In the quest to develop new quantum technologies that could revolutionize various fields, from computing to sensing, scientists are exploring numerous approaches. One of the most promising avenues involves using molecules as the fundamental building blocks of these technologies. Researchers at the California Institute of Technology (Caltech) have recently discovered a groundbreaking way to leverage ultrafast laser pulses to induce a crucial quantum mechanical phenomenon known as superposition, transforming a relatively simple molecule into a quantum sensor capable of measuring chemical phenomena in its surroundings in inherently quantum ways.
Superposition, a cornerstone of quantum mechanics, allows particles like electrons to exist in multiple states simultaneously. This phenomenon is often illustrated by Schrödinger’s cat thought experiment, which imagines a cat inside a box being both alive and dead until an observation is made. Similarly, an electron can exist in a superposition of several states, each corresponding to a different potential outcome, and its actual state will only be revealed when it is measured.
A central feature of many quantum technologies is the use of quantum bits, or qubits, which are the quantum analogs of the classical bits used in today’s computers. Unlike classical bits, which exist in one of two states—either 0 or 1—qubits can exist in multiple states at the same time, thanks to superposition. This ability enables quantum systems to potentially perform calculations far more efficiently than classical systems, offering a significant computational advantage. However, this advantage comes with a major challenge: a superposition is inherently unstable and will collapse into one of its constituent states when it interacts with the surrounding environment. This phenomenon, known as decoherence, makes it difficult to harness the power of quantum systems for practical applications such as quantum computing and quantum sensing.
In a recent paper published in Science, researchers in the lab of Ryan G. Hadt, Assistant Professor of Chemistry at Caltech, have introduced a novel method to overcome this challenge. The researchers have developed a technique using femtosecond laser pulses (pulses lasting just a few millionths of a billionth of a second) to induce and measure superposition in a simple molecule at room temperature. By manipulating the electron spin superposition of potassium hexachloroiridate (IV), or K2IrCl6, the team demonstrated how this molecule could be used as a quantum sensor—a tool capable of measuring chemical and physical properties with quantum precision.
Potassium hexachloroiridate (IV) is a paramagnetic molecule, meaning it contains unpaired electrons that make it particularly suitable for quantum manipulation. In molecular systems, electrons typically occupy discrete energy levels, but in paramagnetic molecules like K2IrCl6, these energy states are arranged in a way that allows efficient manipulation of the electron’s spin. The spin of an electron refers to its intrinsic angular momentum, which can be thought of as the direction of a magnetic field generated by the electron. Manipulating this spin can lead to the creation of a superposition of states, which the researchers use to track quantum phenomena.
The key innovation in the Caltech team’s work is the use of a technique called pump-probe polarization spectroscopy to create and measure electron spin superpositions. In this approach, the researchers expose a sample of K2IrCl6 to an ultrafast laser pulse with a specific polarization. The polarization of light refers to the orientation of the oscillations of the electromagnetic wave as it propagates through space. By carefully selecting the polarization of the light, the researchers were able to excite the electrons in the molecule from one energy state to a higher one, creating a superposition of states.
Following this excitation, a second, weaker laser pulse is passed through the sample, and the researchers measure the change in polarization of the transmitted light. By repeating this process over time, the researchers can track how long the electron spin remains in a superposition before it relaxes back to its original state. This technique allows them to measure the lifetime of the superposition and understand how the system behaves at extremely fast timescales.
Nathanael P. Kazmierczak, a graduate student in Hadt’s lab and co-author of the paper, notes that this technique is not universal, and the design of the molecules used is crucial to its success. The key insights behind the study were twofold: first, the development of the pump-probe spectroscopy method, and second, the discovery that paramagnetic molecules like K2IrCl6 are particularly suitable for this kind of experiment. Sutcliffe, one of the lead researchers on the project, emphasizes that K2IrCl6 is only one example of a broader class of molecules that could be used for quantum sensing and the study of quantum properties. This opens up the possibility of a wide range of molecular probes that could be applied to various quantum technologies.
The potential applications of this research are vast, particularly in the field of quantum sensing. The electron spin superposition created in K2IrCl6 is sensitive to a variety of chemical properties in its environment. For example, the electron spin can be influenced by the viscosity of the surrounding medium or the presence of magnetic nuclei. This sensitivity makes these molecules excellent candidates for detecting subtle changes in the environment, which is a key aspect of quantum sensing. The ability to measure such properties at extremely fast timescales could lead to breakthroughs in fields ranging from materials science to biology.
Moreover, the simplicity of the technique makes it highly versatile. Unlike other methods of quantum sensing, which may require large magnetic fields or microwave radiation, this technique relies solely on light. This means that it could be applied in smaller, more portable devices, allowing for measurements on a much smaller scale. Sutcliffe highlights that this opens up the possibility of using this technique for microscopy, providing a way to measure properties at the molecular level that were previously inaccessible.
The team’s research could also have significant implications for biological studies. Since electron superpositions are sensitive to the surrounding environment, they could be used to study the effects of protein structures and amino acid compositions on spin superpositions. Hadt suggests that this technique could even be used to detect cancer-related mutations in proteins, offering a potential tool for diagnosing diseases at the molecular level.
The ability to measure superposition at room temperature and with femtosecond precision represents a significant advancement in the field of quantum technologies. These new techniques could lead to the development of highly sensitive quantum sensors capable of measuring a wide range of chemical and physical phenomena with unparalleled precision. Moreover, the ability to use light as the primary tool for measurement, rather than large-scale equipment like magnets or microwaves, makes the approach highly adaptable and potentially applicable in various settings, including medical diagnostics and environmental monitoring.
As quantum technologies continue to advance, the work done by the Caltech team could play a crucial role in realizing the full potential of quantum systems. Their ability to create and measure superposition in molecules at room temperature opens up exciting possibilities for the future of quantum sensing, with applications across numerous scientific and technological domains. By harnessing the inherent properties of quantum mechanics, such as superposition, scientists are not only pushing the boundaries of what is possible in fundamental research but are also paving the way for practical technologies that could revolutionize fields as diverse as medicine, materials science, and environmental monitoring.