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The Innovative Approach to Molecular Qubits in Quantum Sensing

The exploration of qubits—a fundamental unit of quantum information—poses numerous challenges for researchers. Two prevalent types of qubits, superconducting qubits and trapped-ion qubits, rely on delicate operational conditions. Superconducting qubits are often comprised of thin aluminum layers, while trapped-ion qubits leverage the energy levels of electron shells in ions to denote binary states of 1 and 0. Both types demand maintenance at temperatures approaching absolute zero (–273 °C), necessitating costly and complex refrigeration systems. Despite notable advancements in recent years, integrating these qubits into larger quantum systems remains a significant hurdle.

Dr. Danna Freedman is leading an innovative venture into molecular qubits through a distinct “bottom-up” methodology. This approach emphasizes the design of novel molecules embedded with specific quantum characteristics for targeted applications. Rather than pursuing a general aim, such as optimizing coherence time—the duration a qubit can retain its quantum state—Freedman concentrates on defining necessary properties for particular applications, like sensors intended to gauge biological activities at the molecular scale. From there, she and her research team work to create customized molecules that meet these precise requirements, tailoring them to their intended environments.

To accurately discern the structure of newly synthesized molecules, Freedman’s team utilizes advanced software to interpret visual data generated by an x-ray diffractometer. One such visualization illustrates an organometallic chromium (IV) complex, characterized by a central chromium atom bonded to four hydrocarbon ligands.

These molecular qubits, composed of a central metallic atom encircled by hydrocarbon atoms, encode information through their spin states. The stored data can be transformed into photons, which are emitted to facilitate information retrieval. Remarkably, these qubits can be finely tuned using lasers, a process akin to adjusting a radio frequency. By manipulating the strength of the bonds linking the hydrocarbons to the metal, researchers can intricately manage the spin of the qubit and the corresponding wavelengths of emitted light. This emitted photon stream serves as a means to gain insights into subtle, atomic-level variations in electrical or magnetic fields.

While many in the field aspire to build robust and scalable quantum computers, Freedman’s group prioritizes the development of custom molecules specifically for quantum sensors. These highly sensitive sensors exist in a balanced state, making them acutely responsive to even the most minor environmental fluctuations, which critically influence their light emission patterns. For instance, one qubit crafted in Freedman’s lab, featuring a chromium atom encased by four hydrocarbon molecules, is designed to respond uniquely to slight alterations in nearby magnetic fields.

A notable advantage of these molecular sensors is their diminutive size—approximately one nanometer—which enables them to make close-range observations of the phenomena they are studying. This proximity results in exceptional precision, especially when assessing surface magnetism in two-dimensional materials, as magnetic field strength diminishes with distance. Freedman explains, “While a molecular quantum sensor might not offer inherently greater accuracy than a traditional sensor, the ability to reduce distance by an order of magnitude can yield a wealth of information.” The potential applications for quantum sensors are extensive, spanning environmental monitoring, medical diagnostics, geolocation, and beyond.

In their design process for quantum sensors, Freedman’s team considers how each molecule will perform in various sensing environments. For instance, a sensor intended for aquatic applications requires a molecule that is compatible with water, whereas sensors meant for low-temperature operations must be optimized for such conditions. By meticulously customizing molecules for diverse uses, the laboratory seeks to enhance the versatility and adaptability of quantum technology.

Fostering Interdisciplinary Collaboration

As Dr. Freedman and her team delve into the intricate task of custom molecule design, she recognizes the crucial role that interdisciplinary collaboration plays in the advancement of quantum science. “Quantum is a broad and heterogeneous field,” she explains, reflecting on the need for diverse expertise. She contends that narrowly defining quantum research can impede progress and stresses the importance of collaboration that extends beyond traditional boundaries. Even in seemingly straightforward applications—such as using quantum computing to address specific chemistry challenges—successful outcomes rely on the coordinated efforts of physicists to craft quantum algorithms, engineers and materials scientists to build computer hardware, and chemists to frame relevant problems and propose solutions.

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