| 研究実績の概要 |
The objective is to study a system of 2 Rydberg atoms, closed enough (typ. 0.5-1 um) for their electronic orbitals to overlap. The plan is (1) to trap Rubidium (Rb) and Cesium (Cs) in submicron-sized optical tweezers, (2) to cool these atoms to < 1 uK (enough to remove oscillation of the atoms in the traps), and (3) to excite the atoms to Rydberg state and perform spectroscopy.
(1) is completed. Laser-cooled cloud of Rb and Cs atoms have been obtained. Trapping and fluorescence imaging of single Rb atoms in an array of up to 800 holographic tweezers optical tweezers have been achieved using a specially-designed high-performance microscope objective. The optical tweezers characteristic (depth, size, polarization) have been characterized using the single Rb atoms as an in-situ probe. Using advanced holographic techniques, two atoms can now be prepared as close as 1.2 micrometer, and a scheme to reach 0.9-1 micrometer is under investigation.
(2) is completed. The atom motion has been successfully reduced close to the motional quantum ground-state (mean quanta number <0.3) along the radial direction of the tweezers by applying the Raman-sideband cooling technique.
(3) is under work. A new excitation scheme has been realized, including optical pumping and a two-step two-color pulse, leading to high efficiency (>80%) preparation of the atoms in Rydberg state (previously <10%). Signatures of the dipole-dipole interaction between atoms at distance >1.2 micrometer have been observed on a nanosecond-timescale and are currently being thoroughly investigated.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
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理由
After installation of the high-NA (0.75) objective, the size of the optical tweezers has been drastically reduced from 1.5 um down to 0.6um, a crucial requirement for trapping two atoms at sub-micrometer distances. The optical tweezers characteristic (depth, size, polarization) have been measured using the single Rb atoms as an in-situ probe.
A series of effort was then conducted to reduce the atoms' motion using “Raman sideband laser cooling”. This technique, making use of motion-assisted spin-flip to remove quanta of motion, requires the coherence time of the atom’s electronic spin to be much longer than the atom’s motional period in the trap (7 microseconds in the radial direction, 30 in the axial direction). This coherence time was initially measured to be ~1 microsecond, limited by large fluctuating ‘effective’ magnetic field created by the trap polarization (vector light shift). After applying several mitigation techniques, the coherence time could be increased up to 100 microseconds and further improvements are being pursued. Followingly, we applied the cooling technique and reduced the radial motion down to a mean motional quantum of <0.3, where the atom position uncertainty (~30 nm) is now limited by quantum rather than thermal fluctuations. The cooling along the axial direction remains challenging due to the slower motion, requiring even longer coherence time.
Finally, as planned, a novel ultrafast excitation scheme to Rydberg states was successfully implemented and is now routinely used to investigate interactions between two Rydberg atoms.
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| 今後の研究の推進方策 |
Currently, we can achieve inter-atomic distance down to 1.2 micrometer using holographic techniques. In the next fiscal year, we will pursue our investigation of new techniques to decrease this distance to the sub-micrometer regime.
Having achieved laser cooling of the radial motion of the atoms, we will now focus on improving further the spin coherence time to succeed in cooling the axial direction. At this point, the quantum uncertainty of the atom position will be limited by the Heisenberg uncertainty is all direction. We then plan to go beyond this limit by ‘squeezing’ the particle wavefunction down to a position uncertainty of 10 nm. Squeezed states of motion are fragile, highly non-classical states, and realizing them with atoms in optical tweezers is a feat not yet realized.
Finally, interactions between neighbouring Rydberg atoms will be further investigated in a wide range of interatomic distance where the interaction should evolve from a perturbative van-der-Walls potential (large distance) to a situation where quantum effects play a key role (overlapping Rydberg orbitals). We will also be interested in the coupling between the external (motional) and internal (electronic) degrees of freedom of the atom upon interaction at such short distance. In particular, we aim to observe interaction-induced momentum kicks giving rise to quantum correlation on the motion of two atoms.
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