A method for diffracting the weak probe beam into unidirectional and higher-order directions is proposed via a novel Rydberg electromagnetically induced grating, providing a new way for the implementations of quantum devices with cold Rydberg atoms. The proposed scheme utilizes a suitable and position-dependent adjustment to the two-photon detuning besides the modulation of the standing-wave coupling field, bringing a in-phase modulation which can change the parity of the dispersion. We observe that when the modulation amplitude is appropriate, a perfect unidirectional diffraction grating can be realized. In addition, due to the mutual effect between the van der Waals (vdWs) interaction and the atom-field interaction length that deeply improves the dispersion of the medium, the probe energy can be counter-intuitively transferred into higher-order diffractions as increasing the vdWs interaction, leading to the realization of a controllable higher-order diffraction grating via strong blockade.
We study an ultracold atom-cavity coupling system, which had been implemented in an experiment to display weak light nonlinearity [S. Gupta, K. L. Moore, K. W. Murch, and D. M. Stamper-Kurn, Phys. Rev. Lett. 99, 213601 (2007)]. The model is described by a noninteracting Bose-Einstein condensate contained in a Fabry-Pérot optical resonator, in which two incommensurate standing-wave modes are excited and thus form a quasiperiodic optical lattice potential for the atoms. Special emphasis is paid to the variation of the atomic wave function induced by the cavity light field. We show that bistability between the atomic localized and extended states can be generated under appropriate conditions.
Rydberg blockaded gate is a fundamental ingredient for scalable quantum computation with neutral Rydberg atoms. However the fidelity of such a gate is intrinsically limited by a blockade error coming from a Rydberg level shift that forbids its extensive use. Based on a dark-state adiabatic passage, we develop a novel protocol for realizing a two-atom blockade-error-free quantum gate in a hybrid system with simultaneous van der Waals (vdWsI) and resonant dipole-dipole interactions (DDI). The basic idea relies on converting the roles of two interactions, which is, the DDI serves as one time-dependent tunable pulse and the vdWsI acts as a negligible middle level shift as long as the adiabatic condition is preserved. We adopt an optimized super-Gaussian optical pulse with kπ (k ≫ 1) area accompanied by a smooth tuning for the DDI, composing a circular stimulated Raman adiabatic passage, which can robustly ensure a faster operation time ∼ 80ns as well as a highly-efficient gate fidelity ∼ 0.9996. This theoretical protocol offers a flexible treatment for hybrid interactions in complex Rydberg systems, enabling on-demand design of new types of effective Rydberg quantum gate devices.
We propose a scheme to realize parity-time (PT) symmetric photonic Lieb lattices of ribbon shape and complex couplings, thereby demonstrating the higher-order exceptional point (EP) and Landau–Zener Bloch (LZB) oscillations in the presence of a refractive index gradient. Quite different from non-Hermitian flatband lattices with on-site gain/loss, which undergo thresholdless PT symmetry breaking, the spectrum for such quasi-one-dimensional Lieb lattices has completely real values when the index gradient is applied perpendicular to the ribbon, and a triply degenerated higher-order EP (EP3) with coalesced eigenvalues and eigenvectors emerges only when the amplitude of the gain/loss ratio reaches a certain threshold value. When the index gradient is applied parallel to the ribbon, the LZB oscillations exhibit intriguing characteristics, including asymmetric energy transition and pseudo-Hermitian propagation, as the flatband is excited. Meanwhile, a secondary emission occurs each time when the oscillatory motion passes through the EP3, leading to distinct energy distribution in the flatband when a dispersive band is excited. Such novel phenomena may appear in other non-Hermitian flatband systems. Our work may also bring insight and suggest a photonic platform to study the symmetry and topological characterization of higher-order EP that may find unique applications in, for example, enhancing sensitivity.
The modulational instability of coherently excited spin waves is studied in an atomic spin chain of spinor Bose-Einstein condensates ͑BECs͒ confined in an optical lattice. We examine the dependence of the stability on the long-range nonlinear interaction of spin waves excited at different lattice sites. The long-range nonlinear spin coupling in the optical lattice is due to light-induced and static magnetic dipole-dipole interaction between atoms. We compare the spin wave dynamics in an atomic spin chain of spinor BECs formed in an optical lattice with that in a Heisenberg-like spin chain in solid-state physics. This reveals the important differences of the spin chain with long-range spin coupling from that with the short-range one. II. HAMILTONIAN OF SPINOR BOSE-EINSTEIN CONDENSATE IN THE OPTICAL LATTICEWe consider a one-dimensional optical lattice formed by two -polarized laser beams counterpropagating along the y axis. The two lattice laser beams are detuned far from atomic resonance, and the condensate confined in the lattice is approximately in its electronic ground state. We assume that PHYSICAL REVIEW B 76, 214408 ͑2007͒
We study cavity optomechanics of a mixture of ultracold atoms with tunable nonlinear collisions. We show that atomic collisions provide linear couplings between fictitious condensate oscillators, leading to possibilities of achieving a globally coupled quantum optomechanical network with an integrated atom chip. Potential applications range from simulating collective nonequilibrium dynamics in fields well past physics to probing unique properties of quantum mixtures.PACS numbers: 03.75.Pp, 03.70.+k Recent years have witnessed rapid advances in the field of cavity optomechanics [1][2][3]. These advances have lead to striking demonstrations of quantum effects in mechanical objects at the mesoscale (single or two independent oscillators) [4], and opened up exciting new possibilities in developing integrated phononic circuits or coherent acoustic analogs of quantum nonlinear optics. Very recently, Lin et al . experimentally realized a direct mechanical coupling between two nano-oscillators and firstly observed remarkable effects of coherent mechanical wave mixing [5]. By establishing nearest-neighbor couplings in an optomechanical array [6], one can even synchronize vibrations of all elements [7]. For current nano-fabrication techniques, however, it remains a challenge to make a globally coupled optomechanical network, which has applications in simulating numerous important situations well past physics [8].In parallel to the approach to cavity optomechanics that relies on the advanced materials and processing techniques of the semiconductor industry and nanoscience, an alternative approach relies on the realization of optomechanics where momentum sideband or particle-hole excitations of an atomic Bose-Einstein condensate (BEC) [9][10][11] or a degenerate Fermi gas [12] play the role of the mechanical oscillator. In view of rapid advances in making and manipulating an ultracold mixture of two or more superfluids [13-18], here we probe the realization of multimode quantum acoustics [19], with linear mechanical couplings between the "BEC mirrors" arising from two-body atomic collisions. In contrast to the recently proposed nano-fabricated arrays with only nearest-neighbor couplings [7], our cold-atom system presents an interesting example of how to generate a multi-component quantum network with global couplings between different "mirror" modes.We consider a cigar-shaped n-component BEC trapped in an optical Fabry-Pérot cavity of length L, with the soft trapping direction parallel to the cavity axis z (see Fig. 1). We further assume that the trapping strength perpendicular to the cavity axis is sufficient to allow us to treat the system as quasi-1D, and to only consider excitations along the cavity axis [18]. Within the dipole and rotating-wave approximations, the atomic part of Hamil-
Floquet Majorana edge modes capture the topological features of periodically driven superconductors. We present a Kitaev chain with multiple time periodic driving and demonstrate how the avoidance of band crossing is altered, which gives rise to new regions supporting Majorana edge modes. A one dimensional generalized method was proposed to predict Majorana edge modes via the Zak phase of the Floquet bands. We also study the time independent effective Hamiltonian at high frequency limit and introduce diverse index to characterize topological phases with different relative phase between the multiple driving. Our work enriches the physics of driven system and paves the way for locating Majorana edge modes in larger parameter space.Topolgical state of matter is an intriguing topic, and has been studied intensively for years. Topological superconductivity is one of the most attractive theme in this subject for its topological excitation, Majorana fermion [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], which is its own antiparticle. The non-abelian statistics [16][17][18][19] of Majorana edge modes empower it as a novel prospect for quantum computation [20,21]. Thus, methods on how to generate Majorana edge modes are gaining increasing attention [22][23][24][25][26][27]. Conventional techniques of creating Majorana edge modes are via unpaired spin polarized fermions in quantum nanowires [28][29][30][31][32] or superconductors in proximity with topological insulators or semiconductors [33,34]. However, topological nontrivial regions with Majorana edge modes gained above are quite limited in parameter space.In recent years, Floquet engineering emerges as a new protocol for designing topological states of matter [35][36][37], which correspondingly brings the concept Floquet Majorana edge modes. The main idea of Floquet engineering lies in driving the physical parameter periodically with time. In contrast to the adiabatic limit, where the system remains in the eigenstate at each instantaneous time, driven system may absorb quantized energy from external fields, featuring non-equilibrium properties. Hence, the Floquet version of system may exhibit fruitful topological properties [38][39][40][41][42][43][44][45][46][47][48][49]. For a driven Kitaev chain, results have shown that Majorana edge modes can be sustained within a larger parameter space [50][51][52][53]. Besides, multiple Majorana edge modes are discovered with low driving frequency [54]. However, all results above are obtained within a frame of single time periodic driving. A natural question appears to us, how will the multiple time periodic driving affect the Majorana edge modes?In this Letter, we demonstrate that due to the avoidance of bands crossing, multiple time periodic driving are capable of generating gaps at different regions, leading to extraordinary different topological phase transitions, and larger parameter spaces with Majorana edge modes. To identify the Majorana edge modes, we propose a one dimensional method based on the Zak phase of Floqu...
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