A survey by the School of Engineering *McKelvey* from Washington University in St. Louis, USA, found a missing piece in the puzzle of optical quantum computing: the two-bit quantum logic gate. The technology, much more efficient than currently used, was developed by Jung-Tsung Shen, a professor in the University’s Department of Electrical and Systems Engineering.

Shen has developed a deterministic, high-fidelity two-bit quantum logic gate that takes advantage of a new form of light. “In the ideal case, loyalty can be as high as 97%,” Shen said. The research was published in May 2021, in the scientific journal *Physical Review A*.

New logic gate is much more efficient than current technology.Source: Jung-Tsung Shen/Reproduction

## Understand quantum computers

The potential of quantum computers is linked to unusual properties of superposition – the ability of a quantum system to contain many distinct properties, or states, at the same time; and entanglement – two particles acting as if they were correlated despite being physically separate from each other.

While voltage determines the value of a bit (1 or 0) in a classic computer, researchers often use individual electrons as “qubits,” the quantum equivalent. Electrons have several characteristics that are well suited to the task: they are easily manipulated by an electric or magnetic field and they interact with each other. This interaction is a benefit when you need two bits to be tangled up – allowing quantum mechanics to manifest.

The problem with electrons is their propensity to interact with everything from lost magnetic fields to energy lines, which can make them very difficult to control. Over the past two decades, however, some scientists have tried to use photons as qubits instead of electrons. “If computers are going to have a real impact, we need to think about creating the platform using light,” explained Shen, in the article.

Photons have no charge, which can lead to opposite problems: they don’t interact with the environment like electrons, but neither do they interact with each other. The challenge was to design and create effective interphoton interactions – at least in traditional thinking.

## Innovation in quantum computing

Less than a decade ago, scientists discovered that even though they weren’t entangled when entering a logic gate, measuring the two photons as they exited caused them to behave as if they already existed. “Quantum mechanics isn’t difficult, but it’s full of surprises,” Shen said. The discovery of measurement was groundbreaking, but it had not yet transformed the technology, because for every million photons, only one pair was entangled.

Since then, researchers have had more success, but as Shen noted, “it’s still not good enough for a computer” – which needs to perform millions to billions of operations per second.

Shen was able to build a two-bit quantum logic gate with this efficiency due to the discovery of a new class of photonic quantum states: photonic dimers, photons entangled in space and frequency. The professor’s prediction of existence was experimentally validated in 2013 and since then he has found more applications for this new form of light.

When a single photon enters a logic gate, nothing notable happens – it goes in and out. But when there are two photons, “that’s when we predicted that the two could create a state, photonic dimers. It turns out that this new state is crucial”, explained the researcher.

## New Logical Gate Could Revolutionize Computing

Hi-fi two-bit logic gate model.Source: Jung-Tsung Shen/Reproduction

Mathematically, there are many ways to design a logic gate for two-bit operations – called equivalent designs. The logic gate designed by Shen and his group is the phase-controlled gate (or Z-controlled gate). Its main difference is that the two outgoing photons are in the negative state of the two incoming photons. “On classic circuits, there’s no minus sign,” Shen said. “But in quantum computing, it turns out that the minus sign exists and is crucial,” he amended.

When two independent photons (representing two optical qubits) enter the logic gate, “the design of the logic gate is such that the two photons can form a photonic dimer,” explained Shen. “It turns out that the new quantum photonic state is crucial, as it allows the output state to have the correct signal, being essential for optical logic operations”, he added.

The researcher is currently working with the University of Michigan on testing a solid-state logic gate that can operate in mild conditions. So far the results look positive. “It’s like a puzzle,” he said. “It can be tricky to do, but once it’s done, just by looking at it, you’ll know it’s correct,” Shen finished.