We investigate magnetic excitations in the spin-ladder compound Sr 14 Cu 24 O 41 using high-resolution Cu L 3 edge resonant inelastic x-ray scattering (RIXS). Our findings demonstrate that RIXS couples to two-triplon collective excitations. In contrast to inelastic neutron scattering, the RIXS cross section changes only moderately over the entire Brillouin zone, revealing high sensitivity also at small momentum transfers, allowing determination of the two-triplon energy gap as 100 AE 30 meV. Our results are backed by calculations within an effective Hubbard model for a finite-size cluster, and confirm that optical selection rules are obeyed for excitations from this spherically symmetric quantum spin-liquid ground state. DOI: 10.1103/PhysRevLett.103.047401 PACS numbers: 78.70.En, 71.10.Pm, 75.25.+z, 75.30.Ds Collective excitations in strongly correlated electron materials remain a pivotal challenge in contemporary solid state physics. It is widely debated whether magnetic excitations provide the pairing interaction in the hightemperature and unconventional superconductors [1,2]. From that perspective quantum spin systems attract considerable interest. While most such materials, e.g., the cuprate superconductors, exhibit enormous complexity, the two-leg spin ladder is easier to tract theoretically [3][4][5][6]. It consists of two parallel chains (legs) with a transverse (rung) exchange coupling. This system features a singlet ground state and dispersive triplet excitations (triplons), that both have quantum mechanical origin without any classical counterpart. To date, mainly two techniques have been established as momentum-and energy-resolved probes of the dispersion of collective excitations: angleresolved photoelectron spectroscopy and inelastic neutron scattering (INS) for charge and spin degrees of freedom, respectively [7,8]. Because of the latest instrumental improvements [9,10], the energy scale of magnetic exchange is becoming readily accessible for resonant inelastic x-ray scattering (RIXS) [11][12][13][14], which is promising to give information on both, spin and charge degrees of freedom, and in addition is an element-specific technique. Furthermore, RIXS requires only small sample volumes (<0:1 mm 3 ). Recent RIXS studies were performed on long-range ordered magnets with spin-wave excitations [15][16][17].In this Letter, we report a study of the two-leg quantum spin ladder Sr 14 Cu 24 O 41 [18,19] by means of momentumresolved high-resolution RIXS at the Cu L 3 edge. Given that Cu L 3 scattering experiments have been already shown to contain valuable information about the charge degrees of freedom [20], an outstanding question we would like to address here is: how can RIXS provide information on magnetic excitations from a quantum ground state. In the ladder system of Sr 14 Cu 24 O 41 no symmetry breaking occurs-neither in spin-nor in real-space-in contrast to, e.g., a magnetically ordered state, where both symmetries are broken and the direction of the ordered moments dictates the quantization axi...

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