Missing piece discovered in optical quantum computing puzzle

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Jung-Tsung Shen, associate professor of electrical systems engineering, has developed a deterministic and faithful 2-bit quantum logic gate that uses a new form of light. This new logic gate is several orders of magnitude more efficient than current technology. Credit: Jung-Tsung Shen

Until now, effective 2-bit quantum logic gates have been out of reach.

A study by the McKelby Institute of Technology at Washington University in St. Louis found a missing part of the photonic computing puzzle.

Jung-Tsung Shen, Associate Professor in the Department of Electrical Systems Engineering, Preston M. Green, has developed a deterministic and faithful 2-bit quantum logic gate that uses a new form of light. This new logic gate is several orders of magnitude more efficient than current technology.

“In the ideal case, loyalty can reach 97%,” says Shen.

His study was published in the journal in May 2021 Physical examination A..

The potential of quantum computers is related to the anomalous properties of superposition (the ability of a quantum system to contain several different properties or states at the same time) and entanglement (two particles that operate in an unclassical correlation). I am. Ways despite being physically separated from each other.

When a voltage determines the value of a bit (1 or 0) in a typical computer, researchers often use individual electrons as “qubits,” which are quantum equivalents. Electrons have several properties that make them suitable for tasks. Electrons are easily manipulated and interact with each other by electric or magnetic fields. Interactions are useful when you need to entangle two bits. In other words, it reveals the wild nature of quantum mechanics.

But their tendency to interact is also a problem. Everything from stray magnetic fields to power lines affects electrons, which can make it difficult to actually control them.

However, over the past two decades, some scientists have tried to use photons as qubits instead of electrons. “If computers have a real impact, we should consider using light to create a platform,” says Shen.

The opposite problem can occur because the photons are not charged. Photons don’t interact with the environment like electrons, but they don’t. It was also difficult to design and create ad hoc (efficient) photon interactions. Or so the traditional way of thinking is gone.

In less than a decade, scientists working on this question appeared to have done so by measuring two photons as they exited, without having to get tangled up when entering the logic gate. I discovered that I was behaving in it. A unique feature of measurement is another wild manifestation of quantum mechanics.

“Quantum mechanics is not difficult, but it is full of surprises,” Shen said.

The discovery of measurements was revolutionary, but not at all. Indeed, for 1,000,000 photons, only one pair was interlaced. Researchers have had more success since then, but Shen says, “it’s still not enough for computers.” Computers must perform millions to billions of operations per second.

With the discovery of a new class of quantum photonic states, photonic dimers intertwined in both space and frequency, Shen was able to construct 2-bit quantum logic gates with such efficiency. His predictions of their existence were tested experimentally in 2013, and since then he has found this new form of using light.

When a single photon enters a logic gate, nothing special happens: it goes in and out. But if you have two photons, “That’s when we predicted they might create a new state, the photonic dimer. We found this new state to be important.

A 2-bit fidelity logic gate designed by Jung-Tsung Shen. Credit: Jung-Tsung Shen

Mathematically, there are many ways to design logic gates for 2-bit operations. These different models are called equivalents. The specific logic gate designed by Shen and his research group is the Control Phase Gate (or Control Z Gate). The main function of the control phase gate is to be in the negative state of two photons containing the two photons coming out of it.

“On a classic track, there is no minus sign,” said Shen. “But in quantum computing, the minus sign exists and is important.”

“Quantum mechanics is not difficult, but it is full of surprises.”

– – Jung Tsun Sheng

When two independent photons (representing two bits of photons) enter the logic gate, “the design of the logic gate allows the two photons to form a photonic dimer,” Shen said. .. “The new quantum photonic state has proven to be very important because it allows the output state to have the correct sign, which is essential for optical logic operations.”

Shen worked with the University of Michigan to test its design as a solid state logic gate that could operate under moderate conditions. So far, he says, the results appear positive.

Shen says this result is confusing to most people, but apparent to those in the know.

“It’s like a puzzle,” he says. “It can be complicated to do, but once you’ve done it, a glance at it will tell you it’s right.”

Reference: May 21, 2021, “Two-photon control phase gate realized by a photonic dimer” by Zihao Chen, Yao Zhou, Jung-Tsung Shen, Pei-Cheng Ku, Duncan Steel Physical examination A..
DOI: 10.1103 / PhysRevA.103.052610

This study is supported by the National Science Foundation and is numbered by ECCS. 1608049 and 1838996. This was also supported by the 2018 NSF Quantum Leap (RAISE) Awards.

The McKelby Institute of Technology at Washington University in St. Louis promotes independent research and education with an emphasis on scientific excellence, innovation and collaboration without borders. McKelvey Engineering offers top-notch research and graduate programs across all departments, particularly in Biomedical Engineering, Environmental Engineering, and Computer Science, and has one of the most selective undergraduate programs in the world. country. With 140 full-time faculty, 1,387 undergraduates, 1,448 graduate students and 21,000 alumni, we are working to solve some of society’s greatest challenges. Students become leaders and prepare to innovate throughout their careers. Catalyze economic development in the Saint-Louis region and beyond.

Missing piece discovered in optical quantum computing puzzle Missing piece discovered in optical quantum computing puzzle

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