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1. Dynamically generated decoherence-free subspaces and subsystems on superconducting qubits

   https://doi.org/10.1088/1361-6633/ad6805  

   Quiroz, Gregory; Pokharel, Bibek; Boen, Joseph; Tewala, Lina; Tripathi, Vinay; Williams, Devon; Wu, Lian-Ao; Titum, Paraj; Schultz, Kevin; Lidar, Daniel (August 2024, Reports on Progress in Physics)

   Abstract Decoherence-free subspaces and subsystems (DFS) preserve quantum information by encoding it into symmetry-protected states unaffected by decoherence. An inherent DFS of a given experimental system may not exist; however, through the use of dynamical decoupling (DD), one can induce symmetries that support DFSs. Here, we provide the first experimental demonstration of DD-generated decoherence-free subsystem logical qubits. Utilizing IBM Quantum superconducting processors, we investigate two and three-qubit DFS codes comprising up to six and seven noninteracting logical qubits, respectively. Through a combination of DD and error detection, we show that DFS logical qubits can achieve up to a 23% improvement in state preservation fidelity over physical qubits subject to DD alone. This constitutes a beyond-breakeven fidelity improvement for DFS-encoded qubits. Our results showcase the potential utility of DFS codes as a pathway toward enhanced computational accuracy via logical encoding on quantum processors. 

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2. Characterizing the magnetic noise power spectrum of dark spins in diamond

   https://doi.org/10.1088/1367-2630/adb2b8  

   Williams, Ethan Q; Ramanathan, Chandrasekhar (March 2025, New Journal of Physics)

   Abstract The coherence times of solid-state spin qubits are often limited by the presence of a spin bath. Characterizing the spectrum of the local magnetic field fluctuations of the bath is key to understanding spin qubit decoherence. Here we use pulsed electron paramagnetic resonance (pEPR) based noise spectroscopy to measure the magnetic noise power spectra for ensembles of substitutional nitrogen (P1) centers in diamond that typically form the bath for nitrogen-vacancy (NV) centers. The Carr–Purcell–Meiboom–Gill (CPMG) dynamical decoupling experiments on the P1 centers were performed on a low [N] chemical vapor deposition (CVD) sample and a high [N] high-temperature, high-pressure (HPHT) sample at 89 mT. We characterize the NV centers of the latter sample using the same 2.5 GHz pEPR spectrometer. All power spectra show two distinct features, a broad component that is observed to scale as approximately $1/{\omega }^{0.7-1.0}$, and a prominent peak at the¹³C Larmor frequency. The behavior of the broad component is consistent with an inhomogeneous distribution of Lorentzian spectra due to clustering of P1 centers, which has recently been shown to be prevalent in HPHT diamond. We develop techniques utilizing harmonics of the CPMG filter function to improve characterization of high-frequency signals, which we demonstrate on the¹³C nuclear Larmor frequency. At 190 mT this is 2.04 MHz, 5.7 times higher than the CPMG modulation frequency ( $<357$kHz, hardware-limited). Understanding the properties of the bath allow us to either exploit it as a quantum resource or optimize decoupling performance, while also informing sample fabrication technologies. The techniques are applicable to ac magnetometry for nanoscale nuclear magnetic resonance and chemical sensing. 

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3. Spin Echo, Fidelity, and the Quantum Critical Fan in ${\mathrm{TmVO}}_4$

   https://doi.org/10.1103/PhysRevLett.132.216502  

   Nian, Y-H; Vinograd, I; Green, T; Chaffey, C; Massat, P; Singh, R_R P; Zic, M P; Fisher, I R; Curro, N J (May 2024, Physical Review Letters)

   Using spin-echo nuclear magnetic resonance in the model transverse field Ising system TmVO4, we show that low frequency quantum fluctuations at the quantum critical point have a very different effect on 51V nuclear spins than classical low-frequency noise or fluctuations that arise at a finite temperature critical point. Spin echoes filter out the low-frequency classical noise but not the quantum fluctuations. This allows us to directly visualize the quantum critical fan and demonstrate the persistence of quantum fluctuations at the critical coupling strength in TmVO4 to high temperatures in an experiment that remains transparent to finite temperature classical phase transitions. These results show that while dynamical decoupling schemes can be quite effective in eliminating classical noise in a qubit, a quantum critical environment may lead to rapid entanglement and decoherence. 

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4. Mapping electronic decoherence pathways in molecules

   https://doi.org/10.1073/pnas.2309987120  

   Gustin, Ignacio; Kim, Chang Woo; McCamant, David W; Franco, Ignacio (December 2023, Proceedings of the National Academy of Sciences)

   Establishing the fundamental chemical principles that govern molecular electronic quantum decoherence has remained an outstanding challenge. Fundamental questions such as how solvent and intramolecular vibrations or chemical functionalization contribute to the decoherence remain unanswered and are beyond the reach of state-of-the-art theoretical and experimental approaches. Here we address this challenge by developing a strategy to isolate electronic decoherence pathways for molecular chromophores immersed in condensed phase environments that enables elucidating how electronic quantum coherence is lost. For this, we first identify resonance Raman spectroscopy as a general experimental method to reconstruct molecular spectral densities with full chemical complexity at room temperature, in solvent, and for fluorescent and non-fluorescent molecules. We then show how to quantitatively capture the decoherence dynamics from the spectral density and identify decoherence pathways by decomposing the overall coherence loss into contributions due to individual molecular vibrations and solvent modes. We illustrate the utility of the strategy by analyzing the electronic decoherence pathways of the DNA base thymine in water. Its electronic coherences decay in $\sim$30 fs. The early-time decoherence is determined by intramolecular vibrations while the overall decay by solvent. Chemical substitution of thymine modulates the decoherence with hydrogen-bond interactions of the thymine ring with water leading to the fastest decoherence. Increasing temperature leads to faster decoherence as it enhances the importance of solvent contributions but leaves the early-time decoherence dynamics intact. The developed strategy opens key opportunities to establish the connection between molecular structure and quantum decoherence as needed to develop chemical strategies to rationally modulate it. 

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5. Modeling Low- and High-Frequency Noise in Transmon Qubits with Resource-Efficient Measurement

   https://doi.org/10.1103/PRXQuantum.5.010320  

   Tripathi, Vinay; Chen, Huo; Levenson-Falk, Eli; Lidar, Daniel A. (February 2024, PRX Quantum)

   Transmon qubits experience open-system effects that manifest as noise at a broad range of frequencies. We present a model of these effects using the Redfield master equation with a hybrid bath consisting of low- and high-frequency components. We use two-level fluctuators to simulate 1/f-like noise behavior, which is a dominant source of decoherence for superconducting qubits. By measuring quantum state fidelity under free evolution with and without dynamical decoupling (DD), we can fit the low- and high-frequency noise parameters in our model. We train and test our model using experiments on quantum devices available through IBM quantum experience. Our model accurately predicts the fidelity decay of random initial states, including the effect of DD pulse sequences. We compare our model with two simpler models and confirm the importance of including both high frequency and 1/f noise in order to accurately predict transmon behavior. 

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