New measurements identifying quantities provide a glimpse into the quantum future

New measurements identifying quantities provide a glimpse into the quantum future

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

Using existing experimental and computational resources, a multi-institutional team has developed an efficient method for measuring high-dimensional qudits encoded in quantum frequency combs, a type of photon source, on a single optical chip.

Although the word “qudit” may sound like a typo, this lesser known cousin of qubit, or Quantum bitIt can carry more information and is more resistant to noise—both of which are key qualities needed to improve the performance of quantum networks, quantum key distribution systems, and eventually a quantum Internet.

Classical computer qubits classify data as ones or zeros, while qubits can hold values ​​of one, zero, or both — simultaneously — due to superposition, a phenomenon that allows multiple quantum states to exist at the same time. The letter “d” in a qudit stands for the number of different levels or values ​​that can be encoded on a photon. Traditional qubits have two levels, but adding more levels turns them into qudits.

Recently, researchers from the US Department of Energy’s Oak Ridge National Laboratory, Purdue University, and the Swiss Federal Institute of Technology in Lausanne, or EPFL, characterized an entangled pair of eight qudits, which made up 64 quantum space dimensions — four times the previous record of frequency modes. discrete. These results have been published in Nature Communications.

“We have always known that it is possible to encode 10, 20, or even higher levels using the colors of photons, or Optical frequenciesBut the problem is that measuring these particles is very difficult, said Hsuan-Hao Lu, a postdoctoral researcher at ORNL. “This is the value of this paper – we found an effective method and new technology It’s relatively easy to do on the experimental side.”

Qudits are difficult to measure when they are intertwined, which means that they share non-classical associations regardless of the physical distance between them. Despite these challenges, frequency container pairs—two groups in the form of photons entangled in their frequencies—are well suited to transmit quantum information because they can follow a specific path through Optical fiber without being significantly modified by their environment.

“We combined state-of-the-art frequency enclosures with state-of-the-art light sources, and then used our technology to describe high-dimensional kodate entanglement with a level of precision never seen before,” said Joseph Lukens, Wigner Fellow and Research Scientist at ORNL.

The researchers began their experiments by shining a laser into a micro-ring resonator — a circular device on a chip made by EPFL and designed to generate non-classical light. This powerful photon source occupies an area of ​​a square millimeter — comparable in size to the point of a sharp pencil — and allowed the team to generate pairs of frequency containers shaped like quantum frequency combs.

Qudit experiments usually require researchers to build a type of quantum circuit called a quantum gate. But in this case, the team used a photovoltaic phase modulator to mix different frequencies of light and a pulse modulator to modulate the phase of these frequencies. These technologies have been extensively studied at the Laboratory of Ultrafast Optics and Fiber Optic Communications led by Andrew Weiner in Purdue, where Low studied before joining ORNL.

These optical devices are common in telecom industryThe researchers randomly performed these operations 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 tangled together.

“This technology, which includes phase modulators and pulse modulators, is being pursued largely in the classical context of ultrafast and broadband optical signal processing, and has been extended to the quantum field of frequency scales,” Weiner said.

To work backwards and infer quantitative states that produced optimal frequency correlations for qudit applications, the researchers developed a data analysis tool based on a statistical method called Bayesian inference and ran computer simulations in ORNL. This achievement builds on previous team work that 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 over optical fibers, they aim to test quantum communication protocols such as teleportation, a method of transmitting quantum information, and entanglement swap, the process of entanglement of two previously unrelated particles.

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

“Now that we have a way to efficiently describe the entangled frequency efficiencies, we can perform other application-oriented experiments,” said Myleswamy.

Researchers are building a transistor-like gate for quantum information processing — using codes

more information:
Hsuan-Hao Lu et al, Bayesian tomography of high-dimensional frequency combs on a two-photon chip with randomized measurements, Nature Communications (2022). DOI: 10.1038 / s41467-022-31639-z

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