Scientists achieve direct experimental realization of dual-type entangling gates

To develop scalable and reliable quantum computers, engineers and physicists will need to devise effective strategies to mitigate errors in their quantum systems without adding complex additional components. A promising strategy to reduce errors entails the use of so-called dual-type qubits.

These are qubits that can encode quantum information in a system across two different types of quantum states. These qubits could increase the flexibility of quantum computing architectures, while also reducing undesirable crosstalk between qubits and enhancing a system’s operational fidelity.

Researchers at Tsinghua University and other research institutes in China recently realized an entangling gate between dual-type qubits in an experimental setting.

Their paper, published in Physical Review Letters, introduces a promising approach for realizing these gates, which could help to boost the performance of quantum computing systems without the need for additional hardware components.

“In ion trap quantum computation, recently it has been realized that encoding two types of qubits in the same ion species is a promising scheme, one type for carrying quantum information and the other type for auxiliary operations,” Luming Duan, senior author of the paper, told Phys.org.

“A crucial gadget in this scheme is to entangle the qubits encoded in these two types. Although in principle one can first convert the qubits into the same type and then perform the same-type entangling gate, in practice it would lead to considerable overhead.”

The main objective of this recent study by Duan and his colleagues was to directly and experimentally realize a dual-type entangling gate. The team achieved this using a single 532 nm laser system to drive Raman transitions, entangling dual-type qubits without employing any additional hardware.

“We carefully design multiple frequency components in the driving laser, such that they couple the two qubit types, one encoded in the hyperfine levels of one ion and the other encoded in the hyperfine levels of another ion, simultaneous to the collective spatial oscillation of the two ions,” explained Duan.

“Using these collective oscillation modes as a quantum bus, we generate entanglement between the dual-type qubits.”

The researchers evaluated their proposed approach’s potential in a series of tests and found that it achieved entanglement between dual-type qubits encoded in the S and D manifolds of ions, respectively, with a remarkable Bell state fidelity of 96.3%. This performance is comparable to that achieved by same-type entangling gates (i.e., S-S or D-D gates).

“Using a single setup, we achieve dual-type and same-type entangling gates with similar gate performance,” said Duan. “This suggests that there is no additional fundamental limitation in realizing a dual-type entangling gate, such that it can be applied in practical quantum circuits to help reduce the overhead of back-and-forth qubit type conversions.”

The findings of this study could soon inspire other research groups to experiment with dual-type qubits to reduce errors in quantum systems without increasing their complexity. Meanwhile, Duan and his colleagues will continue building on their methods, to further boost the performance of the dual-type entangling gate they realized.

“We plan to upgrade the system for better stabilization of the optical paths and better locking of the trap frequency,” added Duan.

“These will help improve the gate performance. We also plan to apply this dual-type entangling gate to demonstrate mid-circuit quantum state detection for quantum error correction, and to demonstrate the functioning of a trapped-ion-based quantum network node.”

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