PHYSICAL REVIEW B 97, 064409 (2018) Editors’ Suggestion Coherent spin dynamics of ytterbium ions in yttrium orthosilicate Hee-Jin Lim,1 Sacha Welinski,2 Alban Ferrier,2,3 Philippe Goldner,2,* and J. J. L. Morton1,4,† 1London Centre for Nanotechnology, University College London, London WC1H 0AH, United Kingdom 2PSL Research University, Chimie ParisTech, CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France 3Sorbonne Universités, UPMC Université Paris 6, 75005 Paris, France 4Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom (Received 22 December 2017; published 14 February 2018) We investigate the electron and nuclear spin coherence properties of ytterbium (Yb3+) ions with nonzero nuclear spin, within an yttrium orthosilicate (Y2SiO5) crystal, with a view to their potential application in quantum memories or repeaters. We find electron spin-lattice relaxation times are maximized at low magnetic field (<100 mT), where g ∼ 6, reaching 5 s at 2.5 K, while coherence times are maximized when addressing ESR transitions at higher fields where g ∼ 0.7, where a Hahn echo measurement yields T2 up to 73 μs. Dynamical decoupling can be used to suppress spectral diffusion and extend the coherence lifetime to over 0.5 ms, close to the limit of instantaneous diffusion. Using Davies electron-nuclear double resonance, we performed coherent control of the 173Yb3+ nuclear spin and studied its relaxation dynamics. At around 4.5 K we measure nuclear spins T1 and T2 of 4 and 0.35 ms, respectively, about 4 and 14 times longer than the corresponding times for the electron spin. DOI: 10.1103/PhysRevB.97.064409 I. INTRODUCTION mainly due to its low natural abundance of nuclear spins which would otherwise lead to spin decoherence [22,23]. Paramagnetic rare-earth (RE) ions in optical crystals are + Yb3 in YSO has already been shown to exhibit good optical rich systems possessing electron and nuclear spins and optical properties like high oscillator strengths, low inhomogeneous transitions [1], making them attractive for coherent interactions linewidths, and favorable branching ratios into narrow tran- with both optical and microwave photons [2–6]. The excel- + + + sitions in comparison to Er3 ,Pr3 , and Eu3 in the same lent coherence properties of RE optical transitions have lent host [20]. The maximal electron g factor (g ∼ 6) of the themselves to photon memories for quantum repeaters [7–9], max Yb:YSO ground state lies somewhere between that of other while their nuclear spin degree of freedom has demonstrated paramagnetic REs such as Nd:YSO (g ∼ 4.2) [5] and the capability for long-term coherent storage of quantum max Er:YSO (g ∼ 15.5) [24]. A larger g factor is beneficial information [5,6,10–12]. Embedded in microwave cavities, the max for enhancing the cooperativity between spins and microwave collective spin dynamics of ensembles of paramagnetic RE cavities, important in developing microwave quantum mem- ions have been investigated with a view to develop efficient and ories [3,13] and microwave-to-optical quantum transducers faithful microwave memories [3,13,14] and microwave-optical [25]. However, larger g factors often come at the expense of conversion [4,15]. increased decoherence rates due to stronger coupling to other Compared to other paramagnetic RE ions used for quantum + + spins, leading to spectral diffusion and instantaneous diffusion memories, like Er3 [9,16] and Nd3 [8,17], ytterbium ions + [21]. A detailed understanding of decoherence and relaxation (Yb3 ) have a number of potential advantages [18–20]. As + + processes for paramagnetic RE ions is therefore essential with Er3 and Nd3 , the optical transitions of ytterbium are for identifying the optimum species, sites, and transitions to accessible via single-mode laser diodes in the near infrared. address the different quantum technological applications. However, the lower nuclear spin quantum number of ytterbium + + Here, we investigate the relaxation and the decoherence (e.g., I = 1/2for171Yb3 and I = 5/2for173Yb3 ) with + + dynamics of electron and nuclear spins of Yb:YSO using respect to 145Nd3 and 167Er3 (for both I = 7/2) is beneficial pulsed electron spin resonance (ESR) at the X band (9.8 for addressing optical transitions between ground and excited GHz) and electron-nuclear double resonance (ENDOR) in the spin states and initialization into a ground spin state. Coherence temperature range 2–8 K. We identify multiple electron spin- times up to 130 μs have also been observed for electron spin + lattice relaxation processes in Yb:YSO, combining our pulsed resonance transitions in Yb3 :CaWO [21]. 4 ESR measurements with optical spectroscopy of the ground- Single-crystal yttrium orthosilicate (Y SiO , commonly 2 5 state multiplet [20]. We find that spectral diffusion [21,26] referred to as YSO) has been a reference host material for is the most common source of decoherence for electron spins, quantum information processing research and applications, further evidenced by stimulated echo decay measurements, and are able to suppress it considerably using dynamical decou- pling [27]. Using Davies ENDOR, we explored the coherent *[email protected] spin properties of the 173Yb nuclear spins, including nuclear †[email protected] spin Rabi oscillations, studying inhomogeneous broadening 2469-9950/2018/97(6)/064409(9) 064409-1 ©2018 American Physical Society LIM, WELINSKI, FERRIER, GOLDNER, AND MORTON PHYSICAL REVIEW B 97, 064409 (2018) (a) 6 1/2 (b) Site I 0 0.8 0 - 1/2 4 0.6 168, 170, 0.4 172,174,176 3+ –1/21 2 Yb 0.2 +1/21 -1/2 +3/21 (I = 0) –5/21112 +5/22–3/2 –1/2 +5/21 0 amplitude (a.u.) 0.0 171Yb 3+ Electron spin echo 900 950 1000 1050 1100 –2 (I = 1/2) B [mT] Energy (GHz) (c) –4 0.5 –1/2 ×0.25 Site II - 1/2 0 +1/2 0 0.4 –6 B 1/2 0.3 Site I, //b +1/2 6 –5/2 0.2 +5/2 –1/2 –3/2 +3/2 –3/2 –5/2 –1/2 ( ) +1/2 0.1 * ( ) ( ) ( ) 4 +3/2 amplitude (a.u.) * * * +5/2 Electron spin echo 60 80 100 120 140 160 180 2 (d) B [mT] 173Yb 3+ 0 I 0.0 B = 989 mT // b ( = 5/2) B -0.2 = 1025 mT // bD1 [0.5˚] B (+) = 1002 mT // bD1 [1˚] +1/2 : +5/2 –2 -0.4 −1 +3 (−) Energy (GHz) +5/2 /2 : /2 +3/2 -0.6 −1 +3 (+) −1 –4 +1/2 −1 +1 (+) /2 : /2 /2 : /2 : /2 (−) –1/2 -0.8 (+) −1 +1 (−) +3 –3/2 −1/2 : +3/2 /2 : /2 /2 –6 –5/2 ENDOR signal (a.u.) -1.0 0 400 800 900 1000 1100 180180 200 220 240 B (mT) Frequency [MHz] FIG. 1. (a) Spin energy-level diagram for the various isotopes of Yb, in site I, as a function of magnetic field B applied along the crystal axis b. Dashed lines indicate allowed ESR transitions (mI = 0) at 9.8 GHz, corresponding to peaks observed in (b), the corresponding electron spin-echo-detected field sweep (EDFS) spectrum measured at 7 K. A similar EDFS spectrum for site II is shown in (c). In both spectra, peaks are labeled according to nominal mI values for convenience, although the spin states show considerable mixing. In addition, peaks labeled with 173 an asterisk correspond to forbidden ESR transitions (mI =±1). (d) Davies electron-nuclear double resonance (ENDOR) spectra of Yb in site I at 5 K. Peaks are labeled according to our best estimate of the mI states involved, with the superscript indicating the upper (+) or lower (−) electron spin manifold. ∗ e through the nuclear spin T2 , nuclear spin relaxation time T1n, dipole strength γx , which varies considerably as a function of and the nuclear spin coherence time T2n. magnetic field and across different sites [Eq. (S3) [31]]. Figure 1(a) illustrates the allowed ESR transitions expected for site I, with the magnetic field B0 applied close to the crystal II. ELECTRON AND NUCLEAR SPIN SPECTROSCOPY axis b and geff equal to 0.70. Due to the imperfect alignment ◦ The YSO crystal sample was grown using the Czochralski with b (estimated to be of the order of 2 ), the spectrum of method [28], cut along the b−D1−D2 principal dielectric axes only one subsite is seen in each plot. At the X band, the + and doped with Yb3 (natural isotopic abundance) at a nominal spin eigenstates are considerably mixed, so mS,I are not concentration of 0.005 at. % (50 ppm). Yb isotopes with a good quantum numbers and are used only as a qualitative nonzero nuclear spin number are 171Yb (I = 1/2) and 173Yb identification of the states. Figures 1(b) and 1(c) show the (I = 5/2), which respectively constitute 14% and 16% of the echo-detected ESR spectra (B0 b) from Yb ions in sites I total Yb concentration, with the remaining 70% composed of and II, highlighting the large difference in geff between the I = 0 isotopes. In YSO, which has a monoclinic structure and two sites. For each site, sets of ESR peaks can be identified 6 from their hyperfine coupling to the 171Yb and 173Yb isotopes C2h (C2/c) space group, Yb can substitute Y located in two crystallographic sites with C1 point symmetry [29], denoted in addition to a single, intense resonance from the family of site I and site II [20,30]. Each site has two subsites which are Yb isotopes with zero nuclear spin. The peaks have intensities magnetically equivalent only when the applied magnetic field consistent with the natural isotopic composition of Yb and arise = is parallel or perpendicular to the C2 symmetry axis (the crystal primarily from allowed ESR transitions (mI 0), although =± axis b).