In Search of Molecular Ions for Optical Cycling: A Difficult Road

Abstract

<jats:p>&lt;div&gt; &lt;div&gt; &lt;div&gt; &lt;p&gt;Optical cycling, a continuous photon scattering off atoms or molecules, plays a central role in the quantum information science. While optical cycling has been experimentally achieved for many neutral species, few molecular ions have been investigated. We present a systematic theoretical search for diatomic molecular ions suitable for optical cycling using equation-of-motion coupled-cluster methods. Inspired by the electronic structure patterns of laser-cooled neutral molecules, we establish the design principles for molecular ions and explore various possible cationic molecular frameworks. The results show identifying a perfect molecular ion for optical cycling is challenging, yet possible. Among various possible diatomic molecules we suggest several candidates, which require further attention from both theory and experiment: YF+, SiO+, PN+, SiBr+, and BO+. &lt;/p&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt;</jats:p>

On the prospects of optical cycling in diatomic cations: Effects of transition metals, spin-orbit couplings, and multiple bonds

“…We solve the electronic structure problem using high-level ab initio methods following the protocols from our previous studies on laser-coolable species. 1,40,41,43 We use the equationof-motion coupled-cluster method for electron attachment (EOM-EA-CC) to describe the ground and excited electronic states of doublet radical cations 50,51 . In these calculations the target states (with +1 charge) are obtained by attaching an electron to a closed-shell +2 reference state described by the coupled-clusters singles doubles (CCSD) method.…”

“…These requirements for molecules suitable for laser cooling are well known from earlier studies. [1][2][3][4][5] Potential applications of laser-coolable species include quantum computing 6,7 and sensing, as well as precision measurements of fundamental physical constants. [8][9][10][11] Charged particles are particularly attractive for experimental applications because they can be controlled by electric fields, motivating intense experimental and theoretical research in this area.…”

“…Earlier, we investigated diatomic ions of the main group elements with a goal to find transitions suitable for cycling. 1 The results were disappointing-compared to the isoelectronic neutral species with good OCCs, the transitions in the cations generally featured less favorable properties. We attributed this difference to more delocalized electronic structure and higher density of states in the cations, which present challenges in the design of cationic OCCs.…”

“…We attributed this difference to more delocalized electronic structure and higher density of states in the cations, which present challenges in the design of cationic OCCs. 1 Other computational studies have shown that BO + and SiBr + are not promising for laser cooling. 32,33 Here, we extend our search for cyclable molecular cations to a series of diatomic radical-cations comprising a d-block metal and a p-block ligand that are isoelectronic (in their valence shell) to the successfully laser-cooled neutral molecules.…”

High-resolution laser-induced fluorescence spectroscopy of $^{28}$Si$^{16}$O$^+$ and $^{29}$Si$^{16}$O$^+$ in a cryogenic buffer-gas cell

“…While several neutral molecules have now been laser cooled, such as SrF [12], CaF [13,14], YbF [15], YO [16], SrOH [17], and CaOH [18], YbOH [7] and CaOCH 3 [19], comparatively little experimental work has been carried out on molecular ions. Recently, we undertook a systematic search for diatomic molecular ions suitable for optical cycling [20] and found, in addition to four candidates already proposed by other workers, SiO + [21,22], BH + [23,24], AlH + [25] and AlCl + [26], only three additional potential candidates: BO + , PN + , and YF + .…”