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The role of frequency-shifting in quantum scalability

Friday, June 8, 2018

Physicists at IQC have developed new methods for preventing leakage errors due to cavity modes, an important obstacle in building a scalable quantum computer.

The Digital Quantum Matter (DQM) lab, led by researcher Matteo Mariantoni, studied two frequency-shifting techniques to prevent a quantum system’s own hardware from interfering with qubit operation.

“As a quantum system is scaled up and gains qubits, it becomes more challenging to maintain the qubit states,” said Thomas McConkey, PhD candidate and lead author on the study.  “And once a qubit system is large enough, its own hardware can cause information to leak out and the chip to stop functioning altogether.”

When a qubit chip is housed in a superconducting box, the box and chip dimensions often create unwanted modes that support different microwave frequencies. If the mode frequency is near the qubit frequency, information held in the qubit will transfer to this mode, causing processing errors.

Stopping the leak

The first solution proposed by DQM, anti-node pinning, is a familiar concept in the engineering world, but has never before been applied to quantum computing.

True to its name, anti-node pinning involves placing a pin at the anti-node of the first mode, resulting in a shift of its frequency. The pinning process is repeated until the resonance frequency of the mode shifts above the frequency of the qubit, lowering the leakage error rate.

While anti-node pinning gives optimal resource return and is compatible with any cavity shape, it results in pin arrangements that aren’t readily compatible with grid arrays of qubits. Since grid pattern chips are the designs that result from pursuing a variety of common quantum computing schemes like the surface code, the research group looked at a second method to shift the frequency: half-wave fencing. This approach creates multiple smaller cavities inside the original cavity, like a faraday cage.

Hlaf-wave fencing and anti-node pinning A) Half-wave fencing.
B) Anti-node pinning.

Simulations testing the effectiveness of both frequency-shifting methods showed the leakage error rates could be easily lowered to orders of magnitude below the best overall error rates achievable in the field currently.

“The quantum socket shows a promising way to suppress spurious modes by strategically placing them in a cavity containing quantum circuits,” said Vivekananda Adiga, Research Engineer at IBM Research. “This can improve the control and readout of qubits by reducing leakage and crosstalk. It enables a potential path forward to extend the qubit lattices to large areas without compromising their performance.”

In the next phase of the project, researchers will test the frequency shifting methods in the lab.

“There are many other scaling issues under consideration, such as scalable qubit controllers, so there is still much more work to do,” said Mariantoni, also a professor in the Department of Physics and Astronomy. “But at least we have methods to mitigate leakage errors.”

Mitigating leakage errors due to cavity modes in a superconducting quantum computer, is published in Quantum Science and Technology.

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