Despite the rapid development of photonic devices and systems, on-chip information technologies are usually limited to two-tier systems due to the lack of sufficient reconfiguration to meet stringent requirements. Even with extensive efforts directed at recently emerging vector lasers and microcavities to expand dimensionality, it remains a challenge to actively tune the diversified, high-dimensional superposition states of light on demand.

Penn Engineering scientists have created a hyper-dimensional, spin-orbit microlaser chip that exceeds the security and robustness of existing quantum communications hardware. Their system uses “qudits” for communication, doubling the quantum information space of previous on-chip lasers.

Sophisticated quantum devices use qubits, units of digital information that can be both 1 and 0 at the same time. In quantum mechanics, this state of simultaneity is called “superposition.” A quantum bit in a superposition state of more than two levels is called a qudit to indicate these extra dimensions.

The new device uses four-level qudits that enable significant advances in quantum cryptography. In addition, the device offers four superimposition levels and opens the door to further dimensional enlargement.

Materials Science and Engineering (MSE) postdoctoral researcher Zhifeng Zhang said: “The biggest challenge was the complexity and non-scalability of the default setup. We already knew how to generate these four-level systems, but it took a lab and many different optical tools to control all the parameters associated with the increase in dimension. Our goal was to achieve this on a single chip. And that is exactly what we have done.”

The hyperdimensional spin-orbit microlaser advances the group’s previous work with vortex microlasers, which sensitively regulate the orbital angular momentum (OAM) of photons. The recent device adds control over photonic spin to the previous laser’s capabilities.

This extra level of control – the ability to manipulate and link OAM and spin – is the breakthrough that allowed them to achieve a four-level system.

The main experimental achievement of the team’s work is the simultaneous control of all parameters that prevented the creation of qudit in integrated photonics.

ESE Ph.D. student Haoqi Zhao said: “Think of the quantum states of our photons as two planets stacked on top of each other. Previously, we only had information about the latitudes of these planets. This would allow us to create a maximum of two levels of superposition. We didn’t have enough information to quarter them. Now we also have the longitude. This is the information we need to manipulate photons in a coupled way and achieve dimensional increase. We coordinate the rotation and rotation of each planet and keep the two planets in strategic relationship to each other.”

Liang Feng, professor in the Departments of Materials Science and Engineering (MSE), said: “There is great concern that mathematical encryption, however complex, will become less and less effective as we advance so rapidly in computer technologies. Quantum communication’s reliance on physical rather than mathematical barriers makes it immune to these future threats. It is more important than ever that we continue to develop and refine quantum communication technologies.”

Magazine reference:

  1. Zhang, Z., Zhao, H., Wu, S. et al. Spin-orbit microlaser emitting in four-dimensional Hilbert space. Nature (2022). DOI: 10.1038/s41586-022-05339-z