Quantum Logic May Enable Single-Molecule NMR

According to Wikipedia, quantum logic (QL) is a set of rules for reasoning about propositions that takes the principles of quantum theory into account. QL is expected to be the next huge breakthrough in computers, including memory, that utilize the many quantum states and interactions to facilitate faster and more reliable computing such as hack-free communication.

When applied to chemistry, QL can be a process that uses the peculiar nature of quantum states of atomic ions to interact with its neighbors. The current state-of-the-art is for atomic ions interacting with other atomic ions. A report from a team at NIST (Boulder, CO) shows that QL can be used to probe a single, multiatom molecular ion (such as ammonium) using a probe ion (see http://www.nature.com/nature/journal/v545/n7653/full/nature22338.html).

The first step is to put a probe ion (40Ca+) and a 40Ca1H+ ion in an ion trap fashioned from a mass spectrometer mass analyzer (step 1, Figure 1). In step 2, the two cations interact with each other repulsively. However, in step 3, the repulsion is quantized. Laser cooling of the monoatomic 40Ca+ also lowers the temperature of the CaH+ ion until it is in the lowest electronic and vibrational state. However, the target ion is still able to rotate randomly between quantized rotational states. In step 4, the target ion is irradiated with a laser pulse tuned to promote a transition in its rotational energy. This transition also is felt by (i.e., coupled) to the probe ion (Ca+), producing synchronized movement. In step 6, a separate laser pulse is aimed at the probe ion, which scatters light due to the movement. The scattered light is measured as the output of the experiment.

Dr. Dietrich Leibfried, lead investigator on the paper, points out that the molecular ion only jiggles if it is in the right state stimulated by the laser in step 4. The probe ion responds via quantum coupling (step 5). The readout laser provides a direct black or white signal. Leibfried expects that current technology is compatible with an m/z range of 2 to about 500. Multiply charged molecular ions resulting from adducts might extend the useful mass range as in ESI-MS.

The absorbance of light in step 4 can be related to the angular momentum of the molecular ion. Potentially, isomers, including optical, could have different quantized rotational energy states and thus be discriminated.

As with most basic experiments, the calcium experiment shows proof of principle. But what about more practical potential applications? Detection of molecular ions in space could answer interesting questions such as: Do the amino acids in space show an enantiomeric bias?

The probe ion can respond to the topography of the target. Thus, Leibfried expects that one could scan the surface structure to produce a connectivity map similar to NMR structure, but with a single molecule target. This would be in a vacuum, however, which might not be representative of the structure of large hydrated molecules such as proteins, which is probably less of a concern for a single small molecule.

I’m certain that we will soon see many more application examples of QL in analytical chemistry.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected].

Related Products

Comments