3) dB, which together with the measured collective spin length of 0.99(1) N/2, translates into an entanglement depth of 640 atoms, primarily limited by the loss of atoms.

We anticipate our results will establish a benchmark for producing entangled atomic BECs, which will provide previously unavailable opportunities to study their properties and to clarify their metrological implications.

We acknowledge the support by MOST (No. 2013CB922004) of the National Key Basic Re-search Program of China, and by NSFC (No. 91121005, No. 91421305, and No. 11374176).

1. E. M. Bookjans et al, Phys. Rev. Lett. 107, 210406 (2011).

2. C. Gross et al, Nature 480, 219 (2011).

3. B. Lucke et al, Science 334, 773 (2011).

# Noise-resilient Control of Nuclear Spins in Interacting Clusters for Diamond-based Quantum Computing and Bio-imaging

*By Zhen-Yu Wang, Ulm University*

# Light Matter Quantum Interface Based on Single Colour Centres in Diamond

*By Fedor Jelezko, Ulm University*

Efficient interfaces between photons and atoms are crucial for quantum networks and enable nonlinear optical devices operating at the single-photon level. In this talk I will highlight properties of single colour centres at low temperatures and show that single SiV and GeV colour centres in diamond are promising candidates for creating such interfaces. I will also show experiments aiming to create technologies allowing realization of fully integrated, scalable nanophotonic quantum devices.

# Precision Spectroscopic Applications of Cold Molecular Ions

*By Xin Tong, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences*

# Nanofiber-trapped Atomic Ensemble Interfaces: Chances and Challenges

*By Jürgen Appel, University of Copenhagen*

In the search for systems suitable for implementing light-matter quantum interfaces the interaction achieved with guided photons has emerged as one of the most prospective candidates. Efficient coupling of guided photons has been achieved with such different systems as atoms or quantum dots coupled to photonic bandgap waveguides or atoms in a hollow core fiber.

In our setup, we trap two strings of atoms in the evanescent field of a tapered optical fiber (TOF) so that they are strongly coupled to the single guided photonic fiber mode. We present 12% coherent Bragg scattering off such a one-dimensional system of approx. 1000 atoms, realized by modulating the light-atom interaction by mechanical motion. We investigate the inhomogeneous coupling of the thermal ensemble, evaluate its importance for quantum technology applications and present tools to operate in its presence to perform tasks such as state tomography and spin squeezing.

# Robust Quantum Computation with Diamond Defects and Trapped Ions

*By Luming Duan, University of Michigan*

In this talk, I will briefly explain how to use phonons in a linear ion chain to realize scalable boson sampling, which provides a possibility to disprove the extended Church-Turing thesis. I will also explain recent experiments to use the opto-mechanical coupling in a diamond to realize quantum teleportation from photons to phonons and to use diamond defects to realize robust geometric quantum computation.