The Puzzle of Optical Quantum Computers: A missing piece

It was impossible to create a two-bit efficient quantum logic gate.

The McKelvey School of Engineering at Washington University has discovered a missing piece of the optical quant computing puzzle.

Jung-Tsung Shen is a Preston M. Green Department Electrical & Systems Engineering associate professor. He has created deterministic and high-fidelity, two-bit quantum logic gates that use a new type of light. This new logic gate is orders of magnitude more efficient than current technology.

Superposition allows a quantum system or state to have many different properties at once, and Entanglement, which is when two particles act as if they are correlated in an unusual way despite being physically separated from one another, are the unique properties that make quantum computers possible.

In a classical computer, voltage determines the value of a bit (a number between 1 and 0). Researchers often call individual electrons “qubits,” the quantum equivalent. The many characteristics of electrons make them well-suited for the task. They can be easily controlled by either an electric or magnetic field, and they interact with one another. Interaction is beneficial when two bits need to be entangled, allowing quantum mechanics to manifest.

Their propensity for interaction is also a problem. It is challenging to control electrons because everything, from power lines to stray magnetic fields, can affect them.

However, scientists have tried to use photons instead of electrons for the past 20 years. Shen stated that computers would have an impact only if they were made using light.

Photons are free of charge. This can cause them to interact with their environment and one another, as they don’t have any control. It also took a lot of work 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 elementary, but it is full of surprises.”

Although the measurement discovery was revolutionary, it wasn’t game-changing. 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 more is needed for computers to perform millions to billions of operations per second.

Shen built a quantum logic gate of two bits with such efficiency thanks to discovering new classes of quantum photonic states, photonic dimers. These are photons that are entangled in space and frequency. He proved 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 simply goes in and out. Two photons can enter a logic gate, and nothing happens. But, when they are combined, it is “when we predicted that the two could create a new state, photonic dimers.” This new state is crucial.

Jung-Tsung Shen designed the high-fidelity, 2-bit logic gate. Credit: Jung-Tsung Shen

Mathematically, there are various ways to design logic gates for two-bit operations. These designs are called equivalents. Shen and his research team created the controlled-phase entrance, controlled-Z gates. The controlled-phase gate’s primary function is to ensure that the photons that come out of it are in the same negative state as those that came in.

Shen stated, “There is no negative sign in classical circuits.” “But quantum computing shows 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 gives the output state a vital sign for optical logic operations, is critical.

Shen has worked closely with the University of Michigan to test his solid-state logic gate design. It can operate under moderate conditions. He says that so far, the results have been positive.

Shen claims that this result is baffling to many but apparent to those in the know.

He said, “It’s almost like a puzzle.” It may not seem easy, but it will be evident once completed.

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