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The TU Wien team’s device consists of two black diamond crystals, on a sapphire chip that includes a superconducting waveguide resonator (wiggly line). The setup allowed the team to establish long-distance coupling of spin states in different nitrogen-vacancy centers in the two diamond samples. [Image: TU Wien]

Diamond nitrogen-vacancy (NV) centers—defects in the diamond crystal lattice, in which a nitrogen atom and a lattice vacancy substitute for a carbon atom—have attracted attention as a possible platform for quantum computing and memory. But it’s been difficult to get different NV centers to couple and interact over distances greater than a few nanometers, a must-have for robust quantum information transfer.

Now, researchers at the Vienna University of Technology (TU Wien) have achieved coherent coupling of distinct NV electron-spin ensembles across distances a million times greater, using a superconducting microwave cavity as a transmission line (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.118.140502). The team believes that the result could boost the feasibility of NV centers as a platform for quantum information devices.

Quantum data bus

One reason that physicists have such a keen interest in diamond NV centers is that they’re good at holding onto quantum information (in this case, electron-spin states), with coherence times on the order of a second in some experiments—a very long timescale for quantum operations. But the coupling between NV centers with different spin states tends to be extremely weak and local, with a reach “of only around 10 nm,” according to Johannes Majer, the team leader in the TU Wien work.

The experiment designed by the TU Wien team got to coherent coupling across much greater distances by using an intermediate microwave resonator; the resonator plays, in Majer’s words, “a similar role to that of a data bus in a regular computer.” The experimental setup consists of two square diamond crystals containing known NV centers, and separated by a distance of around 5 mm, bonded to a superconducting waveguide transmission-line resonator made with a niobium thin film that’s etched into a sapphire substrate. Surrounding the setup is a 3-D Helmholz coil, to provide arbitrary magnetic-field control.

The diamond chunks were placed at a precise angle with respect to one another, and the transition energies in the two ensembles were controlled by tweaking the surrounding magnetic field. This allowed the TU Wien group to use angle-resolved transmission spectroscopy to assess the degree of coupling for the NV centers in the two distantly separated samples—both with the cavity mode, and with each other.

Virtual photons

As the team varied the field’s orientation and magnitude, the cavity transmission spectrum showed a consequently varying pattern of bright and dark states, consistent with calculated values. The pattern indicated that NV centers in both diamond samples were coupling strongly and coherently to the microwave cavity mode and interacting via that mode. And, when the researchers cranked up the magnetic field to move the experiment into the dispersive regime, they found spectroscopic evidence of direct coupling between the separated samples through the exchange of “virtual photons” in the cavity.

The experiment, the TU Wien team concludes, “opens the opportunity for … coherent quantum information transfer between remote solid-state spin ensembles.” The researchers note that in addition to the ability to couple spin states in different NV centers across macroscopic distances, the ability to turn that coupling on and off by varying the orientation and strength of the magnetic field offers the flexibility “to realize many different configurations.”  And they stress, too, that while their experiment linked only two ensembles, the architecture in principle can support coupled ensembles of greater number—a plus for using the transmission-line cavities to build larger quantum information networks.