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arXiv:1109.1139 · Physical Review A

Optically induced conical intersections in traps for ultracold atoms and molecules

Alisdair O. G. Wallis, Jeremy M. Hutson

Published Nov 18, 2011• 5 citations
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Abstract

We show that conical intersections can be created in laboratory coordinates by dressing a parabolic trap for ultracold atoms or molecules with a combination of optical and static magnetic fields. The resulting ring trap can support single-particle states with half-integer rotational quantization and many-particle states with persistent flow. Two well-separated atomic or molecular states are brought into near resonance by an optical field and tuned across each other with an inhomogeneous magnetic field. Conical intersections occur at the nodes in the optical field.

DOI
10.1103/physreva.84.051402

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•Jan 1, 2010•0 citations
We show that conical intersections can be created in laboratory coordinates by dressing a parabolic trap for ultracold atoms or molecules with a combination of optical and static magnetic fields. The resulting ring trap can support single-particle states with half-integer rotational quantization and many-particle states with persistent flow. Two well-separated atomic or molecular states are brought into near resonance by an optical field and tuned across each other with an inhomogeneous magnetic field. Conical intersections occur at the nodes in the optical field.
For two states of opposite parity that cross as a function of an external magnetic field, the addition of an electric field will break the symmetry and induce an avoided crossing. A suitable arrangement of fields may be used to create a conical intersection as a function of external spatial coordinates. We consider the effect of the resulting geometric phase for ultracold polar molecules. For a Bose-Einstein condensate in the mean-field approximation, the geometric phase effect induces stable states of persistent superfluid flow that are characterized by half-integer quantized angular momentum.
Conical intersections of potential energy surfaces widely appear in molecules which consist of more than two atoms. No conical intersections exist in the case of free diatomic molecules. We show here that diatomic molecules interacting with standing laser waves produce periodic arrays of conical intersections. At these laser-induced intersections the non-adiabatic effects are infinitely strong. The translational and rovibrational molecular motions become strongly coupled to each other and energy exchange between the various degrees of freedom influences the dynamics of the system. As an illustrative example, an effect of the laser-induced conical intersections on trapping of ultracold diatomic molecules by light is discussed.
Conical intersections are common in molecular physics and photochemistry, and are often invoked to explain observed reaction products. A conical intersection can occur when an excited electronic potential energy surface intersects with the ground electronic potential energy surface in the coordinate space of the nuclear positions. Theory predicts that the conical intersection will result in a geometric phase for a wavepacket on the ground potential energy surface. Although conical intersections have been observed experimentally, the geometric phase has not been observed in a molecular system. Here we use a trapped atomic ion system to perform a quantum simulation of a conical intersection. The internal state of a trapped atomic ion serves as the electronic state and the motion of the atomic nuclei are encoded into the normal modes of motion of the ions. The simulated electronic potential is constructed by applying state-dependent forces to the ion with a near-resonant laser. We experimentally observe the geometric phase on the ground-state surface using adiabatic state preparation followed by motional state measurement. Our experiment shows the advantage of combining spin and motion degrees of freedom in a quantum simulator.
We combine an RF-dressed magnetic trap with an optical potential to produce a toroidal trapping potential for ultracold 87Rb atoms. We load atoms into this ring trap from a conventional magnetic trap and compare the measured oscillation frequencies with theoretical predictions. This method of making a toroidal trap gives a high degree of flexibility such as a tuneable radius and variable transverse oscillation frequency. The ring trap is ideal for the creation of a multiply connected Bose–Einstein condensate (BEC) and the study of persistent flow and we propose a scheme for introducing a flow of the atoms around the ring.
There is great interest within the physics and chemistry communities in the properties of ultracold molecules. Electromagnetic fields can be used to create, trap, and modify the collisional dynamics of ultracold molecules, and thus the properties of ultracold molecules in electromagnetic fields is of growing importance. This thesis examines some of the effects of externally applied electromagnetic fields on ultracold molecules. Initially, magnetic Feshbach resonances in combined electric and magnetic fields are examined in the collisions of He(1S )+SO(3Σ−). Through detailed quantum scattering calculations, it is then shown that the sympathetic cooling of NH(3Σ−) molecules with Mg atoms has a good prospect of success, a first for a neutral molecular system. Detailed quantum scattering calculations are performed for a wide range of collision energies and magnetic field strengths and it is found that the ratio of elastic to inelastic collisions is large for temperatures below 10 mK, and increases as the collision energy and magnetic field strength decrease. The near threshold collision properties of Mg+NH have been examined using a multichannel quantum defect theory approach. A new type of conical intersection, that is a function of applied electromagnetic fields only, is also demonstrated. For states of opposite parity, brought into degeneracy with a magnetic field, the degeneracy can be resolved by the addition of an electric field, forming a conical intersection. A suitable arrangement of fields could thus be used to create a conical intersection in laboratory coordinates within an ultracold trapped gas. For a BoseEinstein condensate, in the mean-field approximation, the resultant geometric phase effect induces stable states of persistent superfluid flow that are characterized by half-integer quantized angular momentum.
There is great interest within the physics and chemistry communities in the properties of ultracold molecules. Electromagnetic fields can be used to create, trap, and modify the collisional dynamics of ultracold molecules, and thus the properties of ultracold molecules in electromagnetic fields is of growing importance. This thesis examines some of the effects of externally applied electromagnetic fields on ultracold molecules. Initially, magnetic Feshbach resonances in combined electric and magnetic fields are examined in the collisions of He(1S^1S1S)+SO(3Σ−^3\Sigma^-3Σ−). Through detailed quantum scattering calculations, it is then shown that the sympathetic cooling of NH(3Σ−^3\Sigma^-3Σ−) molecules with Mg atoms has a good prospect of success, a first for a neutral molecular system. Detailed quantum scattering calculations are performed for a wide range of collision energies and magnetic field strengths and it is found that the ratio of elastic to inelastic collisions is large for temperatures below 10 mK, and increases as the collision energy and magnetic field strength decrease. The near threshold collision properties of Mg+NH have been examined using a multichannel quantum defect theory approach. A new type of conical intersection, that is a function of applied electromagnetic fields only, is also demonstrated. For states of opposite parity, brought into degeneracy with a magnetic field, the degeneracy can be resolved by the addition of an electric field, forming a conical intersection. A suitable arrangement of fields could thus be used to create a conical intersection in laboratory coordinates within an ultracold trapped gas. For a Bose-Einstein condensate, in the mean-field approximation, the resultant geometric phase effect induces stable states of persistent superfluid flow that are characterized by half-integer quantized angular momentum.