Harnessing the power of qubits, molecule by molecule
Harnessing the power of qubits is notoriously tricky. For instance, two of the most common types—superconducting qubits, typically made of thin aluminum layers, and trapped-ion qubits, which utilize the energy levels of an ion’s electrons to represent binary information—must be maintained at temperatures approaching absolute zero (–273 °C). The special refrigerators required to keep them cool can be both costly and complex. Although researchers have made substantial advancements recently, connecting these types of qubits into larger systems has historically proven challenging.
Eager to explore the potential of molecular qubits, Freedman has pioneered a unique “bottom-up” approach to their creation. She designs novel molecules with specific quantum properties intended for individual applications. Instead of pursuing a broad goal such as maximizing coherence time (the duration a qubit can retain its quantum state), she starts by considering the necessary properties for specific sensors, like those intended to measure biological phenomena at the molecular level. Freedman and her team then work on developing molecules with these desired traits suitable for their intended environments.
To determine the precise structure of a new molecule, Freedman’s team employs software to analyze and process visualizations of data collected by an x-ray diffractometer. The diagram displayed illustrates an organometallic Cr(IV) complex composed of a central chromium atom and four hydrocarbon ligands.
These molecular qubits, containing a central metallic atom surrounded by hydrocarbon molecules, store information in their spin. The encoded information is subsequently translated into photons, emitted to “read out” the data. Researchers can finely tune these qubits with laser precision—similar to adjusting a radio dial—by modifying the strength of the ligands or bonds connecting the hydrocarbons to the metal atom. These bonds act as tiny tuning forks; adjusting their strength allows for precise control over the qubit’s spin and the wavelength of emitted photons, which can reveal atomic-level changes in electrical or magnetic fields.
While many researchers aim to build reliable, scalable quantum computers, Freedman and her group focus predominantly on developing custom molecules for quantum sensors. These ultra-sensitive sensors operate with particles in states so delicately balanced that even minor environmental changes can cause them to emit light differently. For instance, one qubit designed in Freedman’s lab, composed of a chromium atom surrounded by four hydrocarbon molecules, can be customized to change its light emissions in response to slight variations in the strength of a nearby magnetic field.
A significant advantage of employing such molecules for sensing is their small size—approximately a nanometer, allowing them to get extremely close to what they are sensing. This proximity offers an exceptional level of precision when measuring something like the surface magnetism of two-dimensional materials, as the strength of a magnetic field diminishes with distance. “A molecular quantum sensor might not be more inherently accurate than a competing quantum sensor,” Freedman explains, “but by reducing the distance by an order of magnitude, we can extract a wealth of information.” The capacity of quantum sensors to detect electric or magnetic changes at the atomic level and provide extraordinarily precise measurements has potential applications across various fields, including environmental monitoring, medical diagnostics, and geolocation.
When designing molecules as quantum sensors, Freedman’s group also considers their expected behavior in specific sensing environments. For example, creating a sensor for water necessitates a water-compatible molecule, while sensors intended for very low temperatures require molecules optimized for performance in cold conditions. By custom-engineering molecules for diverse applications, Freedman’s lab aspires to enhance the versatility and adaptability of quantum technology.
Embracing interdisciplinarity is essential to Freedman and her group’s work. She recognizes that harnessing the power of quantum science relies on collaborative efforts among scientists from various fields. “Quantum is a broad and heterogeneous field,” she notes, emphasizing that attempts to narrowly define it can hinder collective research. She asserts that scientists should welcome collaboration when their research extends beyond their specific discipline. Even when using a quantum computer to tackle a chemistry problem, contributions are needed from physicists to create quantum algorithms, engineers and materials scientists to build the computer, and chemists to define the problem and develop potential solutions.
