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arXiv:2402.05604 · Physical Review Applied

Protecting logical qubits with dynamical decoupling

Jia-Xiu Han, Jiang Zhang, Guangming Xue, Haifeng Yu, Gui‐Lu Long

Published Jul 8, 2025• 9 citations
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Abstract

Demonstrating that logical qubits outperform their physical counterparts represents a milestone in achieving reliable quantum computation. Here, we propose to protect logical qubits with a dynamical decoupling scheme that implements i gates on nearest-neighbor physical qubits, and we experimentally demonstrate this scheme on superconducting transmon qubits. In our scheme, each logical qubit requires only two physical qubits. A universal set of quantum gates on the logical qubits can be achieved such that each logical gate comprises only one or two physical gates. Our experiments reveal that the coherence time of a logical qubit is extended by up to 357% compared to the better-performing physical qubit. Moreover, we demonstrate—to the best of our knowledge, for the first time—that multiple logical qubits can outperform their physical superconducting counterparts. We illustrate a set of universal gates through a logical Ramsey experiment and the creation of a logical Bell state. Given its scalable nature, our scheme holds promise as a component for future reliable quantum computation.

DOI
10.1103/lm8x-r48q

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Demonstrating that logical qubits outperform their physical counterparts represents a milestone in achieving reliable quantum computation. Here, we propose to protect logical qubits with a dynamical decoupling scheme that implements i gates on nearest-neighbor physical qubits, and we experimentally demonstrate this scheme on superconducting transmon qubits. In our scheme, each logical qubit requires only two physical qubits. A universal set of quantum gates on the logical qubits can be achieved such that each logical gate comprises only one or two physical gates. Our experiments reveal that the coherence time of a logical qubit is extended by up to 357% compared to the better-performing physical qubit. Moreover, we demonstrate—to the best of our knowledge, for the first time—that multiple logical qubits can outperform their physical superconducting counterparts. We illustrate a set of universal gates through a logical Ramsey experiment and the creation of a logical Bell state. Given its scalable nature, our scheme holds promise as a component for future reliable quantum computation.
Quantum computers now show the promise of surpassing any possible classical machine. However, errors limit this ability and current machines do not have the ability to implement error correcting codes due to the limited number of qubits and limited control. Therefore, dynamical decoupling (DD) and encodings that limit noise with fewer qubits are more promising. For these reasons, we put forth a model of universal quantum computation that has many advantages over strategies that require a large overhead such as the standard quantum-error correcting codes. First, we separate collective noise from individual noises on physical qubits and use a decoherence-free subspace that uses just two qubits for its encoding to eliminate collective noise. Second, our bath model is very general as it uses a spin-boson-type bath but without any Markovian assumption. Third, we are able to either use a steady global magnetic field or to devise a set of DD pulses that remove much of the remaining noise and commute with the logical operations on the encoded qubit. This allows removal of noise while implementing gate operations. Numerical support is given for this hybrid protection strategy which provides an efficient approach to deal with the decoherence problems in quantum computation and is experimentally viable for several current quantum computing systems. This is emphasized by a recent experiment on superconducting qubits which shows promise for increasing the number of gates that can be implemented reliably with some realistic parameter assumptions.
Dephasing--phase randomization of a quantum superposition state--is a major obstacle for the realization of high fidelity quantum logic operations. Here, we implement a two-qubit controlled-NOT gate using dynamical decoupling (DD), despite the gate time being more than 1 order of magnitude longer than the intrinsic coherence time of the system. For realizing this universal conditional quantum gate, we have devised a concatenated DD sequence that ensures robustness against imperfections of DD pulses that otherwise may destroy quantum information or interfere with gate dynamics. We compare its performance with three other types of DD sequences. These experiments are carried out using a well-controlled prototype quantum system--trapped atomic ions coupled by an effective spin-spin interaction. The scheme for protecting conditional quantum gates demonstrated here is applicable to other physical systems, such as nitrogen vacancy centers, solid state nuclear magnetic resonance, and circuit quantum electrodynamics.
Crosstalk suppression is a key requirement for scalable implementations of fault-tolerant quantum computing, in particular using superconducting devices. This study demonstrates a simple, effective, and easily scalable scheme for dynamical decoupling to slash crosstalk. The scheme uses existing logic gates and does not require any additional calibrations or complexity. Apart from showing significant improvements in quantum memory experiments with multiqubit commercial processors, this approach also boosts the performance of single- and two-qubit gates. This method is suitable for immediate incorporation into both near-term NISQ and longer-term fault-tolerant systems.
We study the implementation of holonomic quantum gates preserved from arbitrary single-qubit and two-qubit decoherence or errors using a dynamical decoupling approach. To integrate the holonomic feature with the dynamical decoupling technique, we require only one auxiliary qubit for single-qubit gates and no auxiliary qubit for two-qubit gates. Our scheme uses far fewer physical qubits to achieve this integration compared to existing proposals in the literature. Moreover, the scheme is based on system interactions that can be experimentally realized with superconducting qubits. These advantages suggest that our scheme is a promising step toward developing noise-resistant quantum devices.
Implementing precise operations on quantum systems is one of the biggest challenges for building quantum devices in a noisy environment. Dynamical decoupling attenuates the destructive effect of the environmental noise, but so far, it has been used primarily in the context of quantum memories. Here, we experimentally demonstrate a general scheme for combining dynamical decoupling with quantum logical gate operations using the example of an electron-spin qubit of a single nitrogen-vacancy center in diamond. We achieve process fidelities >98% for gate times that are 2 orders of magnitude longer than the unprotected dephasing time T2.
Abstract Unconventional nonadiabatic geometric quantum computation not only possesses the geometric robustness of conventional ones but also avoids the need to remove the dynamical phase. Dynamical decoupling is a promising method to protect unconventional nonadiabatic geometric gates against decoherence. Here, we propose a protocol to implement unconventional nonadiabatic geometric quantum computation protected by dynamical decoupling. By using three physical qubits to encode a logical qubit and choosing a system Hamiltonian that commutes with the decoupling group, a universal set of dynamical-decoupling-protected unconventional nonadiabatic geometric gates can be realized. Our work maintains the robustness of unconventional nonadiabatic geometric gates while protecting quantum gates against both collective and independent decoherence.