Hidden symmetry could be key to more robust quantum systems, researchers say

Researchers have found a way to protect very fragile quantum systems from noise, which could aid the design and development of new quantum devices, such as ultra-powerful quantum computers.

Researchers at the University of Cambridge have shown that microscopic particles can remain intrinsically bound, or entangled, over long distances even if there are random disturbances between them. Using the mathematics of quantum theory, they discovered a simple configuration where entangled particles can be prepared and stabilized even in the presence of noise by taking advantage of a symmetry hitherto unknown in quantum systems.

Their results, reported in the newspaper Physical examination letters, opens a new window to the mysterious quantum world that could revolutionize future technology by preserving quantum effects in noisy environments, which is the biggest obstacle to the development of such technology. Harnessing this capability will be at the heart of ultra-fast quantum computers.

Quantum systems are built on the particular behavior of particles at the atomic level and could revolutionize the way complex calculations are done. While a normal computer bit is an electrical switch that can be set to one or zero, a quantum bit, or qubit, can be set to one, zero, or both at the same time. Moreover, when two qubits are entangled, an operation on one immediately affects the other, regardless of their distance. This dual state is what gives a quantum computer its power. A computer built with entangled qubits instead of normal bits could perform calculations far beyond the capabilities of the most powerful supercomputers.

“However, qubits are extremely picky things and the slightest noise in their surroundings can cause their tangle to break apart,” said Dr Shovan Dutta of the Cavendish Laboratory in Cambridge, first author of the article. “Until we can find a way to make quantum systems more robust, their applications in the real world will be limited.”

Several companies, including IBM and Google, have developed working quantum computers, although so far these have been limited to less than 100 qubits. They require almost total isolation from noise, and even then have very short lifetimes of a few microseconds. The two companies plan to develop 1000-qubit quantum computers in the next few years, although if stability issues are not addressed, quantum computers cannot be used in practice.

Now Dutta and his co-author Professor Nigel Cooper have discovered a robust quantum system where multiple pairs of qubits remain entangled even with a lot of noise.

They modeled an atomic system in a lattice formation, where atoms strongly interact with each other, jumping from one lattice site to another. The authors found that if noise was added in the middle of the grating, it did not affect the entangled particles between the left and right sides. This surprising characteristic results from a special type of symmetry which preserves the number of these entangled pairs.

“We weren’t expecting this kind of stabilized entanglement at all,” Dutta said. “We came across this hidden symmetry, which is very rare in these noisy systems.”

They have shown that this hidden symmetry protects the entangled pairs and allows their number to be controlled from zero to a large maximum value. Similar conclusions can be applied to a wide class of physical systems and can be achieved with ingredients already existing in experimental platforms, paving the way for controllable entanglement in a noisy environment.

“Uncontrolled environmental disturbances are bad for the survival of quantum effects like entanglement, but a lot can be learned by deliberately working out specific types of disturbance and seeing how particles react,” Dutta said. “We have shown that a single form of perturbation can in fact produce – and preserve – many entangled pairs, which is a great incentive for experimental developments in this area.”

The researchers hope to confirm their theoretical findings with experiments during the next year.

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