Even Colder Molecules in Cryocrystals

Future quantum computers promise to solve important problems that are out of reach for todays classical machines, such as designing new materials or simulating complex chemical reactions. However, building a large and reliable quantum computer remains challenging because each separate physical quantum system possesses its own advantages and disadvantages. One promising approach is to combine different quantum systems into a single hybrid platform, where one component provides long-lived quantum states and another enables fast and accurate control. This project develops a new type of hybrid platform based on polar molecules placed in a very cold solid made from inert gases, called a cryocrystal. These cryocrystals can be grown directly on top of superconducting microwave circuits, which are a key technology in modern quantum computing. Polar molecules are attractive because they can interact strongly with microwave fields, allowing fast control and readout. The main challenge is to place molecules close enough to the superconducting circuit and cool them sufficiently so that they remain in their lowest-energy states during experiments. Our team has already demonstrated that cryocrystals can host impurities near superconducting resonators and has successfully embedded ammonia molecules into noble-gas solids. The next step is to move to partially deuterated ammonia molecules, which have microwave transitions in the standard frequency range used in superconducting quantum processors. This enables better compatibility with existing quantum hardware and reduces thermal noise. The project will build and calibrate a new cryocrystal deposition setup inside a dilution refrigerator, the type of cryogenic system used in most superconducting quantum computing laboratories. This will allow cryocrystals and embedded molecules to be cooled continuously to ultra-low temperatures, improving stability and performance. Superconducting microwave resonators will be specially designed and fabricated to maximize the interaction with single molecules. Using spectroscopy and time-resolved microwave measurements, the project will quantify how long molecular quantum states survive and how well they can be controlled. Finally, it will demonstrate basic qubit operations on a single molecule and evaluate the fidelity of these operations with established benchmarking methods. By enabling coherent control of individual molecules in cryocrystals, the project establishes the groundwork for molecular quantum bits that can be integrated with scalable superconducting technology. This approach will open new directions in quantum computing and long-lived quantum memory devices.