We propose several schemes for implementing a fast two-qubit quantum gate for neutral atoms with the gate operation time much faster than the time scales associated with the external motion of the atoms in the trapping potential. In our example, the large interaction energy required to perform fast gate operations is provided by the dipole-dipole interaction of atoms excited to low-lying Rydberg states in constant electric fields. A detailed analysis of imperfections of the gate operation is given.

We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting "dipole blockade" can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as nonclassical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.

In the atomic Bose-Einstein condensate, the interactions that bring a binary atom system to an intermediate state molecule in the Feshbach resonance create a second condensate component of molecules. The atomic and molecular condensates coherently exchange pairs of atoms. We discuss a signature of the coherent intercondensate exchange: Josephson-like oscillations of the atomic and molecular populations in response to a sudden change of the energy detuning. The dependence of the many-body ground state energy on volume suggests that the on-resonant ground state is a dilute condensate with the liquidlike property of a self-determined density. PACS numbers: 03.75.Fi, 05.30.Jp, 32.80.Pj, 67.90. + z As dilute gases, the atomic Bose-Einstein condensates [1-3] are amenable to atomic manipulation techniques. As superfluids [4], these dilute condensates exhibit an unusual degree of flexibility, promising novel studies of macroscopic quantum coherence. For instance, the notion that external fields can alter the interatomic interactions [5] suggests the study of condensates with interparticle interactions that can be experimentally controlled [6]. Signatures of one of the proposed schemes, the low energy Feshbach resonance [7], have been observed recently [8].In this Letter, we point out that this resonance affects the condensate system more profoundly than altering an effective interparticle interaction: the molecules, formed in the intermediate state of the binary atom resonance, occupy a second condensate component. The atomic and molecular condensates interact in part by coherently exchanging pairs of atoms, implying interesting and unusual properties. Below, we discuss two examples: (i) The molecular condensate, even if it is small in the off-resonant regime, can reveal its presence by means of Josephson-like oscillations of the atomic and molecular populations in response to a sudden change of the detuning. (ii) Close to resonance, the coherent intercondensate tunneling binds the dilute manybody Bose-Einstein condensate (BEC) to a system with the liquidlike property of a self-determined density.In the low energy Feshbach resonance, the hyperfine interaction, V hf , rearranges the spins of two alkali atoms interacting in an external magnetic field B, bringing them to an intermediate quasibound molecular state. In this process, a valence electron spin is "flipped" which, in the magnetic field, raises the continuum of the intermediate spin state jS 0 ͘ by an amount D͑B͒, relative to the continuum of the initial binary atom spin state jS͘. At a resonant magnetic field B B m , the jS͘ continuum lines up with the bound state m of the jS 0 ͘ interatomic potential. Near the resonance, the energy difference e of the initial and intermediate states, which we call "detuning," varies with magnetic field, e ഠ ͑≠D͞≠B͒ 3 ͓B 2 B m ͔. The amplitude of the resonant process is characterized by a ͗S 0 jV hf jS͘ 3 R d 3 r w ‫ء‬ m ͑r͒w͑r͒, where w m denotes the vibrational wave function of the m state and w the continuum (jS͘) wave function...

In the laser excitation of ultracold atoms to Rydberg states, we observe a dramatic suppression caused by van der Waals interactions. This behavior is interpreted as a local excitation blockade: Rydberg atoms strongly inhibit excitation of their neighbors. We measure suppression, relative to isolated atom excitation, by up to a factor of 6.4. The dependence of this suppression on both laser irradiance and atomic density are in good agreement with a mean-field model. These results are an important step towards using ultracold Rydberg atoms in quantum information processing.PACS numbers: 32.80. Rm, 03.67.Lx, 34.20.Cf The possibility of a computer that operates according to the principles of quantum mechanics has attracted growing interest from a variety of research fields [1,2]. A number of possible implementations are being investigated, including solid-state systems, nuclear magnetic resonance, cavity quantum electrodynamics, trapped dipolar molecules, trapped ions, and trapped neutral atoms. A key element to any successful system is the ability to control the coherent interactions between the fundamental building blocks (qubits). Highly-excited Rydberg atoms with principal quantum numbers n 30 have the advantage that they interact quite strongly with each other, allowing information to be exchanged quickly [3]. Here we report an important advance towards using ultracold Rydberg atoms in quantum computing. We observe that the laser excitation of a macroscopic sample of ultracold atoms to high-lying Rydberg states can be dramatically suppressed by their strong long-range interactions. This leads to a local blockade effect, where the excitation of one atom prevents excitation of its neighbors. Our observations agree well with a model based on mean-field interactions.In a high-n Rydberg state, the electron spends most of its time quite far from the nucleus [4]. As a result, the energy of this highly-excited state is very sensitive to external perturbations, including those caused by neighboring Rydberg atoms. A system of two ultracold Rydberg atoms, subject to these long-range interactions, has been proposed as a possible realization of a quantum logic gate [3,5]. Rydberg states combine the advantages of long radiative lifetimes and strong long-range interactions, allowing information to be exchanged before decoherence sets in, even when the atoms are sufficiently separated to allow individual addressing. If the atoms are ultracold, they can be highly localized, e.g., in the sites of an optical lattice, allowing control of their interactions and efficient detection of their quantum state. An outstanding challenge is the assurance that at most a single Rydberg atom is produced at a given site. Towards this end, the concept of an excitation blockade has been proposed [6,7]. With multiple atoms occupying a sufficiently localized site, the strong Rydberg-Rydberg interactions allow at most one Rydberg excitation. Further excitations are blocked by the large energy level shifts that push the resonant frequencies outsi...

We have calculated the long-range interaction potential curves of highly excited Rydberg atom pairs for the combinations Li–Li, Na–Na, K–K, Rb–Rb and Cs–Cs in a perturbative approach. The dispersion C-coefficients are determined for all symmetries of molecular states that correlate to the ns–ns, np–np and nd–nd asymptotes. Fitted parameters are given for the scaling of the C-coefficients as a function of the principal quantum number n for all homonuclear pairs of alkali metal atoms.