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Missing Piece Discovered in the Puzzle of Optical Quantum Computing

Jung-Tsung Shen, affiliate professor in the Division of Electrical & Techniques Engineering, has developed a deterministic, high-fidelity, two-bit quantum logic gate that takes benefit of a brand new type of gentle. This new logic gate is orders of magnitude extra environment friendly than the present expertise. Credit score: Jung-Tsung Shen

An environment friendly two-bit quantum logic gate has been out of attain, till now.

Analysis from the McKelvey College of Engineering at Washington College in St. Louis has discovered a lacking piece in the puzzle of optical quantum computing. 

Jung-Tsung Shen, affiliate professor in the Preston M. Inexperienced Division of Electrical & Techniques Engineering, has developed a deterministic, high-fidelity two-bit quantum logic gate that takes benefit of a brand new type of gentle. This new logic gate is orders of magnitude extra environment friendly than the present expertise.  

“In the superb case, the constancy may be as excessive as 97%,” Shen mentioned.

His analysis was revealed in Might 2021 in the journal Bodily Assessment A.

The potential of quantum computer systems is sure to the uncommon properties of superposition —  the means of a quantum system to comprise many distinct properties, or states, at the identical time — and entanglement — two particles performing as if they’re correlated in a non-classical method, regardless of being bodily faraway from one another. 

The place voltage determines the worth of a bit (a 1 or a 0) in a classical pc, researchers usually use particular person electrons as “qubits,” the quantum equal. Electrons have a number of traits that swimsuit them nicely to the activity: they’re simply manipulated by an electrical or magnetic discipline and so they work together with one another. Interplay is a profit while you want two bits to be entangled — letting the wilderness of quantum mechanics manifest.  

However their propensity to work together can also be an issue. Every little thing from stray magnetic fields to energy traces can affect electrons, making them onerous to actually management.  

For the previous 20 years, nonetheless, some scientists have been making an attempt to make use of photons as qubits as a substitute of electrons. “If computer systems are going to have a real influence, we have to look into creating the platform utilizing gentle,” Shen mentioned.

Photons haven’t any cost, which might result in the reverse issues: they don’t work together with the setting like electrons, however additionally they don’t work together with one another. It has additionally been difficult to engineer and to create advert hoc (efficient) inter-photon interactions. Or so conventional pondering went. 

Lower than a decade in the past, scientists engaged on this drawback found that, even when they weren’t entangled as they entered a logic gate, the act of measuring the two photons once they exited led them to behave as if that they had been. The distinctive options of measurement are one other wild manifestation of quantum mechanics.

“Quantum mechanics just isn’t tough, but it surely’s full of surprises,” Shen mentioned.

The measurement discovery was groundbreaking, however not fairly game-changing. That’s as a result of for each 1,000,000 photons, just one pair turned entangled. Researchers have since been extra profitable, however, Shen mentioned, “It’s nonetheless not ok for a pc,” which has to hold out hundreds of thousands to billions of operations per second. 

Shen was capable of construct a two-bit quantum logic gate with such effectivity as a result of of the discovery of a brand new class of quantum photonic states — photonic dimers, photons entangled in each house and frequency. His prediction of their existence was experimentally validated in 2013, and he has since been discovering functions for this new type of gentle.

When a single photon enters a logic gate, nothing notable occurs — it goes in and comes out. However when there are two photons, “That’s once we predicted the two could make a brand new state, photonic dimers. It seems this new state is essential.”

Excessive-fidelity, two-bit logic gate, designed by Jung-Tsung Shen. Credit score: Jung-Tsung Shen

Mathematically, there are numerous methods to design a logic gate for two-bit operations. These completely different designs are referred to as equal. The particular logic gate that Shen and his analysis group designed is the controlled-phase gate (or controlled-Z gate). The principal perform of the controlled-phase gate is that the two photons that come out are in the unfavorable state of the two photons that went in.

“In classical circuits, there is no such thing as a minus signal,” Shen mentioned. “However in quantum computing, it seems the minus signal exists and is essential.” 

“Quantum mechanics just isn’t tough, but it surely’s full of surprises.”

Jung-Tsung Shen

When two unbiased photons (representing two optical qubits) enter the logic gate, “The design of the logic gate is such that the two photons can type a photonic dimer,” Shen mentioned. “It seems the new quantum photonic state is essential because it permits the output state to have the right signal that’s important to the optical logic operations.”

Shen has been working with the College of Michigan to check his design, which is a solid-state logic gate — one that may function below reasonable circumstances. To this point, he says, outcomes appear constructive.

Shen says this outcome, whereas baffling to most, is obvious as day to these in the know. 

“It’s like a puzzle,” he mentioned. “It might be difficult to do, however as soon as it’s completed, simply by glancing at it, you’ll understand it’s right.”

Reference: “Two-photon controlled-phase gates enabled by photonic dimers” by Zihao Chen, Yao Zhou, Jung-Tsung Shen, Pei-Cheng Ku and Duncan Metal, 21 Might 2021, Bodily Assessment A.
DOI: 10.1103/PhysRevA.103.052610

This analysis was supported by the Nationwide Science Basis, ECCS grants nos. 1608049 and 1838996. It was additionally supported by the 2018 NSF Quantum Leap (RAISE) Award.

The McKelvey College of Engineering at Washington College in St. Louis promotes unbiased inquiry and schooling with an emphasis on scientific excellence, innovation and collaboration with out boundaries. McKelvey Engineering has top-ranked analysis and graduate packages throughout departments, notably in biomedical engineering, environmental engineering and computing, and has one of the most selective undergraduate packages in the nation. With 140 full-time school, 1,387 undergraduate college students, 1,448 graduate college students and 21,000 residing alumni, we’re working to resolve some of society’s best challenges; to arrange college students to change into leaders and innovate all through their careers; and to be a catalyst of financial growth for the St. Louis area and past.

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