Observing Quantum Oscillation of Ground States in Single Molecular Magnet Jiahui Yang, Ya Wang, Zixiang Wang, Xing Rong, Chang-Kui Duan, Ji-Hu Su, * and Jiangfeng Du † Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China (Received 23 March 2012; published 4 June 2012) Single molecular magnets (SMMs) are among the potential systems for quantum memory and quantum information processing. Quantum coherence and oscillation are critical for these applications. The ground-state quantum coherence and Rabi oscillations of the SMM V 15 (½V IV 15 As III 6 O 42 ðH 2 OÞ 6 ) have been studied in this context. We have affirmatively measured at 2.4 K the Rabi quantum oscillations and coherence time T 2 for the ground states of the V 15 ion of collective spin S ¼ 1=2, in addition to confirming the previously reported results for the S ¼ 3=2 excited states. The oscillations of S ¼ 3=2 and S ¼ 1=2 states are of different frequencies, and so can be separately selected for purposive manipulations. T 2 of 188  4 ns (S ¼ 3=2) and 149  10 ns (S ¼ 1=2) are much less than T 1  12 s and are further extendible via various approaches for qubit implementations. DOI: 10.1103/PhysRevLett.108.230501 PACS numbers: 03.67.Lx, 03.65.Yz, 03.67.Pp, 76.30.Mi Single molecular magnet (SMM) refers to the class of molecules with nonzero collective spin showing magnetic property similar to a bulk magnet [1,2]. Since the proposal of quantum computing in SMMs in 2001 [3], quantum oscillations and coherence of SMMs have been subjected to considerable studies [4–10] for the purpose of quantum memory [11] and quantum information processing [12]. Most of the studies have focused on SMMs with large collective spins. In comparison, SMMs with small ground-state spins, such as V 15 (½V IV 15 As III 6 O 42 ðH 2 OÞ 6 ) [13] and Cr 7 Ni [14] have the advantage of substantial smaller coupling with each other and the environment [15] and are likely to be more suitable than SMMs with large collective spins for quantum information applica- tions. Although extensive studies have been carried out on V 15 [8,13,16,17], the quantum coherence and Rabi oscillation for the ground spin states (S ¼ 1=2), which are critical for quantum information applications, have not been affirmatively observed [8,18]. The claimed first experimental observation in 2008 [8] was unable to be reproduced [18] and turned out to be the signal of cavity background. In this Letter we report our experimental observations of the ground-state quantum coherence and Rabi oscillation of V 15 at a lower temperature of 2.4 K together with scrutinized validation. The results show the possibility of qubit implementation in the ground states of V 15 , subject to further lowering the temperature and/or applying various dynamic decoupling approaches to extend the quantum coherent time [19,20]. The K 6 ½V IV 15 As III 6 O 42 ðH 2 OÞ single crystals were pre- pared as described [13]. Dark brown crystals were col- lected after three days. One of the crystals was examined by x-ray diffraction (XRD) at room temperature, and the resultant molecular structure is shown in Fig. 1(a) (the K atoms are not depicted). It clearly shows the quasispherical structure with three layers of vanadium atoms, where two nonplanar hexagons (top and bottom) are separated by the central triangle. This agrees with the reported structure of the K 6 ½V IV 15 As III 6 O 42 ðH 2 OÞ single crystal [13]. The mole- cule shows a three-fold axis of symmetry (C 3 ) and the core (a) (c) (b) FIG. 1 (color online). (a) Ball-and-stick diagram of molecular structure of V 15 gaining from the result of single crystal XRD at room temperature (red, V; black, As; silver, O). (b) Cw spectrum of V 15 single crystal at 100 K. Experimental settings: microwave frequency, 9.7 GHz; microwave power, 0.002 mW; modulation frequency, 100 kHz. (c) The energy diagram of V 15 low-lying states with the magnetic field parallel to the C 3 axis [8]. The two lower red arrows denote the transitions in the ground doublet states and the three upper black arrows denote those in the first excited states. The energy gap between the excited states and the ground states is about 3.67 K (see text). PRL 108, 230501 (2012) PHYSICAL REVIEW LETTERS week ending 8 JUNE 2012 0031-9007= 12=108(23)=230501(5) 230501-1 Ó 2012 American Physical Society