Startup achieves first conversion of two different kinds of quantum information

The Kastler Brossel Laboratory in Paris has successfully constructed the first converter between two different types of quantum bit encoding - the equivalent of a converter for classical information, but for different types of quantum data. Specifically, the conversion of quantum information between two main paradigms, namely discrete and continuous variable quantum bits, is demonstrated. In the February 2023 issue of the online journal Nature Photonics [1], they report the first successful demonstration of quantum bit encoding conversion. The first author of this study, Tom Darras, is the CEO and co-founder of the quantum startup Welinq.

Similar to classical analog or digital information encoding, they are preferred for certain tasks and platforms. The team found a way to convert one very unique type of quantum information into another, thus confirming the possibility of interconnecting different quantum devices.

In the race for quantum computing, many platforms are under development that rely on different quantum systems, such as photons, neutral atoms, ions, superconductors and semiconductors. For all these systems, several types of encodings exist and its choice depends on the particular application and the available resources. Resolving this heterogeneity in quantum networks is an urgent problem. It will allow the integration of the best features of each to achieve more powerful and efficient networks.

Addressing heterogeneity early in the development of quantum networks prevents compatibility issues and may allow seamless integration and interconnection of different quantum systems in the future. This task requires the use of a quantum coding converter, a device that changes the writing base while preserving the fragile encoded quantum information signal.

Quantum bit conversion is a complex challenge. A straightforward way to create a quantum bit converter is to measure the information stored in one encoding and recreate that information as another encoding. However, quantum mechanics and the so-called unclonable theorem do not allow this operation to be performed on arbitrary information.

In a way, this inconvenience is a blessing, because this is where the power of quantum cryptography comes from. Nevertheless, this forced the team to take another approach to creating converters: using quantum entanglement. Entanglement describes the non-classical correlation between quantum systems. It was described by Einstein as a "spooky hyperdistance interaction".

First author Tom Darras says [2]: "In fact, the second quantum revolution is driven by the ability to harness and control entanglement at the quantum scale. The ability to create, manipulate, and distribute entanglement opens the door to many new applications and technologies that would not have been possible with classical systems alone." Darras is now the CEO and co-founder of the quantum startup Welinq.

The implementation of a quantum bit converter can be divided into three main steps. First, key resource entanglement must be created. Second, the input quantum bits are sent to the converter. Finally, special measurements called "Bell state measurements" must be performed to invisibly transfer the input information to the output quantum bits. In this process, unlike other invisible transfer protocols, the quantum bits are rewritten into another base.

To create all the resource states, the researchers in Paris used an efficient nonlinear light source called an optical parametric oscillator (OPO). Depending on the chosen crystal, high purity single photons or optical cat states are output at the time of the heralded event. They also rely on highly efficient superconducting single-photon detectors.

This process requires a very specific resource for optical entanglement, i.e., a "hybrid entangled state" between discrete-variable quantum bits and continuous-variable Schrödinger cat quantum bits. To achieve the Bell state measurement, the single photon part of the hybrid entanglement is used to interfere with the input quantum bit, followed by an enhanced single photon detection. For verification, the output quantum bits are characterized using "quantum tomography" to calculate the fidelity between the input and output quantum bits, which is a typical way to assess the quality of the process. For any input quantum bit, a conversion above the classical limit is confirmed.

The success of this process is an important milestone in the quantum technology infrastructure," said Beate Asenbeck, a PhD student and one of the paper's lead authors. Once we are able to interconnect quantum devices, we can build more complex and efficient networks. Using decade-old technology, this task would have been nearly impossible. This is a very exciting time."

Reference links:

[1]https://www.nature.com/articles/s41566-022-01117-5

[2]https://phys.org/news/2023-02-quantum-conversion-path-scale-technology.html