The research was mainly carried out by Tuomas Ollikainen, a PhD student at Aalto university who split his time between carrying out experimental work in Amherst in Massachusetts, and analyzing the data and developing his theories at Aalto.

‘We hadn’t been able to study the dynamics of these sorts of three-dimensional structures experimentally before, so this is the first step to this direction.’ says Ollikainen. ‘The fact that the knot decays is surprising, since topological structures like quantum knots are typically exceptionally stable. It’s also exciting for the field because our observation that a three-dimensional quantum defect decays into a one-dimensional defect hasn’t been seen before in these quantum gas systems’

Controlling quantum gasses

The researchers hope their new study opens up new avenues in experimental research. One of the key breakthroughs in the study was being able to have better control over the state of the quantum gas, which allowed them to detect changes in its structure, like the decay of the knots and the formation of the vortex.

‘Of course one can simulate these things but actually making quantum knots is not that easy. By being able to control the environment better we can explore different effects and get to understand more about these exciting quantum systems.’ tells Ollikainen.

‘When we tied quantum knots in 2016, it was the first realization of three-dimensionally winding topological structures. That was like breathing air another planet for the first time. Amazing.’ says Prof. Mikko Möttönen, head of Quantum Computing and Devices group where Ollikainen works.

‘I know that many researchers have paid attention to our work and got inspiration to try this out in completely different type of systems. It would be great to see this technology being used some day in a practical application, which may well happen. Our latest results show that while quantum knots in atomic gases are exciting, you need to be quick to use them before they untie themselves. Thus the first applications are likely to be found in other systems.’ Möttönen continues.

At present, our global communication infrastructure relies on a hybrid model: electronic circuits are used to store and process information, while optical fibers transmit that information as light across vast distances. Quantum networks are expected to benefit from a similar division of labor. In such networks, information would be encoded not in classical bits, which can only represent a 0 or 1, but in qubits, the quantum counterparts of ordinary bits. Unlike classical bits, qubits can exist in superpositions of states, enabling powerful new modes of computation and communication. However, realizing such a system requires that qubits designed for storage and processing (such as those based on electrons) interact seamlessly with qubits designed for communication and transport (such as those based on photons).

In conventional telecommunications, this challenge is addressed by electro-optic modulators, devices that use electronic signals to modify the properties of light. In the quantum world, however, scientists must find new mechanisms to allow delicate quantum states to influence one another without being destroyed in the process. The recent breakthrough, achieved by the research group of Edo Waks—Fellow at the Joint Quantum Institute (JQI) and Associate Professor of Electrical and Computer Engineering at the University of Maryland—represents a promising interface between individual photons and electrons.

By confining a photon and an electron in the same extremely small cavity, the team has created a system in which the electron can rapidly change the quantum properties of the photon, and conversely, the photon can directly alter the state of the electron. Their research, reported online in Nature Nanotechnology on February 8, 2016, demonstrates an approach that could eventually enable the “quantum wiring” needed for distributed quantum systems.

“Our platform has two major advantages over previous work,” explains Shuo Sun, graduate student at JQI and lead author of the paper. “First, the electronic qubit is integrated directly on a chip, making it highly scalable. Second, the interaction between light and matter is extremely fast, occurring in just a trillionth of a second—about 1,000 times quicker than earlier studies.”

Constructing a Quantum Interface

At the heart of this breakthrough is a carefully engineered photonic crystal. Photonic crystals are microscopic structures built from semiconductor layers patterned with a repeating grid of nanometer-sized holes. These periodic arrangements allow researchers to precisely manipulate the way light propagates through the material. By tailoring the size, shape, and distribution of the holes, scientists can create pathways for light, bend it around corners, or even trap it in tiny cavities where it bounces back and forth.

“These photonic crystals can focus light into an incredibly small volume, down to the fundamental quantum limit where the presence of a single photon is enough to drastically affect the system,” Waks explains. This ability to control light at the quantum level is essential for creating devices that operate reliably with individual quanta of energy rather than large pulses of light.

The experiment builds on another line of research involving quantum dots—engineered nanocrystals that behave like artificial atoms. Quantum dots can confine electrons within a very small region and exhibit discrete energy levels, much like natural atoms. In prior work, JQI researchers demonstrated that quantum dots could strongly influence beams of light, redirecting them or altering their properties.

In their new study, the team combined both approaches: the light-trapping power of photonic crystals with the electron-trapping ability of quantum dots. They fabricated a photonic crystal punctuated by holes only 72 nanometers wide. By intentionally leaving three adjacent holes undrilled, they introduced a controlled “defect” into the lattice. This defect formed a resonant cavity that selectively admitted and confined photons with very specific energies.

Inside this cavity, embedded in layers of semiconductor material, they placed a quantum dot capable of holding a single electron. The quantum property of that electron, known as its spin, then dictated how photons entering the cavity behaved. If the spin pointed upward, photons passed through unchanged. But if the spin pointed downward, every photon that entered emerged with its polarization flipped—its electric field oscillation rotated to the opposite orientation.

Crucially, the process also worked in reverse: a single photon prepared with the appropriate polarization could flip the electron’s spin. This bidirectional coupling between electron spin states and photon polarization demonstrates a fundamental type of quantum switch—a building block that could form the basis for scalable quantum circuits.

Toward Quantum Networking

The successful demonstration of this photon-electron interface has far-reaching implications. A robust quantum network will likely combine the storage and processing strengths of electrons with the long-distance transport abilities of photons. For example, electrons confined in quantum dots or other solid-state systems could hold information locally and perform computations, while photons transmitted through optical fibers could carry that information securely to distant locations.

Such a network would also make possible the distribution of entanglement, the uniquely quantum correlation that links particles across arbitrary distances. Entanglement is the foundation of many proposed quantum technologies, including distributed quantum computation, quantum teleportation of information, and secure communication protocols based on unbreakable quantum keys.

Before these applications become reality, however, more work is required. Sun and his colleagues emphasize that the next challenge is to demonstrate entanglement between the electron and photon qubits in their system—a step that requires even more precise measurements. Only once entanglement is verified and controlled can the platform serve as a reliable node in a future quantum network.

“The ultimate goal is to integrate photon generation, routing, and switching all onto a single chip,” Sun explains. “If we can accomplish that, we will be able to construct increasingly sophisticated quantum devices and circuits, paving the way toward practical quantum computers and secure quantum communication systems.”

A Glimpse Into the Future

This work underscores how progress in nanofabrication and materials engineering is enabling scientists to control light and matter at unprecedented scales. By merging photonic crystals with quantum dots, the JQI researchers have shown a pathway toward functional interfaces that can mediate interactions between photons and electrons, two of the most promising candidates for quantum information carriers.

Although still at an early stage, this research demonstrates that the basic ingredients for building quantum networks are beginning to fall into place. With continued development, such technologies could transform how information is stored, transmitted, and secured—ushering in a new era of communication where the principles of quantum mechanics are harnessed on a global scale.

Journal Reference:

Shuo Sun, Hyochul Kim, Glenn S. Solomon, Edo Waks. A quantum phase switch between a single solid-state spin and a photon. Nature Nanotechnology, 2016; DOI: 10.1038/nnano.2015.334