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Optically addressable molecular spins are a promising qubit platform, combining the chemical versatility, atomistic tunability, nanoscale modularity, and structural diversity of molecules with the coherence, detection sensitivity, and remote connectivity of optically readable spins. Here, we review progress developing and deploying optically readable molecular spin qubits and the key opportunities they present, with a particular focus on quantum sensing. We survey key criteria for realizing optically addressable spin qubits in molecules; discuss existing and emerging platforms-spanning coordination complexes, organic molecules, and both ground and excited-state spins; and outline emerging applications, open challenges, and opportunities for combining the richness of chemical systems with the power of optically readable spin qubits. Optically addressable molecular spins as chemically versatile qubits.
Entanglement is crucial for quantum networks and computation, yet maintaining high-fidelity entangled quantum states is hindered by decoherence and resource-intensive purification methods. Here, we experimentally demonstrate entanglement pumping, utilizing bosonic quantum error correction (QEC) codes as long-coherence-time storage qubits. By repetitively generating entanglement with short-coherence-time qubits and injecting it into error-detectable logical qubits, our approach effectively preserves entanglement. Through error-detection to discard error states and entanglement pumping to mitigate errors within the code space, we extend the existence time of entanglement by nearly 50% compared to the case without entanglement pumping. This entanglement pumping scheme can additionally serve as an erasure detection protocol for the dual-rail code. This Letter highlights the potential of bosonic logical qubits for scalable quantum networks and introduces a novel paradigm for efficient entanglement management.
We report on atomic-scale analyses of oxygen distribution and segregation at grain boundaries (GBs) of Nb and Ta-encapsulated Nb (Ta/Nb) thin films for superconducting qubits using atom probe tomography (APT) and transmission electron microscopy (TEM). We observe oxygen segregation at grain boundaries (GBs) relative to the oxygen concentration within the grains for both Nb and Ta-capped Nb thin films and find that a higher oxygen concentration in the interior of Nb grains leads to greater oxygen segregation levels at GBs. This finding reveals that the formation of a local equilibrium of oxygen concentration between GBs and grain interiors of Nb is the primary driving force of the oxygen segregation behaviors in Nb and Ta-capped Nb. The enrichment factors (CGB/Cgrain) for oxygen segregation at GBs in Nb and Ta-capped Nb range from 2.4 ± 0.3 to 2.7 ± 0.4. The current results also highlight that controlling oxygen impurities in Nb during film deposition and fabrication processing is important to concomitantly reducing the level of oxygen segregation at GBs in Nb. Finally, we find that increases in the oxygen concentration in both Nb grains and GBs correlate with a suppression in the critical temperature for superconductivity (Tc). Together, our comparative chemical and charge transport property analyses provide atomic-scale insights into a potential mechanism, contributing to the decoherence in superconducting qubits.
Manipulation of quantum systems for sensing and transduction rely on controlling the interactions between a quantum system and the many degrees of freedom of the bath. In molecular spin quantum systems, spin-orbit coupling serves as a conduit for energy dissipation via vibrational and phonon modes, which in turn are dictated by changes in oxidation state, metal-ligand covalency, and symmetry of the coordination sphere. The confluence of these factors complicate design strategies however for manipulation of spin qubits for quantum sensing and transduction strategies. Here, we report an investigation of the spin dynamics in isostructural S = 1/2 first-row transition metal complexes in which the spin-orbit coupling is varied between a ls-Co(ii)N4Phen (1-Co) and Cu(ii)N4Phen (1-Cu) complex. Based on free-ion spin-orbit coupling parameters (528 cm-1 for Co(ii) and 829 cm-1 for Cu(ii)), faster spin-lattice relaxation rates (1/T 1) are initially expected for 1-Cuvs.1-Co. However, X-band pulsed EPR and AC susceptibility reveal that both complexes have nearly identical slow spin-lattice relaxation processes. Notably, decoherence (phase memory times, T m) at 60 K is longer for 1-Cu (0.63(1) µs) than for 1-Co (0.56(1) µs). Direct observation of d-d splittings, and determination of anisotropic g-values by EPR spectroscopy reveals an effective decrease in spin-orbit coupling for 1-Cu (λ' = 400-435 cm-1) relative to 1-Co (λ' = 370-400 cm-1) due to greater metal-ligand covalency in the Cu(ii) complex. Computational modelling of spin density distributions (DFT) and the excited state manifolds (CASSCF) support the differences in excited state energies and spin densities that dictate spin dynamics in these complexes. Two sets of nearly degenerate low-frequency modes were identified as possible vibrational relaxation channels via a two-phonon (Raman) process, consistent with contributions from spin-vibrational orbit interactions. This study provides fundamental insight into the role of metal-ligand covalency in modulating spin-orbit coupling contributions to spin-lattice relaxation and decoherence processes. Increased metal-ligand covalency reduces effective spin-orbit coupling, thereby increasing both spin-lattice and coherence time in molecular spin qubits, providing an important strategy for controlling quantum states and spin-vibrational energy transfer processes in molecular qubit platforms for quantum information processing.
We show that chaos-assisted tunneling (CAT) imposes an intrinsic limit to the protection of Kerr-cat qubits. In the static effective description, tunneling between the quasidegenerate cat states can be exponentially suppressed, ensuring long lifetimes. However, our Floquet analysis reveals that when the nonlinearities increase, chaotic states mediate tunneling between the cat states, producing large quasienergy splittings. We compute tunneling rates using both full quantum simulations and semiclassical WKB theory, finding quantitative agreement and confirming that the splittings are directly linked to chaos. These results provide the first evidence of CAT in the Kerr-cat qubit and demonstrate that chaos sets a fundamental bound on the coherence of dynamically protected superconducting qubits.
The scalability and power of quantum computing architectures depend critically on high-fidelity operations and robust and flexible qubit connectivity1-3. In this respect, mobile qubits are particularly attractive as they enable dynamic and reconfigurable qubit arrays. This approach allows quantum processors to adapt their connectivity patterns during operation, implement different quantum error correction codes on the same hardware and optimize resource use through dedicated functional zones for specific operations such as measurement or entanglement generation4-7. Such flexibility also relieves architectural constraints, as recently demonstrated in atomic systems based on trapped ions4,5 and neutral atoms manipulated with optical tweezers6,7. In solid-state platforms, highly coherent shuttling of electron spins was recently reported8,9. A key outstanding question is whether it may be possible to perform quantum gates directly on the mobile spins. Here we demonstrate two-qubit operations between two electron spins carried towards each other in separate travelling potential minima in a semiconductor device. We find that the interaction strength is highly tunable by their spatial separation. When we shuttle the two spins towards the centre by 120 nm each for a total displacement of 240 nm, we achieve an average two-qubit gate fidelity of about 99%. Furthermore, we implement conditional post-selected quantum state teleportation between qubits separated by 320 nm with an average gate fidelity of 87%, showcasing the potential of mobile spin qubits for non-local quantum information processing. We expect that operations on mobile qubits will become a universal feature of future large-scale semiconductor quantum processors.
Qubit readout schemes often deviate from ideal projective measurements, introducing critical issues that limit quantum computing performance. In this Letter, we model charge-sensing-based readout for semiconductor spin qubits in double quantum dots, and identify key error mechanisms caused by the backaction of the charge sensor. We quantify how the charge noise of the sensor, residual tunneling, and g-tensor modulation degrade readout fidelity, induce a mixed postmeasurement state, and cause leakage from the computational subspace. For state-of-the-art systems with strong spin-orbit interaction and electrically tunable g tensors, we identify a readout sweet spot, that is, a special device configuration where readout is closest to projective. Our framework provides a foundation for developing effective readout error mitigation strategies, with broad applications for optimizing readout performance for a variety of charge-sensing techniques, advancing quantum protocols, and improving adaptive circuits for error correction.
Metal-organic frameworks (MOFs) have emerged as promising candidates for quantum information and photonic applications due to their structural tunability and the potential to contain molecular qubits in architectures that support coherent light-matter interactions. In this study, we investigate the ultrafast coherent dynamics of a perylene-based MOF, UMCM-313, and its corresponding perylene-based organic linker crystal using time-resolved two-photon near-field scanning optical microscopy (NSOM). By tracking the electronic quantum coherence, we reveal coherent modes persisting up to several hundred femtoseconds (fs) at room temperature and extending to the picosecond regime (≈0.9-1 ps) at 173 K in the MOF, significantly longer than the coherence time observed in the organic linkers at the femtosecond scale. The enhanced electronic coherence of UMCM-313 is attributed to periodic chromophore separation and a reduced degree of homogeneous broadening. Because electron spin coherence can be several orders of magnitude longer than optical coherence, we further probed the spin coherence of photoexcited triplet states in UMCM-313 by using time-resolved and pulsed electron paramagnetic resonance (TREPR and pulse-EPR) spectroscopy. TREPR spectra reveal multiple triplet species within the framework, while pulse-EPR measurements yield a phase memory time of 237 ± 5 ns at 173 K. These results demonstrate that UMCM-313 supports both extended excitonic coherence and nanosecond spin coherence, underscoring the importance of periodic framework connectivity in sustaining phase-stable quantum states. The coexistence of electronic coherence and long-lived spin coherence highlights the potential of MOFs as hybrid platforms for quantum photonic and spintronic technologies capable of maintaining coherence under operationally accessible conditions.
This Perspective article outlines the design principles, key challenges, and future directions for advancing carbon-based molecular quantum technologies.
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Electronic spin superposition states enable nanoscale sensing through their sensitivity to the local environment, yet their sensitivity to vibrational motion also limits their coherence times. In molecular spin systems, chemical tunability and atomic-scale resolution are accompanied by a dense, thermally accessible phonon spectrum that introduces efficient spin relaxation pathways. Despite extensive theoretical work, there is little experimental consensus on which vibrational energies dominate spin relaxation or how molecular structure controls spin-phonon coupling (SPC). We present a fully experimental method to quantify SPC coefficients by combining temperature-dependent vibrational spectra from inelastic neutron scattering with spin relaxation rates measured by electron paramagnetic resonance. We apply this framework to two model S = 1/2 systems, copper(II) phthalocyanine (CuPc) and copper(II) octaethylporphyrin (CuOEP). Two distinct relaxation regimes emerge: below 40 K, weakly coupled lattice modes below 50 cm-1 dominate, whereas above 40 K, optical phonons above ∼185 cm-1 become thermally populated and drive relaxation with SPC coefficients nearly 3 orders of magnitude larger. Structural distortions in CuOEP that break planar symmetry soften the crystal lattice and enhance anharmonic scattering but also raise the energy of stretching modes at the molecular core where the spins reside. This redistributes vibrational energy toward the molecular periphery and out of plane, ultimately reducing SPC relative to CuPc and enabling room-temperature spin coherence in CuOEP. Although our method does not provide mode-specific SPC coefficients, it quantifies contributions from distinct spectral regions and establishes a broadly applicable, fully experimental link between crystal structure, lattice dynamics, and spin relaxation.
The neutral Cu2+ complex [Cu(dttt)2], in which dttt- is the 1,3,2-dithiazole-4-thione-5-thiolate ligand, is a promising molecular spin qubit where a hydrogen-free and sulfur-rich scaffold has been designed to enhance the spin coherence. In bulk, the structural organization induces strong intermolecular antiferromagnetic exchange couplings up to about 100 cm-1, mediated by van der Waals interactions and propagated along 1D chains of molecules within the crystal structure. Here, the deposition by sublimation in ultra-high vacuum conditions of [Cu(dttt)2] on a graphene surface is studied, focusing on investigating the topology and magnetism of ultrathin films. These deposits are characterized by combining X-ray photoelectron spectroscopy and scanning tunneling microscopy; the latter indicates an ordered chain-like arrangement of the assembled monolayer. Synchrotron-based X-ray absorption techniques flanked by density functional theory and wavefunction-based simulations confirm the molecular ordering. These reveal that the magnetic coupling observed in bulk is also present at the monolayer level, highlighting the persistence of a 1D antiferromagnetic intermolecular coupling of about 50 cm-1 with a non-negligible contribution coming from a through-surface exchange path.
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Designing compact and efficient quantum circuits that are compatible with Noisy Intermediate-Scale Quantum (NISQ) hardware remains a central challenge in quantum computing. Most existing optimization approaches rely on fidelity-based fitness functions that require computing the full unitary matrix of the circuit. However, this quickly becomes computationally intractable beyond 10-12 qubits due to the exponential memory and time complexity [Formula: see text]. In this work, we propose a scalable multi-objective genetic algorithm for quantum circuit optimization tailored to NISQ devices. Although evolutionary algorithms have demonstrated strong potential for circuit synthesis, current methods generally depend on full-unitary fidelity evaluation, which severely limits scalability. To address this bottleneck, we introduce two complementary strategies: (1) an independent block-based evaluation using graph partitioning, and (2) an overlapping sliding-window decomposition approach. Both strategies reduce computational complexity from [Formula: see text] to [Formula: see text], where [Formula: see text], making it possible to optimize circuits with more than 20 qubits in practice. Our methods are integrated into the NSGA-II multi-objective framework, enabling simultaneous optimization of fidelity, circuit depth, and gate cost, while maintaining structural compatibility with NISQ hardware-without requiring full-unitary simulation. Experimental results on benchmark circuits demonstrate high fidelity (above 0.94 for small circuits up to 8 qubits, above 0.85 for medium-scale circuits of 10-16 qubits, and above 0.80 for large-scale circuits up to 32 qubits), up to [Formula: see text] reduction in circuit depth, and a [Formula: see text] speedup compared to exact evaluation at 14 qubits.
Molecular magnets are attractive as spin qubits due to their chemical tunability, addressability through electron-spin resonance techniques, and long coherence times. Clock transitions (CTs), for which the system is immune to the effect of magnetic-field fluctuations to first order, provide a method to enhance the coherence time T 2, and to reveal mechanisms of decoherence that are not due to such fluctuations. Here we investigate two variants of Cr7Mn, a spin-1 molecular nanomagnet, at fields near a zero-field CT. We find that at temperatures ≤2 K, T 2 ∼ 1 μs at the CT using a Hahn-echo pulse sequence. Away from the CT, electron-spin-echo envelope modulation (ESEEM) oscillations due to coupling to nuclear spins are observed and have a T 2 as high as 1.35 μs, indicating a distinct mechanism of coherence preservation. Dynamical decoupling with the CPMG pulse sequence yields T 2 ∼ 2.8 μs at the CT and up to ∼3.6 μs in the ESEEM regime along with a demodulation of the oscillatory behavior. The experimental values of T 2 are largely independent of the degree of dilution of the molecules in solvent or whether the solvent is deuterated, indicating that much of the decoherence and ESEEM arises from sources within the molecules themselves. To account for decoherence, we develop a model that includes not only field fluctuations but also fluctuations in the CT transition frequency itself. Our results can be well explained by treating the environment as a combination of noise at the nuclear Larmor precession frequency and 1/f noise in the transverse anisotropy parameter E. Such information about the microscopic origins of decoherence can aid the rational design of molecular-based spin qubits.
We propose hardware-efficient schemes for implementing logical h and s gates transversally on the rotated surface code with reconfigurable neutral atom arrays. Our protocol for logical s gates utilizes the time dynamics of the data and ancilla qubits during syndrome extraction (SE). In particular, we move beyond traditional schemes where transversal logical gates take place between two SE rounds and instead embed our fold-transversal logical operation within a single SE round, leveraging the fact that data and ancilla qubits can be morphed into an unrotated surface code state halfway through an SE round. Under circuit noise, we observe that the performance of our s gate protocol is comparable to that of the quantum memory. For logical h gates, we first develop a simple strategy to rotate code patches efficiently with two sets of 2D acousto-optic deflectors (2D-AOD). We also propose a new transversal h gate protocol, based on the same principle as our s gate protocol, to remove the need for an additional set of 2D-AOD. Together with transversal logical cnot gates, our protocols complete a transversal logical Clifford gate set on the rotated surface code and allow for efficient implementation on neutral atom array platforms.
The integration of molecular spin qubits in the next generation of quantum devices requires magnetic centers that can be individually addressed while remaining decoupled from the substrate. Envisioning this future perspective here, we introduce a heterobimetallic molecular design strategy that integrates a paramagnetic vanadyl spin center with a built-in inorganic decoupling unit within a single coordination complex, overcoming conventional approaches that rely on inorganic buffer layers such as MgO and thereby limit versatility and scalability. The lantern complex [PtVO(SOCPh)4] (PtVO) embeds a VO2+ qubit spatially shielded by a square-planar PtS4 moiety eliminating the need for external decoupling layers. A submonolayer of PtVO was successfully deposited on a highly oriented pyrolytic graphite substrate via electrospray deposition, yielding a chemically intact and well-defined molecular interface. Combining element and polarization-resolved synchrotron spectroscopies, supported by density functional theory calculations, demonstrates that the vanadyl center remains magnetically isolated at the submonolayer limit. Polarization- and angular-dependent X-ray absorption spectroscopy, flanked by multiplet ligand field theory simulations, provided detailed insight into the adsorption geometry and the electronic structure of PtVO upon deposition. Angular-dependent X-ray magnetic circular dichroism further reveals how the molecular coordination geometry governs the orbital contributions and magnetic anisotropy of square-pyramidal vanadyl systems. These results establish a built-in molecular decoupling system as a viable chemical principle for the scalable integration of addressable molecular spin qubits on low-dimensional materials, paving the way to new routes toward surface-based quantum architectures.
Entanglement dynamics are fundamental to quantum technologies, yet controlling their temporal evolution in a reversible and stable manner remains challenging. We propose a solid-state framework based on the Ruderman-Kittel-Kasuya-Yosida interaction, realizable in gate-defined quantum dots or suspended structures, in which two spin qubits couple to a central spin qudit that mediates an effective, time-dependent exchange. The dynamics are governed by an exchange-time integral that unifies interaction strength and physical time into a single scalar control variable, enabling time-reversible and cyclic navigation of the Hilbert space. Crucially, we show that out-of-phase modulation grants access to higher entanglement subspaces, while introducing damping to the exchange modulation achieves stabilized trajectories that drive the system toward stationary entanglement values. This framework provides a systematic route for shaping entanglement dynamics, particularly in the near-boundary regime, using exchange control alone, overcoming the limitations of monotonic evolution and offering practical strategies for entanglement stabilization in realistic solid-state architectures, with direct relevance to quantum metrology and environment-assisted entanglement engineering.
Pinning down a precise understanding of information flow within physical interactions remains a central concern to fields such as stochastic thermodynamics and quantum information science. In both spheres a careful accounting of bits (or qubits) enables a deeper understanding of the physical nature of information. In this work we propose a measure of information flow as a saddle-point solution of the mutual information. This approach places a lower bound on the channel capacity between a particle and an interacting environment. The measure is given by P_{0}/2E_{0} in nats/sec, with P_{0} the average power flux between the particle and its environment, and E_{0} the initial average energy of the particle, all computed in a frame where the particle has zero average momentum. We use a communication theory lens to suggest an associated channel analogy, in which this bound is interpreted as a signal-to-noise ratio. We find that this measure can also quantify early-time information flow for a particle interacting with a thermal bath.
Silicon carbide (SiC) divacancies are attractive candidates for spin-defect qubits possessing long coherence times and optical addressability. The high activation barriers associated with SiC defect formation and motion pose challenges for their study by first-principles molecular dynamics. In this work, we develop and deploy machine learning interatomic potentials (MLIPs) to accelerate defect dynamics simulations while retaining ab initio accuracy. We employ an active learning strategy comprising symmetry-adapted collective variable discovery and enhanced sampling to compile configurationally diverse training data, calculation of energies and forces using density functional theory (DFT), and training of an E(3)-equivariant MLIP based on the Allegro model. The trained MLIP reproduces DFT-level accuracy in defect transition activation free energy barriers, enables the efficient and stable simulation of multidefect 216-atom supercells, and permits an analysis of the temperature dependence of defect thermodynamic stability and formation/annihilation kinetics to propose an optimal annealing temperature to maximally stabilize VV divacancies.