A team of researchers at the University of Waterloo have made a breakthrough in quantum computing that elegantly bypasses the fundamental "no cloning" problem.
Quantum computing is an exciting technological frontier, where information is stored and processed in tiny units - called qubits. Qubits can be stored, for example, in individual electrons, photons (particles of light), atoms, ions or tiny currents.
Universities, industry, and governments around the world are spending billions of dollars to perfect the technology for controlling these qubits so that they can be combined into large, reliable quantum computers. This technology will have powerful applications, including in cybersecurity, materials science, medical research and optimization.
"This breakthrough will enable quantum cloud storage, like a quantum Dropbox, a quantum Google Drive or a quantum STACKIT, that safely and securely stores the same quantum information on multiple servers, as a redundant and encrypted backup," said Dr. Achim Kempf, the Dieter Schwarz Chair in the Physics of Information and AI in the Department of Applied Mathematics at Waterloo. "It's an important step in enabling the buildup of quantum computing infrastructure."
"Quantum computing has tremendous potential, particularly for solving very hard problems, but it also poses unique challenges. One of the most challenging issues facing quantum computing is called the no-cloning theorem, which states that quantum information cannot be copied, at least not directly. This is because of the delicate way in which quantum information is stored."
Image created by Waterloo researcher Dr. Koji Yamaguchi that shows how encryption enables the first copying mechanism for quantum information. (University of Waterloo)
Kempf, who is also an associate at the Institute for Quantum Computing at Waterloo and an associate member of the Perimeter Institute, further explains that quantum information works a bit like splitting a password. If you have the first half of the password and a friend has the second half, neither of you can use it alone - but if you put your two halves together, you acquire the valuable password.
In a similar sense, qubits are special because they can share information in a way that grows as you combine them. A single qubit doesn't hold much on its own, but when qubits are linked together, they can store a huge amount of information that only appears when they're connected. This unique ability to hold shared information across multiple qubits is called quantum entanglement.
Kempf explains that 100 qubits can share information in 2^100 ways simultaneously. This allows them to share so much entangled information that all of today's classical computers could not store it.
For all the potential of quantum computing, however, the no-cloning theorem limits how it can be applied. This is because, unlike in classical computing, where the copying of information - for sharing and for backups - is a very commonly used tool, there is no simple copy and paste in quantum computing.
"We have found a workaround for the no-cloning theorem of quantum information," explains Dr. Koji Yamaguchi, who co-discovered the new method with Kempf while working as a post-doctoral researcher in Kempf's lab and who is now a research assistant professor at Kyushu University.
"It turns out that if we encrypt the quantum information as we copy it, we can make as many copies as we like. This method is able to bypass the no-cloning theorem because after one picks and decrypts one of the encrypted copies, the decryption key automatically expires, that is the decryption key is a one-time-use key. But even a one-time key enables important applications, such as redundant and encrypted quantum cloud services".
This breakthrough reinforces Waterloo's global leadership in quantum science and commercialization. The Institute for Quantum Computing is internationally recognized for pairing world-leading fundamental research with strong commercialization support - an approach that has already helped launch more than 23 quantum startups transforming sensing, security and computing.
The research, "Encrypted Qubits can be Cloned," appeared in Physical Review Letters.
