A technique for manipulating molecules uses an intermediary atom to query a nearby molecule's energy state and produces 'quantum superpositions' of these states, a prerequisite for extremely high-precision spectroscopy. See Letter p.203
Laser cooling and related techniques are routinely used to produce pure quantum states of atoms in the gas phase that enable the atoms to be measured and controlled with unprecedented precision. Put two atoms together in a molecule, however, and things become more than twice as difficult — the complexity of even simple molecules severely diminishes the effectiveness of tools based on repeated light scattering. But if an atomic ion is confined in the same electromagnetic trap as a molecular ion, as demonstrated last year by Wolf et al.1, the atom can be cleverly used as an envoy to facilitate indirect communication with the molecule. On page 203, Chou et al.2 extend this idea by cornering a molecule into a single quantum state using an atomic emissary, and then preparing and measuring quantum superpositions (in which the molecule is simultaneously in more than one energy state), while retaining the molecule for subsequent measurements. This type of control over quantum superpositions is the gold standard for manipulating isolated quantum systems, and is a necessary ingredient of extremely high-precision spectroscopy.
Spectroscopy allows precision measurement of the energy states of atoms and molecules. It typically involves shining light on a sample and looking for a response that depends on the frequency of that incident light. To ensure that the energies being measured correspond to the species of interest (rather than its interaction with a solvent or substrate, which tends to shift these energies), the preferred method for recording high-resolution spectra uses gas-phase molecules isolated in a vacuum. However, because such molecules are free to rotate and vibrate, absorption of energy (whether from the probe light or, for instance, thermal radiation from the container) tends to chase molecules out of the energy state being probed, in a process called bleaching.
Bleaching creates a headache for the spectroscopist, who must then choose between sacrificing precision by working with a large, hot sample to guarantee that a few molecules can be found in each energy state, or working with a small, cold sample of molecules that can provide high precision, but only a few photons of signal. This conundrum is essentially unique to molecules, because their states pack the energy 'landscape' much more densely than even the most complicated atoms. New cooling methods3,4 might soon obviate this dilemma, but the strength of the atomic-envoy technique (known as quantum-logic spectroscopy5) is that it can produce plenty of signal while keeping the molecule inside the trap — the atom acts as a spokesperson to the outside world, but its conversations with the molecule occur strictly in whispers.
Chou and colleagues demonstrate their spectroscopy technique using a calcium atomic ion (Ca+) and a calcium hydride molecular ion (CaH+). Following the approach of Wolf et al.1, the authors confine the two ions in an electromagnetic trap in an ultrahigh vacuum (Fig. 1a). The ions are coupled by their mutual electromagnetic repulsion, and therefore have shared motion. The authors cool the atomic ion using a laser, which causes this motion to stop (Fig. 1b). They then apply 'modulated' laser light to the molecular ion, changing its internal quantum state (Fig. 1c). This change is accompanied by a restoration of the ions' shared motion. Subsequently applied laser light scatters off the atomic ion only if the ions are moving and therefore only if the molecular ion is in the desired quantum state (Fig. 1d).
By applying more laser light to the molecular ion, the authors produce quantum superpositions of molecular states. They are able to control the relative phase between the states — a key requirement for high-precision spectroscopy that the authors verify by repeating the state-preparation technique outlined above. The probability that the molecular ion is found in one of the two states used in the superposition oscillates as a function of time, which illustrates the close connection between the authors' spectroscopy technique and the pendulum of a clock. Crucially, repeated trials of this protocol are possible because the detection method leaves the molecule not only in the trap, but also in the energy state revealed by the detection scheme itself.
Chou and colleagues' technique is particularly powerful because it does not require laser light that has a frequency anywhere near the resonance frequency associated with a molecular electronic transition. This means that other molecules should be amenable to the same method, with few changes. Scattering of light near the envoy atom's electronic resonance allows the state of the molecule to be determined, whereas the laser light that is applied directly to the molecule is hundreds of terahertz (1 THz is 1012 Hz) from the closest electronic resonance. A further advantage of working with such far-off-resonant light is that the authors' technique should be applicable to the hydrogen molecular ion H2+, which does not have a (strongly bound) excited electronic state for near-resonant spectroscopy techniques. This is exciting because, although H2+ is conceptually the simplest molecule possible — and is described by high-precision theoretical calculations — it has been difficult to test these calculations using conventional tools.
Although the type of quantum-logic spectroscopy reported by Chou et al. shows promise for its applicability to other molecules, its technological aspects are demanding. Because the far-off-resonant light interacts with the molecule only extremely weakly, the technique could require high laser power, potentially generating unintended laser-light scattering that must be avoided. Additionally, any high-precision measurement of this kind will, by definition, require relatively long interaction times. This imposes some constraints on the immediate applicability of the authors' technique to molecules other than diatomic hydrides — more-complex species might be bleached by room-temperature thermal radiation faster than the requisite interaction time, and would therefore need a cryogenic environment to reach high resolution.
Finally, a crucial ingredient of Chou and colleagues' technique is a fully functioning, co-trapped envoy atom, which is effectively a simple quantum computer5 and challenging to construct. Nevertheless, growing numbers of research groups have access to these tools, as applications proliferate for well-controlled quantum systems. It might therefore not be long before this type of molecular spectroscopy can be used to probe fundamental physics and to realize other promising proposed applications for cold molecules6,7.Footnote 1
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Chou, C. et al. Nature 545, 203–207 (2017).
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