Strong Atom-atom Interaction Research Articles

Spontaneous emergence of temporal structures in a continuously driven many-body system

Abstract The spontaneous emergence of temporal structures challenges the conventional understanding that systems governed by time-invariant laws remain unchanged over time. Recent experiments have observed this time translation symmetry breaking in quantum atomic systems that either exhibit strong atom-atom interactions or have low dissipation rates. While current theoretical frameworks reveal the importance of strong atom-atom interactions, they fall short in explaining this phenomenon observed in low-dissipation atomic systems. Here, we present a theoretical study on the spontaneous breaking of time translation symmetry in materials with low
dissipation rates. By constructing phase diagrams for a system of four-level atoms driven by a continuous-wave optical field, we identify the essential requirements for self-sustained temporal motions. These include a driven open system, nonlinear interactions, and sufficient degrees of freedom that facilitate competing processes. Our findings contribute to a better understanding of the emergence of spontaneous time translation symmetry breaking in these materials.

Proposal of ensemble qubits with two-atom decay

We propose and analyze a novel approach to implement ensemble qubits. The required anharmonicity is provided by a simultaneous decay of two atoms (i.e. two-atom decay), which is achieved by fully quantum degenerate parametric amplification. For an atomic ensemble, the two-atom decay generates and stabilizes a 2D quantum manifold, which is spanned by the ground and single-excited superradiant states. Moreover, this nonlinear decay process can strongly suppress transitions to higher-excited superradiant states, and convert residual transitions into an effective decay from the single-excitation superradiant state to the ground state. Our method does not require Rydberg dipole blockade and, thus, strong atom-atom interactions, compared to previous work. This indicates that it can be applied to typical atomic or spin ensembles in simple experimental setups. Remarkably, our idea is compatible with the cavity protection mechanism, and therefore spin dephasing due to inhomogeneous broadening can be strongly suppressed. The presented ensemble qubit provides a new platform for quantum information processing, and also extends the range of applications of atomic or spin ensembles.

High-fidelity and controllable cloning of high-dimensional optical beams with a Rydberg atomic gas

High-fidelity and controllable optical cloning of high-dimensional (high-D) optical beams is very important for the development of novel techniques for optical imaging, lithography, and communications, etc. Here we propose a scheme to realize the cloning of high-D optical beams with a Rydberg atomic gas via electromagnetically induced transparency. We show that strong atom-atom interaction can map to two probe laser fields, which may acquire giant nonlocal Kerr nonlinearities supporting the formation of stable high-D optical solitons and vortices at very low light power. We also show that such optical solitons and vortices prepared in one probe field can be cloned onto another one with high fidelity, and the cloning may be actively manipulated through the tuning of the nonlocality degree of the Kerr nonlinearities. Moreover, we demonstrate that based on such a cloning scheme multitimes and multicomponents cloning of high-D optical beams are also possible, which allows us to acquire multiple copies of high-D optical beams. The results on the optical cloning reported here are not only of fundamental interest for nonlocal nonlinear optics but also promising for practical applications in optical information processing and transmission.

Control of electromagnetically induced transparency and Fano resonances in a hybrid optomechanical system

We study the control of electromagnetically induced transparency and Fano resonances in a hybrid optomechanical system based on Bose-Einstein condensate trapped inside a high finesse Fabry Perot cavity coupled with two charged nanomechanical resonators. The system is driven by a pump field and probed by a probe field. The nanomechanical resonators are connected to an external bias voltage V1(−V2) and generated electrostatic Coulomb coupling gc. In this study, we show that the electromagnetically induced transparency can be controlled by the s-wave scattering frequency ωsw as well as by the coupling frequencies of the system. Further, studying the effect of Bogoliubov mode, the transparency window splits from double to triple windows. At strong atom-atom interaction, we explain the asymmetric windows’ and explain their physical understanding.

Control of Rydberg-atom blockade by dc electric-field orientation in a quasi-one-dimensional sample

Rydberg atoms possess a strong atom-atom interaction, which limits its density in an atomic sample. Such effect is known as a Rydberg-atom blockade. Here, we present a way to control such effect by directly orienting the induced atomic dipole moment using a dc external electrical field. To demonstrate it, we excite the $50{S}_{1/2}$ Rb atomic state in a quasiunidimensional sample held in a quasielectrostatic trap. A pure $nS$ state holds only van der Waals interaction at long range, but in the presence of an external electric field the state mixing leads to strong dipole-dipole interactions. We have measured the Rydberg-atom population as a function of ground-state atoms density for several angles between the electric field and the main axis of the unidimensional sample. The results indicate that the limit on the final Rydberg density can be controlled by electric-field orientation. Besides, we have characterized the sample by using a single-atom ion imaging technique, demonstrating that it does behave as a unidimensional sample.

Interaction- and filling-induced quantum anomalous Hall effect in ultracold neutral Bose-Fermi mixtures on a hexagonal lattice

We investigate the quantum anomalous Hall effect in a mixture of ultra-cold neutral bosons and fermions held on a hexagonal optical lattice. In the strong atom-atom interaction limit, composite fermions composed of one fermion with bosons or bosonic holes in the mixture are formed. Such composite fermions have already been generated successfully in experiment [Nat. Phys. {\bf 7}, 642 (2011)]. Here we predict that this kind of composite fermions may provide a realization of the quantum anomalous Hall effect by tuning the atom-atom interaction or the filling of the bosons in the mixture. We also discuss the corresponding experimental signatures of the quantum anomalous Hall effect in the Bose-Fermi mixture on hexagonal optical lattice.

Atom-pair tunneling and quantum phase transition in the strong-interaction regime

We propose a Hamiltonian of ultracold spinless atom in optical lattices including the two-body interaction of nearest neighbors, which reduces to the Bose-Hubbard model in weak-interaction limit. An atom-pair hopping term appearing in the Hamiltonian explains naturally the recent experimental observation of correlated tunneling in a double-well trap with strong atom-atom interactions and moreover leads to a dynamic process of atom-pair tunneling where strongly interacting atoms can tunnel back and forth as a fragmented pair. Finally a dynamics of oscillations induced by the atom-pair tunneling is found in the strong interaction regime, where the Bose-Hubbard model gives rise to the insulator state with fixed time-averaged value of atom-occupation number only. Quantum phase transitions between two quantum phases characterized by degenerate and nondegenerate ground states are shown to be coinciding with the Landau second-order phase-transition theory. In the system of finite atom number the degeneracy of ground states can be removed by quantum tunneling for the even number of atoms but not for the odd number.

Single-atom box: Bosonic staircase and effects of parity

We have developed a theory of a Josephson junction formed by two tunnel-coupled Bose-Einstein condensates in a double-well potential in the regime of strong atom-atom interaction for an arbitrary total number N of bosons in the condensates. The tunnel resonances in the junction are shown to be periodically spaced by the interaction energy, forming a single-atom staircase sensitive to the parity of N even for large N. One of the manifestations of the staircase structure is the periodic modulation with the bias energy of the visibility of the interference pattern in lattices of junctions.

An effective quasi-one-dimensional description of a spin-1 atomic condensate

Within the mean field theory we extend the effective quasi-1D non-polynomial Schr\"{o}dinger equation (NPSE) approach to the description of a spin-1 atomic condensate in a tight radial confinement geometry for both weak and strong atom-atom interactions. Detailed comparisons with full time dependent 3D numerical simulations show excellent agreement as in the case of a single component scalar condensate, demonstrating our result as an efficient and effective tool for the understanding of spin-1 condensate dynamics observed in several recent experiments.

Atomic Bose-Fermi mixtures in an optical lattice.

A mixture of ultracold bosons and fermions placed in an optical lattice constitutes a novel kind of quantum gas, and leads to phenomena, which so far has been discussed neither in atomic physics, nor in condensed matter physics. We discuss the phase diagram at low temperatures, and in the limit of strong atom-atom interactions, and predict the existence of quantum phases that involve pairing of fermions with one or more bosons, or, respectively, bosonic holes. The resulting composite fermions may form, depending on the system parameters, a normal Fermi liquid, a density wave, a superfluid liquid, or an insulator with fermionic domains. We discuss the feasibility for observing such phases in current experiments.

Collision-broadened spectral line profiles in the limit of high perturber density.

In the present research we have used a Fourier spectral analysis to study line profiles due to strong atom-atom interactions in the limit of high perturber density. The presence of multiple satellites in this case is well explained. Moreover, for decreasing interactions and densities we may extrapolate our results and explain the disappearance of the satellites. The use of ``a crude potential shape''\char22{}here a square-well\char22{}proves to be sufficient to explain the presence of multiple satellites and determine the necessary physical conditions for their appearance.