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A Tango With Quantum Entanglement

Scientists have created a ‘universal entangler’ which not only could advance our understanding of the quantum realm, but also pave the way for the construction of quantum computers. Read more from Asian Scientist Magazine at:

AsianScientist (Mar. 22, 2019) – One of the most iconic and intriguing concepts in quantum physics is “entanglement,” a phenomenon where two or more quantum systems become so inextricably linked such that we can no longer learn about their collective state by simply observing each element individually. Einstein, who never really accepted the existence of entanglement, famously nicknamed it “spooky action at a distance.”

Thanks to both the theoretical and experimental developments in quantum physics over the past two decades, scientists have become increasingly proficient in taming this “spooky action” in their laboratories. In a new study recently published in Nature, my team and I have taken the art of entanglement one step further by developing a ‘universal entangler’ that allows any configurations of microwave light to be completely intertwined at will.

This powerful new mechanism is called the exponential-SWAP gate. It is capable of simultaneously instigating an exchange operation between two quantum elements and doing nothing at all to them, leading to the two elements being entangled regardless of their exact configurations. To demonstrate this ‘universal entangler,’ we built a device consisting of two 3D superconducting microwave cavities, which are tightly-sealed boxes that can coherently store complex quantum states of confined microwave light.

Unboxing physical phenomena
The states contained in each superconducting box are well-isolated from the each other during idle times and can become intertwined on demand by enacting the exponential-SWAP gate. This operation sets itself apart from the previously implemented mechanisms to create entanglement in two crucial aspects: it is deterministic—the outcome of entanglement is guaranteed each time this operation is activated, and it is universal—the operation is compatible with any desired configurations of light contained in each superconducting box.

The ability to deterministically create entanglement between such quantum states of light has far-reaching impacts on our understanding of quantum physics. We often think of quantum mechanics as the laws that govern the extremely small particles. Indeed, quantum mechanical effects such as superposition and entanglement are typically only observed between single excitations at the microscopic level, such as an atom or an electron.

However, scientists like myself have always been keen to find out whether it is possible to re-create these quantum effects on more massive objects. Our work on building a ‘universal entangler’ offers an exciting avenue to answer such a question. The operation we engineered is able to create these quantum effects between macroscopic systems, namely the collective states of microwave light consisting of a large number of excitations. This will provide valuable insight into the many riveting physics phenomena at the boundary of the quantum and classical world.

Aside from inciting our scientific curiosity, this ‘universal entangler’ also has profound technological implications. In particular, it is an invaluable tool for realizing robust quantum computation, a powerful new paradigm of information processing that harnesses the unique features of quantum mechanics. A robust quantum computer has the potential to completely transform our way of performing computation and tackle many classically intractable tasks such as molecular simulation for drug discoveries, cryptography, financial analysis, logistics optimization and big data search.

Making the quantum leap in computing
In order to fulfil these promises, a quantum computer must be able to carry out complex calculations faithfully. Unfortunately, the basic bits of quantum information, also called qubits, are highly delicate and prone to errors arising from minute environmental noise. In order for quantum computation to generate trustworthy results, we must instead compute with logical qubits—an encoded bit of quantum information that can be protected against errors.

One way to realize these protected qubits is via the bosonic codes, the use of clever configurations of microwave light confined in superconducting boxes, which have shown tremendous promise for error correction in a hardware-efficient manner. My team at Yale University, US, led by Professor Robert J. Schoelkopf, has been at the forefront of developing the necessary hardware components for implementing quantum computation using these systems.

Thanks to the guidance and support from Schoelkopf and our theory collaborators, we are able to tackle the technical challenges involved in realizing this powerful ‘universal entangler.’ More importantly, our work offers the flexibility of choosing any desired bosonic codeword for each logical qubit without having to worry about re-wiring the operation. This is a significant step towards making robust quantum computation a reality.

This research outcome is a culmination of years of technical expertise in my group, especially in engineering robust quantum systems and manipulating the interactions between them. I am heartened by its acceptance in Nature and look forward to the myriad of future experiments that it will spur. ——— Copyright: Asian Scientist Magazine; Photo: Shutterstock. Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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About the Author
Yvonne Gao Yvonne is an experimental physicist specializing in building robust hardware for quantum computation using microwave circuits. She received her B.A in Physics from the University of Oxford and PhD from Yale University. Upon returning to Singapore in 2018, she joined the A*STAR quantum computing initiative where she focuses on developing hybrid quantum systems. Outside the lab, Yvonne is also playing an active role in building the quantum community in Singapore through public talks, panels discussions and academic lectures. Read more from Asian Scientist Magazine at:

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