We demonstrate coherent control of the fine-structure qubit in neutral strontium atoms. This qubit is encoded in the metastable ^{3}P_{2} and ^{3}P_{0} states, coupled by a Raman transition. Using a magnetic quadrupole transition, we demonstrate coherent state initialization of this THz qubit. We show Rabi oscillations with more than 60 coherent cycles and single-qubit rotations on the μs scale. With spin echo, we demonstrate coherence times of tens of ms. Our results pave the way for fast quantum information processors and highly tunable quantum simulators with two-electron atoms.
We demonstrate coherent control of the fine-structure qubit in neutral strontium atoms. This qubit is encoded in the metastable ^{3}P_{2} and ^{3}P_{0} states, coupled by a Raman transition. Using a magnetic quadrupole transition, we demonstrate coherent state initialization of this THz qubit. We show Rabi oscillations with more than 60 coherent cycles and single-qubit rotations on the μs scale. With spin echo, we demonstrate coherence times of tens of ms. Our results pave the way for fast quantum information processors and highly tunable quantum simulators with two-electron atoms.
We report on the first realization of a novel neutral atom qubit encoded in the spin-orbit coupled metastable states ^{3}P_{0} and ^{3}P_{2} of a single ^{88}Sr atom trapped in an optical tweezer. Raman coupling of the qubit states promises rapid single-qubit rotations on par with the fast Rydberg-mediated two-body gates. We demonstrate preparation, readout, and coherent control of the qubit. In addition to driving Rabi oscillations bridging an energy gap of more than 17 THz using a pair of phase-locked clock lasers, we also carry out Ramsey spectroscopy to extract the transverse qubit coherence time T_{2}. When the tweezer is tuned into magic trapping conditions, which is achieved in our setup by tuning the tensor polarizability of the ^{3}P_{2} state via an external control magnetic field, we measure T_{2}=1.2 ms. A microscopic quantum mechanical model is used to simulate our experiments including dominant noise sources. We identify the main constraints limiting the observed coherence time and project improvements to our system in the immediate future. Our Letter opens the door for a so-far-unexplored qubit encoding concept for neutral atom-based quantum computing.
Neutral atoms are a ubiquitous platform for various quantum sciences with applications in quantum computing, simulation, and metrology across many different atomic species. In these systems, control over the electronic states is given by suitable laser pulses typically applied at the single-particle level. Here, we present fast coherent control over the meta-stable clock states in bosonic strontium-88 using two- and three-photon couplings. Compared to the direct single-photon drive on these transitions, the realistically achievable Rabi frequencies are substantially higher at significantly relaxed magnetic field strength. By applying a magnetic quantization axis under a judiciously chosen angle with respect to the linear trap polarization, we realize an optical qutrit comprising the ground state 1S0 as well as both clock states 3P0 and 3P2, and characterize in detail the coherence of the system. Remarkably, the observed coherence time on the fine-structure qubit (given by the 3P0 and 3P2,mj=0 states) of 715(30) ms demonstrates, for the first time, that state-of-the-art coherence times are compatible with tweezer-based polarizability-engineered trapping potentials, which is applicable across a broad range of wavelengths and atomic species. We identify laser phase noise as the main limitation of the current operation fidelity and discuss methods for characterizing, modelling, and compensating optical phase noise. The multi-photon couplings not only allow for substantially faster Rabi frequencies under realistic experimental conditions, but additionally offer controlled dissipation via the 3P1 state, which we use to demonstrate a measurement-free qubit reset. Combined with three-photon couplings to the 3P2 clock state, we realize resolved sideband cooling in an optical lattice potential and benchmark the fine-structure qubit at various infrared wavelengths. In addition, we introduce a hybrid lattice-tweezer setup and demonstrate the preparation of programmable atom configurations in a single plane of the two-dimensional optical lattice potential. Using single-site addressing with optical tweezers, we present a protocol for the iterative assembly of large-scale atom arrays. Furthermore, we use the lattice-tweezer combination to continuously operate a configurable atom array, which provides means to overcome scaling limitations by repeatedly adding atoms to the system and enables entirely novel approaches to operating neutral-atom arrays. To enable reliable image processing with a fast reconstruction of the lattice occupation, we introduce a novel algorithm based on a convolutional neural network, which provides a rapid and noise-resilient reconstruction compatible with arbitrary lattice geometries. Moreover, we benchmark a new phase-only spatial light modulator based on a micromirror array, which paves the way towards holographic beam shaping at fast update rates compatible with a broad range of wavelengths, including the ultraviolet spectral range.
We report on the coherent excitation of the ultranarrow ^{1}S_{0}-^{3}P_{2} magnetic quadrupole transition in ^{88}Sr. By confining atoms in a state insensitive optical lattice, we achieve excitation fractions of 97(1)% and observe linewidths as narrow as 58(1) Hz. With Ramsey spectroscopy, we find coherence times of 14(1) ms, which can be extended to 266(36) ms using a spin-echo sequence. We determine the lifetime of the ^{3}P_{2} level for spontaneous emission of magnetic quadrupole radiation to be 110(31) min, confirming long-standing theoretical predictions. These results establish an additional clock transition in strontium and pave the way for applications of the metastable ^{3}P_{2} state in quantum computing and quantum simulations.
We present a detailed investigation of the ultranarrow magnetic-quadrupole $^{1}\mathrm{S}_{0}\text{\ensuremath{-}}^{3}\mathrm{P}_{2}$ transition in neutral strontium and show how it can be made accessible for quantum simulation and quantum computation. By engineering the light shift in a one-dimensional optical lattice, we perform high-resolution spectroscopy and observe the characteristic absorption patterns for a magnetic quadrupole transition. We measure an absolute transition frequency of 446,647,242,704(2) kHz in 88Sr and an $^{88}\mathrm{Sr}\text{\ensuremath{-}}^{87}\mathrm{Sr}$ isotope shift of +62.91(4) MHz. In a proof-of-principle experiment, we use this transition to demonstrate local addressing in an optical lattice with 532 nm spacing with a Rayleigh-criterion resolution of 494(45) nm. Our results pave the way for applications of the magnetic quadrupole transition as an optical qubit and for single-site addressing in optical lattices.
Neutral atoms trapped in optical tweezers and excited to Rydberg states, together with trapped ions, are among the most advanced platforms for quantum simulation and quantum computing. Current experiments often rely on additional atoms in neighboring traps to encode ancilla qubits for local manipulation and readout. Here, we demonstrate a dual ion-Rydberg system comprising two qubits encoded in two individually controlled electrons of the same alkaline-earth atom. The first, a microwave qubit, is encoded in a pair of circular Rydberg states, while the second, an optical qubit, is encoded on a narrow quadrupole transition of the Rydberg atom's ionic core. We demonstrate coherent control of the optical qubit and achieve coherence times of several hundred microseconds under dynamical decoupling. Furthermore, we realize coherent coupling between the two electrons via electrostatic quadrupole interactions over the large separation between the Rydberg electron and the ionic core, and map out its angular tunability. Finally, we demonstrate a two-qubit operation, reminiscent of a Mølmer-Sørensen gate, that evolves through an entangled state of the two qubits driven by the quadrupole coupling. Our work opens a pathway to exploit a pair of individually controlled electronic qubits with tunable coupling for quantum simulation and quantum metrology.
The optical clock states of alkaline earth and alkaline earthlike atoms are the fundament of state-of-the-art optical atomic clocks. An important prerequisite for the operation of optical clocks is the magic trapping conditions where electronic and motional dynamics decouple. Here, we identify and experimentally demonstrate simultaneous magic trapping for two clock transitions in ^{88}Sr, realizing so-called triple-magic conditions at a specially chosen magic angle. Under these conditions, we operate an all-optical qutrit comprising the ground state ^{1}S_{0}, and the two metastable clock states ^{3}P_{0} and ^{3}P_{2}. We demonstrate fast optical control in an atom array using two- and three-photon couplings to realize high-fidelity manipulation between all qutrit states. At the magic angle, we probe the coherence achievable in magic-angle-tuned traps and find atom-atom coherence times between the metastable states as long as 715(30) ms. Our work opens several new directions, including qutrit-based quantum metrology on optical transitions and high-fidelity and high-coherence manipulation on the ^{88}Sr fine-structure qubit.