In a laboratory at Purdue University, a laser beam strikes a crystal of hexagonal boron nitride, and in response, a single point of light flashes. This is not just luminescence; it is a signal from a spin defect, a pair of electron spins, one of which is coupled to a carbon-13 nuclear spin. So begins the story of how, in a two-dimensional material at room temperature, two quantum objects were entangled and made to work in harmony, like a courier and an archivist.

The electron spin is a speedy courier: it quickly gathers information from the surrounding magnetic field, but it tires quickly. Its coherence, the ability to maintain a quantum phase, fades in 141 nanoseconds. The nuclear spin is a reliable archivist: it is slow, but it stores quantum information for up to 1 millisecond. Previously, this tandem in hexagonal boron nitride did not work: the dense nuclear environment destroyed the courier's coherence before it could pass on the data. The physicists found a way: a sequence of 1024 refocusing pulses, like a series of short shouts that keep the courier on track. The coherence time grew to 38 microseconds, 270 times longer.

Entanglement was prepared with a fidelity of 0.89 for the Bell state Ψ+. This is not an abstract trick: the electron spin, after measuring a fluctuating magnetic field, transferred its accumulated phase to the nuclear spin, which carefully stored it while the courier rested. Correlation spectroscopy with such a quantum memory achieved a spectral resolution of 0.7 kHz, seven times better than without it. Previously, entanglement in two-dimensional materials at room temperature had not been demonstrated, and the short coherence of the electron spin in hBN limited its use in quantum sensors. Now it has been shown that electron and nuclear spins in hBN can be entangled, the coherence extended to 38 μs, and the nuclear spin used as a quantum memory to improve spectral resolution sevenfold.

This result places hexagonal boron nitride alongside diamond and silicon carbide as a platform for quantum technologies. Two-dimensional sensors based on hBN could map magnetic fields with nanometer resolution, probe two-dimensional magnetism, and investigate moiré structures. In the future, one could scale to multiple nuclear spins, create a quantum register, and integrate into van der Waals heterostructures. The courier and the archivist no longer work alone: their tandem opens the door to hybrid quantum devices compatible with photonics.