Harnessing the power of qubits is notoriously tricky, though. For example, two of the most common types—superconducting qubits, which are often made of thin aluminum layers, and trapped-ion qubits, which use the energy levels of an ion’s electrons to represent 1s and 0s—must be kept at temperatures approaching absolute zero (–273 °C). Maintaining special refrigerators to keep them cool can be costly and difficult. And while researchers have made significant progress recently, both types of qubits have historically been difficult to connect into larger systems.
Eager to explore the potential of molecular qubits, Freedman has pioneered a unique “bottom-up” approach to creating them: She designs novel molecules with specific quantum properties to serve as qubits targeted for individual applications. Instead of focusing on a general goal such as maximizing coherence time (how long a qubit can preserve its quantum state), she begins by asking what kinds of properties are needed for, say, a sensor meant to measure biological phenomena at the molecular level. Then she and her team set out to create molecules that have these properties and are suitable for the environment where they’d be used.
COURTESY OF DANNA FREEDMAN
Made of a central metallic atom surrounded by hydrocarbon atoms, molecular qubits store information in their spin. The encoded information is later translated into photons, which are emitted to “read out” the information. These qubits can be tuned with laser precision—imagine adjusting a radio dial—by modifying the strength of the ligands, or bonds, connecting the hydrocarbons to the metal atom. These bonds act like tiny tuning forks; by adjusting their strength, the researchers can precisely control the qubit’s spin and the wavelength of the emitted photons. That emitted light can be used to provide information about atomic-level changes in electrical or magnetic fields.
While many researchers are eager to build reliable, scalable quantum computers, Freedman and her group devote most of their attention to developing custom molecules for quantum sensors. These ultrasensitive sensors contain particles in a state so delicately balanced that extremely small changes in their environments unbalance them, causing them to emit light differently. For example, one qubit designed in Freedman’s lab, made of a chromium atom surrounded by four hydrocarbon molecules, can be customized so that tiny changes in the strength of a nearby magnetic field will change its light emissions in a particular way.
A key benefit of using such molecules for sensing is that they are small enough—just a nanometer or so wide—to get extremely close to the thing they are sensing. That can offer an unprecedented level of precision when measuring something like the surface magnetism of two-dimensional materials, since the strength of a magnetic field decays with distance. A molecular quantum sensor “might not be more inherently accurate than a competing quantum sensor,” says Freedman, “but if you can lose an order of magnitude of distance, that can give us a lot of information.” Quantum sensors’ ability to detect electric or magnetic changes at the atomic level and make extraordinarily precise measurements could be useful in many fields, such as environmental monitoring, medical diagnostics, geolocation, and more.
When designing molecules to serve as quantum sensors, Freedman’s group also factors in the way they can be expected to act in a specific sensing environment. Creating a sensor for water, for example, requires a water-compatible molecule, and a sensor for use at very low temperatures requires molecules that are optimized to perform well in the cold. By custom-engineering molecules for different uses, the Freedman lab aims to make quantum technology more versatile and widely adaptable.
Embracing interdisciplinarity
As Freedman and her group focus on the highly specific work of designing custom molecules, she is keenly aware that tapping into the power of quantum science depends on the collective efforts of scientists from different fields.
“Quantum is a broad and heterogeneous field,” she says. She believes that attempts to define it narrowly hurt collective research—and that scientists must welcome collaboration when the research leads them beyond their own field. Even in the seemingly straightforward scenario of using a quantum computer to solve a chemistry problem, you would need a physicist to write a quantum algorithm, engineers and materials scientists to build the computer, and chemists to define the problem and identify how the quantum computer might solve it.
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