Photonic Quantum Computing Advances

Purdue University electrical and computer engineering professors Alexandra Boltasseva, left, and Vladimir Shalaev are conducting research that could help scale up quantum photonic integrated circuitry, which is the fundamental technology of quantum computers based on photons.
Photo Credit: Purdue University/Kevin Crisp

Scientific Frontline: Extended "At a Glance" Summary: Quantum Photonic Integrated Circuits

The Core Concept: Quantum photonic integrated circuits utilize single photons as quantum bits, or qubits, to process information. This approach enables computers to manipulate data simultaneously via superposition while transmitting it through circuit gates at the speed of light.

Key Distinction/Mechanism: Unlike other qubit architectures that require highly energy-intensive super cold temperatures to operate, photon-based qubits are resilient against heat and environmental disturbances, allowing them to remain stable at room temperature.

Origin/History: Engineers Alexandra Boltasseva and Vladimir Shalaev at Purdue University's Elmore Family School of Electrical and Computer Engineering recently advanced this technology by developing solutions to create reliable single-photon sources for large-scale circuits.

Major Frameworks/Components:

• Single-photon sources for scalable qubit generation.
• Quantum superposition for simultaneous data processing.
• Room-temperature stability reducing energy consumption.
• Photonic qubit scaling for practical calculations.

Branches of Science: Quantum Photonics, Optoelectronics, Quantum Computing, Quantum Information Science.

Future Application: Developing highly scalable, room-temperature quantum computers capable of solving complex problems exponentially faster than traditional supercomputers without requiring massive cooling infrastructure.

Why It Matters: Overcoming the barrier of reliable single-photon generation brings the industry significantly closer to realizing photon-based quantum processors, which promise unprecedented computational speed and energy efficiency.

Purdue professor Vladimir Shalaev works in the lab where his research group has demonstrated new methods for generating single photons and manipulating them on a chip.
Photo Credit: Purdue University/Kevin Crisp

Quantum computers have the potential to perform calculations more quickly than supercomputers, but they could be even faster and more energy-efficient if they were built with photons as their basic units, called qubits.

Challenges remain, however, in fabricating the chips that allow photons to function as qubits in circuits. Purdue University engineers have developed potential solutions to several major issues with building these chips, known as quantum photonic integrated circuits, on a large scale.

A qubit is a quantum version of a "bit" that a conventional computer uses to process information as zeros and ones. Whereas bits can only take the state of zero or one at a time, a qubit represents both zero and one simultaneously through a phenomenon called superposition, enabling the evaluation of multiple states simultaneously and accelerating complex calculations.

Photons, the smallest possible particles of light, have several advantages over other quantum particles that could be used for building qubits. For example, individual photons would enable qubits to move through the gates of a circuit at the speed of light, transferring data faster and minimizing delays in algorithm performance. Since photons aren’t easily disturbed by heat or other changes in their surroundings, they can also offer qubits stability at room temperature. Other types of qubits require cryogenic temperatures to operate, consuming significantly more energy.

Quantum photonic integrated circuits can work with a small number of qubits, but to perform practical calculations, quantum computers would need hundreds if not thousands of qubits, a scale that is not yet possible with these chips.

"Single photons are the building blocks of quantum photonics. The biggest challenges in scaling up a quantum photonic integrated circuit have to do with creating good single-photon sources," said Vladimir Shalaev, the Bob and Anne Burnett Distinguished Professor in Electrical and Computer Engineering.

Shalaev and Alexandra Boltasseva lead work in Purdue’s Elmore Family School of Electrical and Computer Engineering, Birck Nanotechnology Center, and the Purdue Quantum Science and Engineering Institute on key challenges with building quantum photonic integrated circuits. Their past discoveries on how to manipulate light on the nanoscale have been foundational to the fields of photonics and optics. For multiple years, they have each been recognized on the Clarivate Highly Cited Researchers list for work ranking in the top 1% of citations in the Web of Science.

Boltasseva, the Ron and Dotty Garvin Tonjes Distinguished Professor in Electrical and Computer Engineering, also was featured on last year’s Quantum 100 list as part of the United Nations-designated International Year of Quantum Science and Technology.

Using this experimental setup, Purdue researchers discovered how to create a single-photon switch, which a quantum photonic integrated circuit would need to process information. The researchers shot two lasers through the lens and pinhole of a spatial filter on the left side of this setup to a commercial device called a single-photon avalanche diode, seen on the far right with a green line on its surface. This device allowed a single photon from one light beam to switch on and off another more powerful light beam.
Photo Credit: Purdue University/Kevin Crisp

A more reliable way to produce single photons

One of the difficulties with building a quantum photonic integrated circuit is generating one photon at a time to take the form of a qubit.

In 2021, Shalaev and Boltasseva published a study in Science Advances demonstrating a way to emit single photons using silicon nitride, a material that has become the industry standard for constructing quantum photonic platforms.

But the process the industry has been using for photon emission, called spontaneous parametric down-conversion, generates single photons inconsistently and with low efficiency.

The method Shalaev and Boltasseva’s team developed emits exactly one photon at room temperature from dozens of locations on a silicon nitride platform every time a laser beam shoots through it.

The sources of photon emission are modifications to atomic structure, called "defects," that the researchers created within the platform. These atomic defects limit light emission to a single photon at a time.

Shalaev and Boltasseva have also since shown in a study presented at the 2022 Conference on Lasers and Electro-Optics that they can control the placement and number of defects by embedding them in nanosized pillars on the silicon nitride platform, which would allow the platform to be manufactured more uniformly for use in a quantum photonic integrated circuit.

A single-photon switch

Another issue Shalaev and Boltasseva are tackling is that single photons don’t readily interact with each other, making them hard to control on a chip using switches.

In a conventional computer, switches called transistors toggle bits between zero and one states to process information. Similarly, switches in a quantum photonic integrated circuit would enable photons to act as qubits.

Shalaev and Boltasseva’s research team reported in a Nature Nanotechnology paper last year that they built a switch capable of operating on the level of single photons.

The proof-of-concept switch adapts a mechanism that the industry has used to modulate light with another high-intensity light beam for applications such as fiber-optic communications. This mechanism, called optical nonlinearity, allows a "control" light beam to interact with a "probe" beam but at power levels that would be too high to work with single photons.

With the Purdue-developed method, a single photon from the control beam strikes silicon, creating an electron that then triggers an avalanche of electrons. This avalanche alters the material's refractive index, a property that controls light propagation. By changing silicon’s refractive index, the probe beam passes through the device only when a single photon initiates the electron avalanche process, effectively allowing the photon to switch the beam on and off.

A tool for predicting quantum decoherence

Even though qubits based on photons would be less affected by their surroundings, a single disturbance could still cause qubits to fall out of superposition or entanglement, creating errors in a quantum computer’s calculations.

This kind of disturbance is called decoherence. Purdue electrical and computer engineering professor Alexander Kildishev and Purdue graduate student Pranshu Maan led a team that included Shalaev and Boltasseva in demonstrating an approach that could make it possible to prevent decoherence by anticipating it before it happens. The team published findings in Nature Communications this year.

"Decoherence is seemingly random, but in fact there is a certain pattern that can be recognized. This study takes an important step in identifying such a pattern using machine learning," Boltasseva said.

The team found through statistical-physics analysis that the way decoherence happens over time presents a pattern. They then used machine learning to validate that these patterns have information that could be predicted. The researchers turned these components into a framework that learns and could anticipate the dynamics of a quantum system, which would then enable proactive control of decoherence rather than only responding to it after it occurs.

With this framework, the researchers demonstrated a way to predict and reduce one of the major causes of decoherence, called spectral diffusion. This phenomenon occurs when an emitted photon shifts across the electromagnetic spectrum due to disorder that naturally arises in the surrounding environment, changing the photon’s frequency and therefore its energy level. Because superposition only works if all photons in a quantum system are indistinguishable from one another, a change in a single photon’s energy level would cause superposition to fall apart.

The researchers discovered that spectral shifts retain a memory of how an emitted photon’s frequency has wandered over time, revealing a predictable temporal pattern. The team developed an algorithm with a built-in memory mechanism and trained it on data from individual quantum emitters to predict future spectral shifts. Using these predictions, they showed that an anticipatory-control approach could reduce spectral shifts by as much as 20-fold in the quantum systems studied.

The ability to predict decoherence could significantly improve quantum computational performance.

"You can’t get rid of decoherence—it’s the law of nature that everything fluctuates. But if you recognize the pattern of decoherence and learn how to control it, then you can minimize it," Shalaev said. "Let’s say you need a hundred qubits to run a quantum algorithm, but typically far more than that, say 10,000 qubits, to deal with errors caused by decoherence. If I minimize those errors somehow, then the number of qubits needed to run my algorithm would also be minimized."

In principle, this approach could lay the foundation for making it possible to anticipate decoherence across multiple quantum systems, which is needed for building the scalable quantum architecture that quantum computers would use. Those predictions could then be combined with active tuning to keep different emitters more closely aligned, improving the indistinguishability and synchronization of the photons they generate.

Next, the team plans to use tools from statistical physics to extract the hidden properties of a quantum system’s fluctuating environment from its measured dynamics. These physical insights will be incorporated into machine-learning models to improve their ability to predict decoherence. The team will also investigate whether the models can identify the microscopic processes responsible for spectral shifts, with the long-term goal of predicting and suppressing decoherence before it significantly disrupts the quantum state.

Previous research: 

Room-temperature single-photon emitters in silicon nitride (Science Advances 2021)

All-optical modulation with single photons using an electron avalanche (Nature Nanotechnology 2025)

Funding: The researchers’ work on the single-photon emitter and the algorithm for predicting decoherence was supported by the U.S. Department of Energy’s (DOE) Quantum Science Center, a National Quantum Information Science Research Center under the DOE’s Office of Science. The algorithm’s development also received support from the Purdue-led Quantum Photonic Integrated Design Center, an Energy Frontier Research Center funded by the DOE’s Office of Science through the Basic Energy Sciences program (award number DE-SC0025620). The team’s work on the single-photon switch was funded by the U.S. Army Research Laboratory’s Army Contracting Command—Aberdeen Proving Ground, Research Triangle Park division (grant number W911NF-25-2-0060).

Disclosure: Shalaev and Boltasseva’s work related to quantum photonic integrated circuits is based on multiple technologies disclosed to the Purdue Innovates Office of Technology Commercialization, which applied for patents to protect the intellectual property.

Published in journal: Nature Communications

Title: Anticipating decoherence in quantum systems

Authors: Pranshu Maan, Yuheng Chen, Sean Borneman, Benjamin Lawrie, Alexander Puretzky, Hadiseh Alaeian, Alexandra Boltasseva, Vladimir M. Shalaev, and Alexander V. Kildishev

Source/Credit: Purdue University | Kayla Albert

Edited by: Scientific Frontline

Reference Number: qs111226_01

Privacy Policy | Terms of Service | Contact Us