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The Journal of Physical Chemistry Letters

Near-Room-Temperature Phase Gate in a Molecular High-Spin Qudit of Cocrystallized Endofullerene

Qi Xiong, Yihan Wang, Peng‐Xiang Fu, Cong‐Hui Wu, Bing-Cai Chen, Jian-Yu Liu, Song Wei Gao, Zheng Liu, Ye-Xin Wang, Shen Lin Zhou, Shang‐Da Jiang

Published Jul 23, 2025• 1 citation
Publisher

Abstract

Paramagnetic molecules with high-spin centers can accommodate multilevel spin manipulations. In the cocrystal PdF 20 TPP·C 60 /N@C 60 ·8Benzene (ENF-PdTPP), 14 N@C 60 ( S = 3/2) exhibits distinct transition addressability in the frequency domain and a long quantum coherence time ( T 2 ≈ 5.47 μs at 250 K), making it a promising candidate for multilevel qu d it applications. A three-level superposition state was prepared at near-room temperature (250 K) and validated by quantum phase interference. By varying the evolution time (Δ τ ) of the three-level superposition state, a controllable quantum phase effect has been observed. We found that such a phenomenon can be harnessed as a resource for coherent manipulation for the realization of a fast controlled-phase (C-phase) gate (23.5 MHz), enriching the potential of this high-spin system for quantum algorithm implementations.

DOI
10.1021/acs.jpclett.5c01611

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Paramagnetic molecules with high-spin centers can accommodate multilevel spin manipulations. In the cocrystal PdF 20 TPP·C 60 /N@C 60 ·8Benzene (ENF-PdTPP), 14 N@C 60 ( S = 3/2) exhibits distinct transition addressability in the frequency domain and a long quantum coherence time ( T 2 ≈ 5.47 μs at 250 K), making it a promising candidate for multilevel qu d it applications. A three-level superposition state was prepared at near-room temperature (250 K) and validated by quantum phase interference. By varying the evolution time (Δ τ ) of the three-level superposition state, a controllable quantum phase effect has been observed. We found that such a phenomenon can be harnessed as a resource for coherent manipulation for the realization of a fast controlled-phase (C-phase) gate (23.5 MHz), enriching the potential of this high-spin system for quantum algorithm implementations.
Quantum sensors harnessing multi-quantum coherence offer a promising route to precision metrology beyond the standard quantum limit. Although molecular spin qudits provide engineerable high-dimensional Hilbert spaces, the rapid decoherence of high-order spin states typically restricts their operation to cryogenic environments. Here, we synthesize a high-spin molecular magnet N@C60-C9H8O (electron spin S = 3/2), via a single-step gas-phase ion implantation technique. Leveraging its ultralong room-temperature coherence time (~38 μs), N@C60-C9H8O achieves an AC magnetic sensing sensitivity of 4⋅10-8 T Hz-1/2. Based on full coherent control over the four-level electron spin manifold, we demonstrate the sensing enhancement beyond the standard quantum limit utilizing molecular spin cat states at room temperature. These results exhibit the immense application potential of high-spin molecular qudits in ambient quantum metrology.
Molecular spin qudits based on lanthanide complexes offer a promising platform for quantum technologies, combining chemical tunability with multi-level encoding. However, experimental demonstrations of their envisaged capabilities remain scarce, posing the difficulty of achieving precise control over coherences between qudit states in long pulse sequences. Here, we implement in 173Yb(trensal) qudit the Quantum Fourier Transform (QFT), a core component of numerous quantum algorithms, storing quantum information in the phases of coherences. QFT provides an ideal benchmark for coherence manipulation and an unprecedented challenge for molecular spin qudits. We address this challenge by embedding a full-refocusing protocol for spin qudits in our algorithm, mitigating inhomogeneous broadening and enabling a high-fidelity recovery of the state. Complete state tomography demostrates the performance of the algorithm, while simulations provide insight into the physical mechanisms behind inhomogeneous broadening. This work shows the feasibility of quantum logic on molecular spin qudits and highlights their potential.
Abstract High-spin magnetic molecules are promising candidates for quantum information processing because their intrinsic multiplicity facilitates information storage and computational operations. However, due to the absence of suitable sublevel splittings, their susceptibility to environmental disturbances and limitation from the selection rule, the arbitrary control of the quantum state of a molecular electron multiplet has not been realized. Here, we exploit the photoexcited triplet of C70 as a molecular electron spin qutrit with pulsed electron paramagnetic resonance. We prepared the system into 3-level superposition states characteristic of a qutrit and validated them by the tomography of their density matrices. To further elucidate the coherence of the operation and the nature of the system as a qutrit, we demonstrated the quantum phase interference in the superposition. The interference pattern is further interpreted as a map of possible evolution paths in the space of phase factors, representing the quantum nature of the 3-level system.
Maximizing the number of addressable spins within a fixed volume can improve ensemble quantum sensor sensitivity, but dense packing usually increases dipolar interactions and excited-state transport, shortening coherence and suppressing optical readout. Here we report a 2:1 cocrystal of 6,13-dihydropentacene and pentacene (DHP/Pc) containing 33.3 mol% pentacene (3.3×1053.3\times10^{5}3.3×105 ppm; 9.51×10209.51\times10^{20}9.51×1020 cm−3^{-3}−3), a volumetric spin-site density more than two orders of magnitude above previous benchmarks, NV-diamond and pentacene-doped p-terphenyl (PDP). Despite this density, DHP/Pc exhibits microsecond spin coherence, room-temperature optically detected magnetic resonance and coherent control. Time-resolved measurements indicate that the long-lived triplet population is generated predominantly by intersystem crossing and exhibits strong, non-thermal sublevel polarization. Density-functional theory calculations further suggest that the native cocrystal geometry weakens electronic coupling between neighboring pentacenes, while the higher DHP triplet energy creates a barrier to triplet migration. These results identify molecular packing and coformer triplet energetics as complementary design parameters for preserving coherence in high spin-site density systems, demonstrating cocrystallization as a promising route to dense, optically addressable spin materials for room-temperature ensemble quantum sensing.
A radio frequency (rf) superconducting quantum interference device (SQUID) is a macroscopic quantum object consisting of a superconducting loop interrupted by a Josephson junction. Superconducting phase quantum bits (qubits) based on rf SQUIDs have been proven to be one of the most promising candidates for building a quantum computer. They exploit the unique resources of quantum superposition and entanglement and are exponentially faster than classical computers in solving certain problems, such as factoring. Compared to other approaches to quantum computing, superconducting phase qubits allow stronger and more flexible inter-qubit coupling and thus are easier to scale up. However, phase qubits couple to the environment and are subject to considerable decoherence. The resulting coherence time (also called decoherence time) is on the order of 100 ns, about two orders of magnitude shorter than that required for fault-tolerant quantum computing. One possible solution is to develop faster quantum gates in phase qubits. In this dissertation, coherent manipulation of multi-partite quantum states via Landau-Zener (LZ) transitions was investigated in a phase qubit, which was coupled to two microscopic two-level systems (TLSs) embedded in the tunnel barrier of the Josephson junction. The qubit chip was measured at temperatures below 30 mK in an ultra-low noise environment with excellent electrical and magnetic filtering and shielding. All parameters of the phase qubit were calibrated independently. The phase qubit's decoherence times have been carefully measured as well. Fast and precise coherent control of the tripartite quantum states has been successfully demonstrated by the observation of the Landau-Zener-Stückelberg (LZS) interference in the coupled qubit-TLS system. Furthermore, it is shown that utilizing LZ transitions to create multi-partite entangled states, such as the W state, is significantly more efficient than conventional methods which require a sequence of single-qubit and two-qubit gates. Hence, coherent manipulation of multi-partite quantum states via LZ transitions is a promising basis for a new family of fast multi-qubit quantum gates.
We show that [VO(TPP)] (vanadyl tetraphenylporphyrinate) is a promising system to implement quantum computation algorithms based on encoding information in multi-level (qudit) units. Indeed, it embeds a nuclear spin 7/2 coupled to an electronic spin 1/2 by hyperfine interaction. This qubit-qudit unit can be exploited to implement quantum error correction and quantum simulation algorithms. Through a combined theoretical and broadband nuclear magnetic resonance study, we demonstrate that the elementary operations of such algorithms can be efficiently implemented on the nuclear spin qudit. Manipulation of the nuclear qudit can be achieved by resonant radio-frequency pulses, thanks to the remarkably long coherence times and the effective quadrupolar coupling induced by the strong hyperfine interaction. This approach may open new perspectives for developing new molecular qubit-qudit systems.