H. H. Jen

Spectral analysis for cascade-emission-based quantum communication in atomic ensembles

The ladder configuration of atomic levels provides a source for telecom photons (signal) from the upper atomic transition. For rubidium and caesium atoms, the signal field has the range around 1.3–1.5 μm that can be coupled to an optical fibre and transmitted to a remote location. Cascade emission may result in pairs of photons, the signal entangled with the subsequently emitted infrared photon (idler) from the lower atomic transition. This correlated two-photon source is potentially useful in the DLCZ (Duan–Lukin–Cirac–Zoller) protocol for the quantum repeater. We implement the cascade emission to construct a modified DLCZ quantum repeater and investigate the role of the time-frequency entanglement in the protocol. The dependence of the protocol on the resolving and non-resolving photon-number detectors is also studied. We find that the frequency entanglement deteriorates the performance but the harmful effect can be diminished by using shorter pump pulses to generate the cascade emission. An optimal cascade-emission-based DLCZ scheme is realized by applying a pure two-photon source in addition to using detectors of perfect quantum efficiency.

Phase-imprinted multiphoton subradiant states

We propose to generate the multiphoton subradiant states and investigate their fluorescences in an array of two-level atoms. These multiphoton states are created initially from the timed-Dicke states. Then we can use either a Zeeman or Stark field gradient pulse to imprint linearly increasing phases on the atoms, and this phase-imprinting process unitarily evolves the system to the multiphoton subradiant states. The fluorescence engages a long-range dipole-dipole interaction which originates from a system-reservoir coupling in the dissipation. We locate some of the subradiant multiphoton states from the eigenmodes, and show that an optically thick atomic array is best for the preparation of the state with the most reduced decay rate. This phase-imprinting process enables quantum state engineering of the multiphoton subradiant states, and realizes a potential quantum storage of the photonic qubits in the two-level atoms.

Cooperative single-photon subradiant states

We propose a set of subradiant states which can be prepared and detected in a one-dimensional optical lattice. We find that the decay rates are highly dependent on the spatial phases imprinted on the atomic chain, which gives systematic investigations of the subradiance in the fluorescence experiments. The time evolution of these states can have long decay time where up to hundred milliseconds of lifetime is predicted for one hundred atoms. They can also show decayed Rabi-like oscillations with a beating frequency determined by the difference of cooperative Lamb shift in the subspace. Experimental requirements are also discussed for practical implementation of the subradiant states. Our proposal provides a novel scheme for quantum storage of photons in arrays of two-level atoms through the preparation and detection of subradiant states, which offer opportunities for quantum many-body state preparation and quantum information processing in optical lattices.

Theory of electromagnetically induced transparency in strongly correlated quantum gases

We develop a general theory to study the electromagnetically induced transparency (EIT) in ultracold quantum gases, applicable for both Bose and Fermi gases with arbitrary inter-particle interaction strength. We show that, in the weak probe field limit, the EIT spectrum is solely determined by the single particle Greens function of the ground state atoms, and reflects interesting quantum many-body effects when atoms are virtually coupled to the low-lying Rydberg states. As an example, we apply our theory to 1D Luttinger liquid, Bose-Mott insulator state, and the superfluid state of two-component Fermi gases, and show how the many-body features can be observed non-destructively in the unconventional EIT spectrum.

Electromagnetically induced transparency and slow light in quantum degenerate atomic gases

We systematically investigate the electromagnetically induced transparency (EIT) and slow light properties in ultracold Bose and Fermi gases. It shows a very different property from the classical theory, which assumes frozen atomic motion. For example, the speed of light inside the atomic gases can be changed significantly near the Bose–Einstein condensation temperature, while the presence of the Fermi sea can destroy the EIT effect even at zero temperature. From an experimental point of view, such quantum EIT property is mostly manifested in the counterpropagating excitation schemes in either the low-lying Rydberg transition with a narrow linewidth or in the D2 transitions with a weak coupling field. We further investigate the interaction effects on the EIT for a weakly interacting Bose–Einstein condensate, showing an inhomogeneous broadening of the EIT profile and nontrivial change of the light speed due to the quantum depletion other than mean-field energy shifts.

Spectral shaping of cascade emissions from multiplexed cold atomic ensembles

We investigate the spectral properties of the biphoton state from the cascade emissions of cold atomic ensembles, which are composed of a telecommunication photon (signal) followed by an infrared one (idler) via four-wave mixing. With adiabatic conditions for Gaussian driving pulses of width

Spin-incoherent one-dimensional spin-1 Bose Luttinger liquid

\ensuremath{\tau}

Entropy of entanglement in the continuous frequency space of the biphoton state from multiplexed cold atomic ensembles

, the spectrum of the biphoton state has the form of a Gaussian that conserves signal and idler photon energies within

Cooperative single-photon subradiant states in a three-dimensional atomic array

\ensuremath{\hbar}/\ensuremath{\tau}

Topological superfluid by blockade effects in a Rydberg-dressed Fermi gas

modulated by a Lorentzian with a superradiant linewidth. Multiplexing the atomic ensembles with frequency-shifted cascade emissions, we may manipulate and shape the spectrum of the biphoton state. The entropy of entanglement is derived from Schmidt decomposition, which can be larger if we multiplex the atomic ensembles in a way that conserves signal and idler photon central energies. The eigenvalues in Schmidt bases are degenerate in pairs for symmetric spectral shaping in which the mode probability densities show interference patterns. We also demonstrate the excess entropy of entanglement that comes from continuous frequency space, which scales up the total entropy. The scheme of the multiplexed cascade-emitted biphoton state provides multimode structures that are useful in long-distance quantum communication and multimode quantum information processing.