Deterministic, High-Fidelity, Two-Bit Quantum Logic Gate

Lacking Piece Found within the Puzzle of Optical Quantum Computing


Deterministic, High-Fidelity, Two-Bit Quantum Logic Gate

Jung-Tsung Shen, affiliate professor within the Division of Electrical & Methods Engineering, has developed a deterministic, high-fidelity, two-bit quantum logic gate that takes benefit of a brand new type of mild. 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 Faculty of Engineering at Washington College in St. Louis has discovered a lacking piece within the puzzle of optical quantum computing. 

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

“Within the ideally suited case, the constancy will be as excessive as 97%,” Shen stated.

His analysis was printed in Could 2021 within the journal Bodily Assessment A.

The potential of quantum computer systems is certain to the weird properties of superposition —  the power of a quantum system to include many distinct properties, or states, on the similar time — and entanglement — two particles appearing 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 laptop, researchers typically use particular person electrons as “qubits,” the quantum equal. Electrons have a number of traits that go well with them properly to the duty: they’re simply manipulated by an electrical or magnetic discipline they usually work together with one another. Interplay is a profit once 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 thing from stray magnetic fields to energy strains can affect electrons, making them onerous to actually management.  

For the previous twenty years, nonetheless, some scientists have been attempting to make use of photons as qubits as an alternative of electrons. “If computer systems are going to have a real affect, we have to look into creating the platform utilizing mild,” Shen stated.

Photons haven’t any cost, which might result in the other 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 2 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 will not be troublesome, however it’s stuffed with surprises,” Shen stated.

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 grew to become entangled. Researchers have since been extra profitable, however, Shen stated, “It’s nonetheless not ok for a pc,” which has to hold out thousands and thousands to billions of operations per second. 

Shen was in a position to construct a two-bit quantum logic gate with such effectivity due to the invention 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 purposes for this new type of mild.

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 2 could make a brand new state, photonic dimers. It seems this new state is essential.”

Jung-Tsung Shen Designed Two-Bit Quantum Logic Gate

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

Mathematically, there are various methods to design a logic gate for two-bit operations. These completely different designs are known as equal. The precise logic gate that Shen and his analysis group designed is the controlled-phase gate (or controlled-Z gate). The principal operate of the controlled-phase gate is that the 2 photons that come out are within the unfavorable state of the 2 photons that went in.

“In classical circuits, there isn’t a minus signal,” Shen stated. “However in quantum computing, it seems the minus signal exists and is essential.” 

“Quantum mechanics will not be troublesome, however it’s stuffed with surprises.”

Jung-Tsung Shen

When two impartial photons (representing two optical qubits) enter the logic gate, “The design of the logic gate is such that the 2 photons can kind a photonic dimer,” Shen stated. “It seems the brand new quantum photonic state is essential because it allows the output state to have the proper 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 beneath average circumstances. Up to now, he says, outcomes appear constructive.

Shen says this end result, whereas baffling to most, is obvious as day to these within the know. 

“It’s like a puzzle,” he stated. “It might be sophisticated to do, however as soon as it’s accomplished, simply by glancing at it, you’ll realize 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 Could 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 Faculty of Engineering at Washington College in St. Louis promotes impartial inquiry and training 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 vital selective undergraduate packages within the nation. With 140 full-time college, 1,387 undergraduate college students, 1,448 graduate college students and 21,000 residing alumni, we’re working to unravel a few of society’s best challenges; to organize college students to grow to be leaders and innovate all through their careers; and to be a catalyst of financial improvement for the St. Louis area and past. 

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