Physicists have created a spin qubit for quantum sensing based on a yellow fluorescent protein. The sensor's sensitivity to a magnetic field was 98 picoteslas at room temperature, and its coherence time was approximately 16 microseconds. The scientists introduced the resulting qubit into human embryonic kidney cells without reducing the coherence time or losing the sensor's sensitivity. A preprint of the study is available on arXiv.org.
Qubits are not only components of quantum computers, but also quantum sensors, the operating principle of which is based on interaction with the environment (for example, with a small change in temperature, the coherence time and spectral characteristics of the qubit change sharply). Such sensors allow scientists to measure nanoscale electric and magnetic fields with high accuracy, as well as temperatures close to absolute zero, but the use of quantum sensors in life sciences remains at the conceptual level today.
Most often, for biological sensing, researchers use spin qubits based on nitrogen vacancies in diamond (we talked about qubits on NV centers in more detail in the material “Quantum Technologies. Module 4”), which are easily tuned by optical methods and maintain coherence at room temperature. At the same time, nanodiamond qubits suffer from several disadvantages that are critical for biological research: firstly, their large size, and secondly, their morphological heterogeneity, which makes it difficult to label them.
Physicists from the United States led by Peter Maurer from the University of Chicago have proposed using a fluorescent protein qubit as a quantum sensor. To do this, the scientists used a yellow fluorescent protein obtained from the jellyfish Aequorea victoria, in which the photoactive organic fluorophore is in a metastable state and can be used as a spin triplet. The physicists initialized the spin in the protein using optical pulses at a wavelength of 488 nanometers, and used confocal microscopy to characterize the triplet state.
It turned out that in this experimental configuration, the protein is in a coherent state for a very short time, which is not enough for a detailed study. Therefore, the scientists changed their approach: using an optical pulse at a wavelength of 912 nanometers, they transferred the qubit from the triplet state T1 to a higher-energy triplet T2, the coherence time of which the authors of the work measured using the spin echo method and found that, depending on the applied magnetic field, the coherence time ranged from 140 nanoseconds to 16 microseconds. The physicists also demonstrated the potential of the developed qubit as a magnetic field sensor with an accuracy of 98 picotesla at room temperature (for comparison, a proton at a distance of five nanometers from the qubit creates a field of 20 nanotesla), which was possible due to the linear sensitivity of the spin contrast to the external magnetic field.
In addition, the scientists studied the possibility of embedding the created qubit into mammalian cells using human embryonic kidney cells as an example. Fluorescent visualization showed that the protein remained localized inside the cells and was sensitive to an external magnetic field.
The authors of the paper noted that their results allowed them to create a promising platform for quantum sensors that can be used in biology. However, additional methods must be used to increase the qubit coherence time and its sensitivity to the magnetic field in order to achieve an advantage over existing technologies.
We wrote earlier about how a diamond-based quantum sensor helped measure neural activity in the brain on a microscale.
