18th September 2023

New record length for quantum coherence

A new record time for quantum coherence is reported, with a single-photon qubit encoded for 34 milliseconds. This is 55% longer than the previous record set in 2020.

In classical computing – such as the PC, smartphone, or other device you are currently using – information is processed with bits, which exist in a binary state of either a 0 or a 1. Quantum computing, by contrast, involves the processing of information with quantum bits, or qubits, which can exist in a "superposition" of both 0 and 1 simultaneously. This allows quantum computers to do certain types of calculations much faster than classical computers.

The total number of qubits available in these machines has increased steadily in recent years. With further scaling up, quantum computers have the potential for revolutionary applications in the future. IBM, for example, recently demonstrated a 433-qubit processor and is planning a 100,000-qubit quantum supercomputer by 2033. If successfully developed, these unimaginably vast calculating abilities will benefit many areas of science and technology. Faster and more accurate simulations of molecular interactions might accelerate the discovery of new drugs, chemicals, and materials. Great improvements in astronomy, climate forecasts, financial models and supply chain optimisations are also likely.

But while higher qubit numbers are crucial, an oft-overlooked metric for quantum computing is the coherence time. This can be thought of as the "lifespan" of a qubit's superposition state, during which it can perform useful calculations. In other words, coherence time is the window of opportunity within which a qubit remains stable and uncorrupted by external noise. The longer the coherence time, the more complex and accurate calculations a quantum computer can perform.

When scientists first began demonstrating quantum algorithms, the qubits had lifespans of mere nanoseconds. More than two decades on, the isolation of qubits from their surrounding environment has substantially improved, thanks to advances in materials and shielding techniques. However, there is still a long way to go before they can operate continuously and with negligible errors. As of 2020, the record duration for superposition stood at 22 milliseconds (ms), achieved at the University of Chicago.

Now, a team of researchers has extended that record by 55% and developed a system capable of 34 ms. In addition to maintaining the long-term trend of coherence increases, their work should improve several other aspects of qubit management. The study authors, from the Weizmann Institute of Science, Israel, have published a paper this month in the peer-reviewed journal PRX Quantum.

The scientists' innovation came from using a specialised design and materials (such as high-purity niobium) for the superconducting cavity holding the qubit (pictured below). Additionally, they fine-tuned the interaction between the cavity and another quantum element, known as a "transmon", which is essential for qubit manipulation.

This new arrangement can serve as a highly efficient "quantum memory" for complex computations and quantum networks, and can also be used to support "bosonic quantum error correction," a way to correct errors that may occur during calculations.

In their paper, the team also explain how they successfully created and measured a "Schrödinger cat state" with a total of 1,024 photons, an order of magnitude more than previous efforts. Schrödinger cat state – named after the famous thought experiment – is a quantum state where particles can exist in superposition (multiple states simultaneously), akin to being both alive or dead like the hypothetical cat. This result serves as a proof-of-concept that their cavity can hold more complex quantum information.

"Further improvements to the single-photon lifetime are within reach," they conclude. "For example, using a vacuum annealing step to remove surface oxides has been shown to enhance the quality factor of niobium cavities. These improved cavities can be coupled to superconducting qubits with longer coherence times, reducing the hybridisation-induced photon loss and potentially enabling superconducting cavity qubits with coherence times approaching one second."

If these even longer coherence times are achieved in the future, it could facilitate an in-depth study of subtle decoherence processes that are typically masked by photon loss, the team adds.

In summary, their new superconducting cavity offers a way to improve the stability of qubits, allowing more accurate and complex operations. This brings us a step closer to practical and powerful quantum computers.

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