New measurements provide a glimpse into the quantum future

The micro-ring resonator, shown here as a closed loop, generated high-dimensional photon pairs. The researchers examined these photons by manipulating the phases of different frequencies, or colors, of light and mixing the frequencies, as shown by the crisscrossing multicolored lines. Credit: Yun-Yi Pai/ORNL, US Department. or Energy

A multi-institutional team has created an efficient method for measuring high-dimensional qudits encoded in quantum frequency combs, a kind of photon source, on a single optical chip using already available experimental and computational resources.

While the word “qudit” may seem like a typo, this lesser-known relative of the qubit, or quantum bit, has the ability to carry more data and is more resistant to noise, two key characteristics needed to improve the performance of quantum networks. of quantum key distribution systems and, finally, of the quantum internet.

Unlike traditional computer bits, which classify data as one or zero, qubits can contain values ​​of one, zero, or both. This is due to superposition, a phenomenon that allows several quantum states to exist simultaneously. The “d” of Qudit refers to the variety of levels or values ​​that can be encoded on a photon. Traditional qubits have only two levels, but as you add more levels, they become qudits.

Hsuan Hao Lu and Joseph Lukens

From left, Hsuan-Hao Lu and Joseph Lukens work in an ORNL quantum laboratory. Credit: Genevieve Martin/ORNL, US Dept. or Energy

Researchers from the Swiss Federal Institute of Technology in Lausanne, or EPFL, Purdue University, and the US Department of Energy’s Oak Ridge National Laboratory recently completed the characterization of an entangled pair of eight-level qudits that formed a 64-dimensional quantum space, quadrupling the previous record for discrete-frequency modes. Their findings were recently published in the journal

“We’ve always known that it’s possible to encode level 10 or 20 or even higher qudits using photon colors or optical frequencies, but the problem is that measuring these particles is very difficult,” said Hsuan-Hao Lu, a research associate. postdoctoral at ORNL. “That’s the value of this article: we have found an efficient and innovative technique that is relatively easy to implement experimentally.”

Qudits are even more difficult to measure when they are entangled, meaning they share non-classical correlations regardless of the physical distance between them. Despite these challenges, frequency-bin pairs—two photon-forming qudits that are trapped in their own frequencies—are well suited to carrying quantum information because they can follow a predetermined path through the optical fiber without being significantly changed by their environment.

“We combined state-of-the-art frequency bin manufacturing with state-of-the-art light sources, then used our technique to characterize high-dimensional qudit entanglement with a level of precision never shown before,” said Joseph Lukens, Wigner Fellow and researcher at ORNL.

The researchers began their experiments by shining a laser into a micro-ring resonator, a circular on-chip device manufactured by EPFL and designed to generate non-classical light. This powerful photon source takes up 1 square millimeter of space – about the size of the tip of a sharp pencil – and allowed the team to generate pairs of frequency bins in the form of quantum frequency combs.

Typically, qudit experiments require researchers to build a type of quantum circuit called a quantum gate. But in this case, the team used an electro-optical phase modulator to mix different frequencies of light and a pulse shaper to change the phase of these frequencies. These techniques are being studied extensively at the Ultrafast Optics and Optical Fiber Communications Laboratory led by Andrew Weiner at Purdue, where Lu studied before joining ORNL.

These optical devices are commonplace in the telecommunications industry, and researchers have performed these operations at random to capture many different frequency correlations. According to Lu, this process is like rolling a pair of six-sided dice and recording how many times each combination of numbers appears, but now the dice are intertwined with each other.

“This technique, involving phase modulators and pulse shapers, is being heavily pursued in the classical context for ultrafast, broadband photonic signal processing and has been extended to the quantum pathway of frequency qudits,” Weiner said.

To work backwards and infer which quantum states produced ideal frequency correlations for qudit applications, the researchers developed a data analysis tool based on a statistical method called Bayesian inference and ran computer simulations at ORNL. This result builds on the team’s previous work focused on performing Bayesian analyzes and reconstructing quantum states.

The researchers are now fine-tuning their measurement method to prepare for a series of experiments. By sending signals across optical fiber, they aim to test quantum communication protocols such as teleportation, which is a method of carrying quantum information, and entanglement exchange, which is the process of entangling two previously unrelated particles.

Karthik Myilswamy, a Purdue graduate student, plans to bring the micro-ring resonator to ORNL, which will allow the team to test these capabilities on the lab’s quantum local area network.

“Now that we have a method to efficiently characterize frequency-entangled qudits, we can perform other application-oriented experiments,” said Myilswamy.

Reference: “Bayesian tomography of high-dimensional on-chip biphoton frequency combs with randomized measurements” by Hsuan-Hao Lu, Karthik V. Myilswamy, Ryan S. Bennink, Suparna Seshadri, Mohammed S. Alshaykh, Junqiu Liu, Tobias J. Kippenberg , Daniel E. Leaird, Andrew M. Weiner, and Joseph M. Lukens, July 27, 2022, Nature communications.
DOI: 10.1038/s41467-022-31639-z

The study was funded by the US Department of Energy, the National Science Foundation, the Air Force Office of Scientific Research and the Swiss National Science Foundation.

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