Passive Two-Photon Quantum Dots Enable Secure Photonics

University of Innsbruck engineers, alongside a team of scientists from several prestigious institutions, unveiled a novel method to create dual-state quantum dots. These unique creations are capable of achieving a biexciton state without active switching elements (EOMs), marking a significant milestone for quantum technologies.

The team’s work combines years of research in quantum optics, semiconductor physics, and photonic engineering to open the door for next-generation quantum computers and communications. Here’s what you need to know.

Photonic Quantum Computing

Photonic quantum computing utilizes light photons to achieve a new level in computational performance. These systems function by transferring photons through purpose-built optical elements. These components enable the device to calculate algorithms much faster than other quantum computing approaches.

Notably, the photonics firm Xanadu announced Aurora on January 22, 2025—a modular, networked photonic prototype built from four rack-mounted modules linked over fiber (12 physical qubits, 35 chips, ~13 km of fiber). It demonstrates a practical path to scalability, not fault tolerance.

Despite its better performance, Aurora is still subject to many of the traditional roadblocks to quantum computing, including high costs, technical requirements, and specialized hardware.

Source – University of Innsbruck

Why Quantum Dots Are Ideal Single-Photon Sources

To overcome these obstacles, engineers have begun researching quantum dots. These semiconductors can provide deterministic multi-photon state generation, making them the ideal building blocks for qubits in quantum computing applications.

Notably, quantum dots are nanostructures. They often measure under 10 nanometers in size, which is around 1/10,000 the size of a human hair. These microscopic units can have crucial characteristics such as the ability to emit single photons on demand, which makes them ideal for quantum applications.

To excite the quantum dots, engineers utilize an optical excitation method driven by ultra-fast active polarization-switching modules.  This action produces the multi-photon state that makes these units core components of today’s most advanced medical imaging devices, microscopy, next-gen solar cells, LEDs, lasers, quantum computers, and more.

Why Active EOM Demultiplexing Limits QD Scalability

One of the biggest problems with quantum dots today is that each dot is unique, emitting a slightly different color. As such, the use of high-tech modules is needed to ensure multi-photon generation. These devices are complicated and expensive. Additionally, they require specialized engineers to operate.

Another limiting factor is the hardware performance of these switches. To date, all switches have been limited by their physical design characteristics. The devices have max switching speeds that can slow over time as the hardware ages. These scenarios led to unwanted performance losses and inefficiencies.

New Study: Passive Demultiplexed Two-Photon States from a Single QD

University of Innsbruck scientists worked with a team of engineers from across the globe to complete the “Passive demultiplexed two-photon state generation from a quantum dot” study¹ published in the scientific journal npj Quantum Information this month.

The paper describes a passive quantum dot demultiplexing technique that offers improved performance and stability compared to its predecessors. Their work demonstrates a more stable, scalable, and efficient method to produce high-performance quantum dots, which could power tomorrow’s most advanced technologies.

Stimulated Two-Photon Excitation (sTPE): The Core Idea

At the core of the work is an optical technique called stimulated two-photon excitation.  This process enables the researchers to dictate to the quantum dot exactly how to emit light and when, enabling excitation limited only by the life of the quantum dot and not hardware performance.

As part of the excitation process, engineers precisely tuned laser pulses to generate photon streams in different polarization states. Notably, this method eliminates the need for extra hardware like active switching components.

Notably, this approach creates excitation directly from a quantum dot without requiring any active switching components. The new approach combines laser pulse shaping, polarization-tailored pulse-pair generation, and a cryogenic microscope to eliminate many of the technical roadblocks hindering past quantum dot excitation attempts.

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Inside the Setup: Wavelengths, Delays, and Polarization Control

The first step in the process is to isolate the quantum dots. From there, a femtosecond Ti:Sapphire laser is applied in pulses at a resonance of 780.3 nm. Two 4f pulse shapers begin spectral shaping of quantum dots, driving them into a biexciton state and triggering photon emissions.

As part of the process, a polarizing beam splitter (PBS) works in tandem with the purpose-built pulse-pair generator to create both H and V-polarized photons. Notably, engineers utilized a fiber optic delay, detectors, and interferometers to ensure the time differences between the states were accurately tracked.

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Experimental Results: Purity, HBT, and Two-Photon Output

To test their concept, the team started by creating quantum dots. The paper describes how a sample of GaAs/AlGaAs quantum dots was grown using molecular beam epitaxy with local droplet etching as the first part of their testing phase.

From there, the engineers conducted several tests to measure the quantum dots’ performance and quality, including registering the single photon purity of the exciton emissions. These tests were conducted using a fiber beam splitter and sensitive scanning devices.

The scientists made exact notes describing the quality of the two independent, orthogonally polarized single photons obtained. This step required the team to measure the second-order correlation function of the dots.

To accomplish this task, the engineers decided to utilize a Hanbury Brown and Twiss (HBT) setup. This test was specifically designed to study the photon statistics of various light sources.

The dual state quantum dots test results show promise for the technology. The team set the stimulation pulse to arrive ~6 ps after the TPE pulse and showed that the switching rate is limited by the exciton lifetime rather than the EOM hardware. This result confirmed the engineers’ belief that their new method was only constrained by quantum dot life span as opposed to hardware switching capabilities.

Why It Matters for QKD and Multi-Photon Interference

There are many benefits that this study brings to the market. For one, it opens the door for a deeper understanding of quantum computing and its capabilities. This discovery could help drive further innovations in quantum optics, semiconductor physics, and photonic engineering.

Faster: GHz-Rate Operation Limited by Exciton Lifetime

This approach creates usable quantum dots faster than traditional methods that require special switching equipment to be customized and fine-tuned. This strategy directly excites the quantum dots utilizing lasers rather than electronics.

Cheaper: No EOMs, Lower Loss & Complexity

The elimination of previous switching equipment also reduces the overall costs of creating quantum dots. Developing, operating, and maintaining these devices all added to the overall costs of quantum computer research.

Applications (QKD, Photonic QC) and Realistic Timeline

There are several applications for dual-state quantum computer dots today. These units represent a leap forward for quantum technologies. Keenly, this process could help drive innovation across several industries and help push the adoption of this game-changing tech.

Research: Multi-Photon Interference & Source Benchmarking

One of the key applications for this study is that it will drive innovation in the quantum computing sector forward. This latest development will help to power future experiments into multi-photon interference, deepening scientists’ understanding of the fundamental principles that drive quantum mechanics.

Secure Communications: Multi-Party QKD & Networking

Another major application for this technology is in the communications sector. Quantum communications promise to offer high security, massive data transmission capabilities, and lower costs. These ultra-secure communication networks can provide near real-time communication to multiple sources simultaneously.

Adoption Timeline: From Lab Demos to Field Pilots

It could be around 10 years before this technology makes its way to the general public. Quantum computers are seen by many as the natural evolution of computing. However, they are still very expensive and require hardware like cryogenic chambers, which remain out of the budget for most people.

Dual State Quantum Dots Researchers

The Dual State Quantum Dots study was hosted by the Photonics Group at the Department of Experimental Physics of the University of Innsbruck. The research also lists participants from the University of Cambridge, Johannes Kepler University Linz, and other institutions.

Specifically, Vikas Remesh was the study’s lead researcher. He received additional assistance from Gregor Weihs, Yusuf Karli, and Iker Avila Arenas. Funding for the study came from the Austrian Science Fund (FWF), the Austrian Research Promotion Agency (FFG), and the European Union’s research programs.

What’s Next: Engineered Dots with Arbitrary Linear Polarization

The future of dual-state quantum dots couldn’t be brighter. This technology has seen accelerated research following the successful launch of the Aurora computer this year. Now, this team will seek to create high-performance engineered dots with arbitrary linear polarization states.

Investing in Quantum Computing

There are several industry leaders in the quantum computers sector. These companies continue to pour millions into research and development, hoping to make the technology more accessible and capable moving forward. Here’s one company that continues to innovate in new and exciting ways.

Quantum Computing Inc

Virginia-based Quantum Computing Inc. (QUBT +0.05%) entered the market in 2018 with the goal of driving quantum technologies and adoption forward. The company found immediate support from investors, and in 2021, it listed on NASDAQ. Today, the firm is a leading provider of quantum computing hardware and software.

Quantum Computing Inc. currently offers several quantum computing solutions. For example, the Qatalyst system can complete complex financial and logistical algorithms, providing a competitive edge to users. The company also offers access to its Reservoir Photonic Computer, which is capable of solving the most complex equations using light photons.

Latest Quantum Computing Inc. (QUBT) Stock News and Developments

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Dual State Quantum Dots  Conclusion

The dual state quantum dot study highlights how a collaborative effort between leading scientific institutions can help to solve some of the world’s most advanced problems. Quantum computing is set to reshape the future in many ways, and these engineers have done their part to help ensure these devices reach new performance levels. For these reasons and many more, these scientist deserve a salute.

Learn about other Computing Breakthroughs here.

References:

^{1. Karli, Y., Avila Arenas, I., Schimpf, C. et al. Passive demultiplexed two-photon state generation from a quantum dot. npj Quantum Inf 11, 139 (2025). https://doi.org/10.1038/s41534-025-01083-0}