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Laser Physics

Single rotational state preparation via coherent control of laser pulse

Suyu Li, Fuming Guo, Anmin Chen, Yang Yu-Jun, Mingxing Jin

Published Aug 1, 2014• 2 citations
Publisher

Abstract

We present a scheme to prepare a single rotational state with high probability from an arbitrary superposition state. The NO molecule is used as a computational example. By using a series of linearly and circularly polarized laser pulses and adjusting their parameters, a ground rotational state with probability of 99.982% can be obtained. Starting from the ground rotational state prepared, we can prepare any desired single rotational state with high probability and some rotational superposition states. On this basis, the molecular orientation with initial thermal distribution can be considerably enhanced and well controlled.

DOI
10.1088/1054-660x/24/10/105202

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We present a scheme to prepare a single rotational state with high probability from an arbitrary superposition state. The NO molecule is used as a computational example. By using a series of linearly and circularly polarized laser pulses and adjusting their parameters, a ground rotational state with probability of 99.982% can be obtained. Starting from the ground rotational state prepared, we can prepare any desired single rotational state with high probability and some rotational superposition states. On this basis, the molecular orientation with initial thermal distribution can be considerably enhanced and well controlled.
The lack of a direct map between control fields and desired control objectives poses a significant challenge in applying quantum control theory to quantum technologies. Here, we propose an analytical framework to precisely control a limited set of quantum states and construct desired coherent superpositions using a well-designed laser pulse sequence with optimal amplitudes, phases, and delays. This theoretical framework that corresponds to a multilevel pulse-area theorem establishes a straightforward mapping between the control parameters of the pulse sequence and the amplitudes and phases of rotational states within a specific subspace. As an example, we utilize this approach to generate 15 distinct and desired rotational superpositions of ultracold polar molecules, leading to 15 desired field-free molecular orientations. By optimizing the superposition of the lowest 16 rotational states, we demonstrate that this approach can achieve a maximum orientation value of | 〈 cos θ 〉 | max above 0.99, which is very close to the global optimal value of 1 that could be achieved in an infinite-dimensional state space. This work marks a significant advancement in achieving precise control over multilevel subsystems within molecules. It holds potential applications in molecular alignment and orientation, as well as in various interdisciplinary fields related to the precise quantum control of ultracold polar molecules, opening up considerable opportunities in molecular-based quantum techniques.
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We propose a scheme to coherently control the field-free orientation of NO molecule whose rotational temperature is above 0 K. It is found that the maximum molecular orientation is affected by two factors: one is the sum of the population of M = 0 rotational states and the other is their distribution, however, their distribution plays a much more significant role in molecular orientation than the sum of their population. By adopting a series of linearly polarized pulses resonant with the rotational states, the distribution of M = 0 rotational states is well rearranged. Though the number of pulses used is small, a relatively high orientation degree can be obtained. This scheme provides a promising approach to the achievement of a good orientation effect.
We present an experimental and theoretical investigation of the quantum control of ground-state rotational coherence in a linear molecule. A sequence of two temporally separated laser pulses creates a rotational superposition state in CO2{\mathrm{CO}}_{2}CO2​ whose evolution is monitored through a polarization technique. We study the influence of the phase difference between the two pulses. We show that the overlapping of the two wave packets, produced by each pulse, gives rise to quantum interference that affects the orientational anisotropy of the sample. Because of the large number of coherently excited levels, the interference produces well-separated temporal structures, whose magnitude can be controlled through the relative delay of the two pulses.
We show that the nonadiabatic orientation of diatomic polar molecules can be controlled by polarization-gated ultrashort laser pulses. By finely adjusting the time interval between two circularly polarized pulses of different wavelengths but the same helicity, the orientation direction of the molecules can be twirled. A cloverlike potential is created by using two circularly polarized laser pulses of different wavelengths and opposite helicity, leading to multidirectional molecular orientation along the potential wells, which can be well revealed by a high-order statistics metric of $\ensuremath{\langle}\ensuremath{\langle}\mathrm{cos}{}^{3}{\ensuremath{\theta}}_{d}\ensuremath{\rangle}\ensuremath{\rangle}$.