Meng Khoon Tey

Bose-Einstein Condensation of Strontium

We report on the attainment of Bose-Einstein condensation with ultracold strontium atoms. We use the (84)Sr isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. Accumulation in a metastable state using a magnetic-trap, narrowline cooling, and straightforward evaporative cooling in an optical trap lead to pure condensates containing 1.5 x 10(5) atoms. This puts (84)Sr in a prime position for future experiments on quantum-degenerate gases involving atomic two-electron systems.

Qutrit state engineering with biphotons.

The novel experimental realization of three-level optical quantum systems is presented. We use the polarization state of biphotons to generate a specific sequence of states that are used in the extended version of four-state QKD protocol quantum key distribution protocol. We experimentally verify the orthogonality of the basic states and demonstrate the ability to easily switch between them. The tomography procedure is employed to reconstruct the density matrices of generated states.

Second sound and the superfluid fraction in a Fermi gas with resonant interactions

Superfluidity is a macroscopic quantum phenomenon occurring in systems as diverse as liquid helium and neutron stars. It occurs below a critical temperature and leads to peculiar behaviour such as frictionless flow, the formation of quantized vortices and quenching of the moment of inertia. Ultracold atomic gases offer control of interactions and external confinement, providing unique opportunities to explore superfluid phenomena. Many such (finite-temperature) phenomena can be explained in terms of a two-fluid mixture comprising a normal component, which behaves like an ordinary fluid, and a superfluid component with zero viscosity and zero entropy. The two-component nature of a superfluid is manifest in ‘second sound’, an entropy wave in which the superfluid and the non-superfluid components oscillate with opposite phases (as opposed to ordinary ‘first sound’, where they oscillate in phase). Here we report the observation of second sound in an ultracold Fermi gas with resonant interactions. The speed of second sound depends explicitly on the value of the superfluid fraction, a quantity that is sensitive to the spectrum of elementary excitations. Our measurements allow us to extract the temperature dependence of the superfluid fraction, a previously inaccessible quantity that will provide a benchmark for theories of strongly interacting quantum gases.

Phase shift of a weak coherent beam induced by a single atom.

We report on a direct measurement of a phase shift on a weak coherent beam by a single 87Rb atom in a Mach-Zehnder interferometer. By strongly focusing the probe mode to the location of the atom, a maximum phase shift of about 1 degree is observed experimentally.

Statistical reconstruction of qutrits

We discuss a procedure of measurement followed by the reproduction of the quantum state of a three-level optical system - a frequency- and spatially degenerate two-photon field. The method of statistical estimation of the quantum state based on solving the likelihood equation and analyzing the statistical properties of the obtained estimates is developed. Using the root approach of estimating quantum states, the initial two-photon state vector is reproduced from the measured fourth moments in the field . The developed approach applied to quantum states reconstruction is based on the amplitudes of mutually complementary processes. Classical algorithm of statistical estimation based on the Fisher information matrix is generalized to the case of quantum systems obeying Bohrs complementarity principle. It has been experimentally proved that biphoton-qutrit states can be reconstructed with the fidelity of 0.995-0.999 and higher.

Deterministic entanglement generation from driving through quantum phase transitions

Transitional approach to entanglement In an entangled many-particle system, changing the state of one constituent affects the rest of the system. This property can be used as a resource in quantum information processing, but getting many particles to participate in entanglement is tricky. Luo et al. used another collective phenomenon, a quantum phase transition, to entangle more than 900 atoms in a Bose-Einstein condensate. The size of the entangled ensemble remained stable, making the approach practical for precision measurements. Science, this issue p. 620 A large entangled 87Rb atom ensemble robust to number fluctuations was created. Many-body entanglement is often created through the system evolution, aided by nonlinear interactions between the constituting particles. These very dynamics, however, can also lead to fluctuations and degradation of the entanglement if the interactions cannot be controlled. Here, we demonstrate near-deterministic generation of an entangled twin-Fock condensate of ~11,000 atoms by driving a rubidium-87 Bose-Einstein condensate undergoing spin mixing through two consecutive quantum phase transitions (QPTs). We directly observe number squeezing of 10.7 ± 0.6 decibels and normalized collective spin length of 0.99 ± 0.01. Together, these observations allow us to infer an entanglement-enhanced phase sensitivity of ~6 decibels beyond the standard quantum limit and an entanglement breadth of ~910 atoms. Our work highlights the power of generating large-scale useful entanglement by taking advantage of the different entanglement landscapes separated by QPTs.

Double-degenerate Bose-Fermi mixture of strontium

Sr.This symmetry can lead to new quantum phases in opti-cal lattices [3–6], like the chiral spin liquid. Non-Abeliangauge potentials can be realized by engineering state de-pendent lattices [7]. In addition, the nuclear spin canbe used to robustly store quantum information, whichcan be manipulated using the electronic structure [8, 9].Double-degenerate Bose-Fermi mixtures extend the pos-sibilities evenfurther, allowingto study phase-separationand the effects of mediated interactions.Evaporative cooling of ultracold atoms to quantumdegeneracy relies on elastic collisions to thermalize thesample. Identical fermions do not collide at low tem-peratures, therefore mixtures of spins [10–14], isotopes[15–17], or elements [18–20] are used for evaporation.

Collective Modes in a Unitary Fermi Gas across the Superfluid Phase Transition

We provide a joint theoretical and experimental investigation of the temperature dependence of the collective oscillations of first sound nature exhibited by a highly elongated harmonically trapped Fermi gas at unitarity, including the region below the critical temperature for superfluidity. Differently from the lowest axial breathing mode, the hydrodynamic frequencies of the higher-nodal excitations show a temperature dependence, which is calculated starting from Landau two-fluid theory and using the available experimental knowledge of the equation of state. The experimental results agree with high accuracy with the predictions of theory and provide the first evidence for the temperature dependence of the collective frequencies near the superfluid phase transition.

Bose-Einstein condensation of 86Sr

We report on the attainment of Bose-Einstein condensation of 86Sr. This isotope has a scattering length of about +800 a0 and thus suffers from fast three-body losses. To avoid detrimental atom loss, evaporative cooling is performed at low densities around 3x10^12 cm^-3 in a large volume optical dipole trap. We obtain almost pure condensates of 5x10^3 atoms.

Observation of broad p -wave Feshbach resonances in ultracold Rb85−Rb87 mixtures

Broad entrance-channel