Chen-Lung Hung

In situ observation of incompressible Mott-insulating domains in ultracold atomic gases.

The observation of the superfluid to Mott insulator phase transition of ultracold atoms in optical lattices was an enabling discovery in experimental many-body physics, providing the first tangible example of a quantum phase transition (one that occurs even at zero temperature) in an ultracold atomic gas. For a trapped gas, the spatially varying local chemical potential gives rise to multiple quantum phases within a single sample, complicating the interpretation of bulk measurements. Here we report spatially resolved, in-situ imaging of a two-dimensional ultracold atomic gas as it crosses the superfluid to Mott insulator transition, providing direct access to individual characteristics of the insulating, superfluid and normal phases. We present results for the local compressibility in all phases, observing a strong suppression in the insulator domain and suppressed density fluctuations for the Mott insulator, in accordance with the fluctuation–dissipation theorem. Furthermore, we obtain a direct measure of the finite temperature of the system. Taken together, these methods enable a complete characterization of multiple phases in a strongly correlated Bose gas, and of the interplay between quantum and thermal fluctuations in the quantum critical regime.

Quantum many-body models with cold atoms coupled to photonic crystals

Using cold atoms to simulate strongly interacting quantum systems is an exciting frontier of physics. However, because atoms are nominally neutral point particles, this limits the types of interaction that can be produced. We propose to use the powerful new platform of cold atoms trapped near nanophotonic systems to extend these limits, enabling a novel quantum material in which atomic spin degrees of freedom, motion and photons strongly couple over long distances. In this system, an atom trapped near a photonic crystal seeds a localized, tunable cavity mode around the atomic position. We find that this effective cavity facilitates interactions with other atoms within the cavity length, in a way that can be made robust against realistic imperfections. Finally, we show that such phenomena should be accessible using one-dimensional photonic crystal waveguides in which coupling to atoms has already been experimentally demonstrated.

Observation of scale invariance and universality in two-dimensional Bose gases.

The collective behaviour of a many-body system near a continuous phase transition is insensitive to the details of its microscopic physics; for example, thermodynamic observables follow generalized scaling laws near the phase transition. The Berezinskii–Kosterlitz–Thouless (BKT) phase transition in two-dimensional Bose gases presents a particularly interesting case because the marginal dimensionality and intrinsic scaling symmetry result in a broad fluctuation regime and an extended range of universal scaling behaviour. Studies of the BKT transition in cold atoms have stimulated great interest in recent years, but a clear demonstration of critical behaviour near the phase transition has remained elusive. Here we report in situ density and density-fluctuation measurements of two-dimensional Bose gases of caesium at different temperatures and interaction strengths, observing scale-invariant, universal behaviours. The extracted thermodynamic functions confirm the existence of a wide universal region near the BKT phase transition, and provide a sensitive test of the universality predicted by classical-field theory and quantum Monte Carlo calculations. Our experimental results provide evidence for growing density–density correlations in the fluctuation region, and call for further explorations of universal phenomena in classical and quantum critical physics.

Subwavelength vacuum lattices and atom–atom interactions in two-dimensional photonic crystals

Quantum simulation with cold atoms in optical lattices is an attractive avenue for explorations of quantum many-body physics. A principal challenge in the field is to increase the energy and length scales in current set-ups, thereby reducing temperature and coherence-time requirements. Here, we present a new paradigm for high-density, two-dimensional optical lattices in photonic crystal waveguides. Specially engineered two-dimensional photonic crystals provide a practical platform to trap atoms and engineer their interactions in ways that surpass the limitations of current technologies and enable investigations of novel quantum many-body matter. Our schemes remove the constraint on the lattice constant set by the free-space optical wavelength in favour of deeply sub-wavelength atomic arrays. We further describe possibilities for atom–atom interactions mediated by photons in two-dimensional photonic crystal waveguides with energy scales several orders of magnitude larger than for exchange interactions in free-space lattices and with the capability to engineer strongly long-range interactions.

Nanowire photonic crystal waveguides for single-atom trapping and strong light-matter interactions

We present a comprehensive study of dispersion-engineered nanowire photonic crystal waveguides suitable for experiments in quantum optics and atomic physics with optically trapped atoms. Detailed design methodology and specifications are provided, as are the processing steps used to create silicon nitride waveguides of low optical loss in the near-IR. Measurements of the waveguide optical properties and power-handling capability are also presented.

Slow mass transport and statistical evolution of an atomic gas across the superfluid-Mott-insulator transition.

We study near-equilibrium thermodynamics of bosonic atoms in a two-dimensional optical lattice by ramping up the lattice depth to convert a superfluid into an inhomogeneous mixture of superfluid and Mott insulator. Detailed study of in situ density profiles shows that, first, locally adiabatic ramps do not guarantee global thermal equilibrium. Indeed, full thermalization for typical parameters only occurs for experiment times which exceed one second. Secondly, ramping non-adiabatically to the Mott insulator regime can result in strong localized cooling at short times and global cooling once equilibrated. For an initial temperature estimated as 20 nK, we observe local temperatures as low as 1.5 nK, and a final global temperature of 9 nK. Possible cooling mechanisms include adiabatic decompression, modification of the density of states near the quantum critical regime, and the Joule-Thomson effect. **NOTE: Following submission of arXiv:0910.1382v1, a systematic correction was discovered in the density measurement, stemming from three-body losses during the imaging process. New measurements were performed, and the result is in support of the claim on the slow global dynamics. Due to the substantially altered methods and analysis, a new text has been posted as arXiv:1003.0855.We study transport dynamics of ultracold cesium atoms in a two-dimensional optical lattice across the superfluid-Mott-insulator transition based on in situ imaging. Inducing the phase transition with a lattice ramping routine expected to be locally adiabatic, we observe a global mass redistribution which requires a very long time to equilibrate, more than 100 times longer than the microscopic time scales for on-site interaction and tunneling. When the sample enters the Mott-insulator regime, mass transport significantly slows down. By employing fast recombination loss pulses to analyze the occupancy distribution, we observe similarly slow-evolving dynamics, and a lower effective temperature at the center of the sample.

Accelerating evaporative cooling of atoms into Bose-Einstein condensation in optical traps

We demonstrate a simple scheme to achieve fast, runaway evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient. Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth, which preserves atomic density during the evaporation process. Using this scheme, we show that Bose-Einstein condensation with ~10^5 cesium atoms can be realized in 2~4 s of forced evaporation. The evaporation speed and energetics are consistent with the three-dimensional evaporation picture, despite the fact that atoms can only leave the trap in the direction of tilt.

Observation of Quantum Criticality with Ultracold Atoms in Optical Lattices

Critically Cold Atoms Unlike classical phase transitions, such as the freezing of water into ice, which is driven by lowering the temperature of the system, quantum phase transitions occur at absolute zero and are driven by other parameters, including magnetic field or pressure. In the vicinity of a quantum phase transition, a critical region forms where physical observables obey scaling laws as a consequence of the self-similarity of the system. Quantum phase transitions and quantum criticality are usually observed in solid state, but Zhang et al. (p. 1070, published online 16 February) used an optical lattice filled with a cold gas of atoms to simulate a quantum phase transition—from an insulator to a superflnuid in two dimensions. They observed the characteristic scaling of the equation of state, a finding that will facilitate the building of a platform in a tunable system for further investigations of quantum criticality. Trapped low-temperature atoms model the transition between insulating and superfluid states in a more complex material. Quantum criticality emerges when a many-body system is in the proximity of a continuous phase transition that is driven by quantum fluctuations. In the quantum critical regime, exotic, yet universal properties are anticipated; ultracold atoms provide a clean system to test these predictions. We report the observation of quantum criticality with two-dimensional Bose gases in optical lattices. On the basis of in situ density measurements, we observe scaling behavior of the equation of state at low temperatures, locate the quantum critical point, and constrain the critical exponents. We observe a finite critical entropy per particle that carries a weak dependence on the atomic interaction strength. Our experiment provides a prototypical method to study quantum criticality with ultracold atoms.As the temperature of a many-body system approaches absolute zero, thermal fluctuations of observables cease and quantum fluctuations dominate. Competition between different energies, such as kinetic energy, interactions or thermodynamic potentials, can induce a quantum phase transition between distinct ground states. Near a continuous quantum phase transition, the many-body system is quantum critical, exhibiting scale invariant and universal collective behavior [1, 2]. Quantum criticality has been actively pursued in the study of a broad range of novel materials [3–6], and can invoke new insights beyond the Landau-Ginzburg-Wilson paradigm of critical phenomena [7]. It remains a challenging task, however, to directly and quantitatively verify predictions of quantum criticality in a clean and controlled system. Here we report the observation of quantum critical behavior in a two-dimensional Bose gas in optical lattices near the vacuum-to-superfluid quantum phase transition. Based on in situ density measurements, we observe universal scaling of the equation of state at sufficiently low temperatures, locate the quantum critical point, and determine the critical exponents. The universal scaling laws also allow determination of thermodynamic observables. In particular, we observe a finite entropy per particle in the critical regime, which only weakly depends on the atomic interaction. Our experiment provides a prototypical method to study quantum criticality with ultracold atoms, and prepares the essential tools for further study on quantum critical dynamics.

Extracting density?density correlations from in situ images of atomic quantum gases

We present a complete recipe to extract the density–density correlations and the static structure factor of a two-dimensional (2D) atomic quantum gas from in situ imaging. Using images of non-interacting thermal gases, we characterize and remove the systematic contributions of imaging aberrations to the measured density–density correlations of atomic samples. We determine the static structure factor and report the results on weakly interacting 2D Bose gases, as well as strongly interacting gases in a 2D optical lattice. In the strongly interacting regime, we observe a strong suppression of the static structure factor at long wavelengths.

From Cosmology to Cold Atoms: Observation of Sakharov Oscillations in a Quenched Atomic Superfluid

Tinyverse Quantum many-systems, such superfluids, are difficult to study in equilibrium, and thus understanding their dynamics poses a particular challenge. When the strength of interactions was suddenly changed in a two-dimensional gas of cesium atoms, Hung et al. (p. 1213, published online 1 August; see the Perspective by Schmiedmayer and Berges) observed oscillations in the time and space domains analogous to the peaks in the spectrum of the cosmic microwave background. Abrupt interaction changes in an ultracold gas reveal oscillations analogous to peaks in the cosmic microwave background. [Also see Perspective by Schmiedmayer and Berges] Predicting the dynamics of many-body systems far from equilibrium is a challenging theoretical problem. A long-predicted phenomenon in hydrodynamic nonequilibrium systems is the occurrence of Sakharov oscillations, which manifest in the anisotropy of the cosmic microwave background and the large-scale correlations of galaxies. Here, we report the observation of Sakharov oscillations in the density fluctuations of a quenched atomic superfluid through a systematic study in both space and time domains and with tunable interaction strengths. Our work suggests a different approach to the study of nonequilibrium dynamics of quantum many-body systems and the exploration of their analogs in cosmology and astrophysics.