Jung-Tsung Shen is an associate professor in the Department of Electrical & Systems Engineering. He has created a deterministic and high-fidelity two-bit quantum logic gates that exploits a new type of light. This new logic gate is orders-of-magnitude more efficient than current technology. Credit: Jung-Tsung ShenThe McKelvey School of Engineering at Washington University, St. Louis discovered a missing piece to the puzzle of optical quantum computing.Jung-Tsung Shen is an associate professor in the Department of Electrical & Systems Engineering. He has created a deterministic and high-fidelity, two-bit quantum logic gates that uses a new type of light. This new logic gate is orders-of-magnitude more efficient than current technology.Shen stated, "In an ideal case, the level of fidelity could be as high as 97%."His research was published by Physical Review A in May 2021.Quantum computers have the potential to be more than just computers.In a classical computer, voltage is what determines the value for a bit (a number between 1 and 0). Researchers often refer to individual electrons as "qubits" or the quantum equivalent. They are easy to manipulate by magnetic or electric fields and interact with one another. When two bits need to interact, it is called interaction. This allows quantum mechanics to manifest.Their propensity for interaction is also a problem. It is difficult to control electrons because everything, from power lines to stray magnetic field, can affect them.However, scientists have tried to use photons instead of electrons for the past 20 years. Shen stated that computers will have an impact only if they are made using light.Photons are free of charge. This can cause them to interact with their environment, but also with one another, as they don't have any charge. It was also difficult to engineer inter-photon interactions that were effective and efficient. So, traditional thinking was.Scientists working on this problem found that even though they were not entangled when they entered a logic gate they behaved as if they were. Another wild manifestation of quantum mechanics is the unique characteristics of measurement.Shen stated, "Quantum mechanics can be very simple, but it is full of surprises."Although the measurement discovery was revolutionary, it wasn't game-changing. This is because only one pair of photons can become entangled for every 1,000,000. Although researchers have been more successful in the past, Shen stated that it is still not enough for computers to perform millions to billions operations per second.Shen was able build a quantum logic gate of two bits with such efficiency thanks to the discovery of new quantum photonic statesphotonic dimers. These are photons that are entangled in space and frequency. He was able to prove their existence experimentally in 2013 and has been looking for applications for this new type of light ever since.Nothing happens when a single photon enters the logic gate. It just goes in and out. Two photons can enter a logic gate and nothing happens. But, when they are combined, it is when we predict that the two will create a new state called photonic dimers. This new state turns out to be crucial."Mathematically, there is a variety of ways to design logic gates for two-bit operations. These designs are known as equivalent. Shen and his research team designed the controlled-phase gate, also known as controlled-Z gates. The controlled-phase gate's main function is to ensure that the photons that come out of it are in the same negative state as the photons that came in.Shen stated, "In classical circuits there is no negative sign." "But quantum computing turns out that the minus sign is important and exists."Shen stated that two independent photons, representing two optical qubits, enter the logic gates. "The logic gate design is such that the two photos can form a photonic dimer." The new quantum photonic state, which allows the output state to have an essential sign for optical logic operations, is critical.Shen is currently working with the University of Michigan in order to test his design. It's a solid-state logic gateone, capable of operating under moderate conditions. He says that so far, the results have been positive.Shen claims that this result is baffling to many, but clear to those who are in the know.He said, "It's almost like a puzzle." It may seem complicated, but once you've completed it, it will be obvious.Zihao Chen and colleagues, Two-photon controlled phase gates enabled by photonic diers, Physical Review A (2021). Information from the Journal: Physical Review A Zihao Chen and colleagues, Two-photon controlled phase gates enabled by photonic diers, (2021). DOI: 10.103/PhysRevA.103.052610.
