Alkaline-earth Rydberg atoms are very promising tools for quantum technologies. They benefit from the remarkable properties of Rydberg atoms and, notably, a huge coupling to external fields and to other Rydberg atoms while the ionic core retains an optically active electron. However, low angular-momentum Rydberg states suffer almost immediate autoionization when the core is excited. Classically, the two electrons “collide” and one is expelled. However, this is not the case for circular states. Here, we demonstrate that Strontium circular Rydberg atoms with a core excited in a 4d metastable level are stable for more than a few milliseconds. We verify this property while the ionic core is either in a 4d, j = 3/2 state or a 4d, j = 5/2 state. Furthermore, we measure the energy difference between the Zeeman sublevels of metastable 4d states of the ionic-core electron, resulting from the electric-quadrupole coupling between the two valence electrons, with one prepared in a Rydberg state. We show that the energy difference is a function of the principal and magnetic quantum numbers of the Rydberg electron. We show that this coupling between the two valence electrons leads to control the state of one depending on the state of the other. This opens the way to detect non-destructively the state of the Rydberg atom. Using the fluorescence emitted by the core electron allows us to envision a selective, optical detection of the circular states with remarkable sensitivity. The results presented in this work are important in the context of the realization of a Rydberg quantum simulator, exploiting the circular states of Strontium to build the qubits.
Alkaline-earth Rydberg atoms are very promising tools for quantum technologies. They benefit from the remarkable properties of Rydberg atoms and, notably, a huge coupling to external fields and to other Rydberg atoms while the ionic core retains an optically active electron. However, low angular-momentum Rydberg states suffer almost immediate autoionization when the core is excited. Classically, the two electrons “collide” and one is expelled. However, this is not the case for circular states. Here, we demonstrate that Strontium circular Rydberg atoms with a core excited in a 4d metastable level are stable for more than a few milliseconds. We verify this property while the ionic core is either in a 4d, j = 3/2 state or a 4d, j = 5/2 state. Furthermore, we measure the energy difference between the Zeeman sublevels of metastable 4d states of the ionic-core electron, resulting from the electric-quadrupole coupling between the two valence electrons, with one prepared in a Rydberg state. We show that the energy difference is a function of the principal and magnetic quantum numbers of the Rydberg electron. We show that this coupling between the two valence electrons leads to control the state of one depending on the state of the other. This opens the way to detect non-destructively the state of the Rydberg atom. Using the fluorescence emitted by the core electron allows us to envision a selective, optical detection of the circular states with remarkable sensitivity. The results presented in this work are important in the context of the realization of a Rydberg quantum simulator, exploiting the circular states of Strontium to build the qubits.
Couplage quadripolaire électrique entre l'électron du noyau ionique et l'électron circulaire de Rydberg d'un atome alcalino-terreux Les atomes de Rydberg alcalino-terreux sont des outils très prometteurs pour les technologies quantique. Ils profitent des propriétés remarquables des atomes de Rydberg et, notamment, un énorme couplage aux champs externes ou à d’autres atomes de Rydberg tandis que le noyau ionique conserve un électron optiquement actif. Cependant, les états de Rydberg de faible moment angulaire subissent une auto-ionisation presque immédiate lorsque le noyau est excité. Classiquement, les deux électrons « entrent en collision » et un est expulsé. Cependant, ce n’est pas le cas pour les états circulaires. Ici, nous démontrons que les atomes de Strontium où un électron est dans un état de Rydberg circulaire, avec un noyau excité dans un niveau métastable 4d sont stables pendant plus de quelques millisecondes. Nous vérifions cette propriété pour un noyau ionique soit dans un état 4d, j = 3/2, soit dans un état 4d, j = 5/2. Aussi, nous mesurons la différence d’énergie entre les états 4d métastables de l’électron du noyau ionique, résultant du couplage électrostatique entre les deux électrons de valence, avec l’un préparé dans un état Rydberg. Nous montrons que cette différence d’énergie est fonction des nombres quantiques principaux et magnétiques de l’électron de Rydberg. Nous montrons que le couplage électrostatique entre les deux électrons de valence amène à contrôler l’état de l’un par l’état de l’autre. Cela ouvre la voie à la détection non destructive de l’état de l’atome de Rydberg. L’utilisation de la fluorescence émise par l’électron du coeur permet d’envisager la détection optique selective des états circulaires avec une sensibilité remarquable. Les résultats présentés dans ce travail sont importants dans le cadre de la réalisation d’un simulateur quantique Rydberg, exploitant les états circulaires du Strontium pour construire les qubits.
Alkaline earth Rydberg atoms are very promising tools for quantum technologies. Their highly excited outer electron provides them with the remarkable properties of Rydberg atoms and, notably, with a huge coupling to external fields or to other Rydberg atoms while the ionic core retains an optically active electron. However, low angular-momentum Rydberg states suffer almost immediate autoionization when the core is excited. Here, we demonstrate that strontium circular Rydberg atoms with a core excited in a 4D metastable level are impervious to autoionization over more than a few millisecond time scale. This makes it possible to trap and laser-cool Rydberg atoms. Moreover, we observe singlet to triplet transitions due to the core optical manipulations, opening the way to a microwave to optical quantum interface.
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.
Divalent atoms provide excellent means for advancing control in Rydberg atom-based quantum simulation and computing due to the second optically active valence electron available. Particularly promising in this context are circular Rydberg atoms, for which long-lived ionic core excitations can be exploited without suffering from detrimental autoionization. Here, we report the implementation of electric quadrupole coupling between the metastable 4D_{3/2} level and a very high-n (n=79) circular Rydberg qubit, realized in doubly excited ^{88}Sr atoms prepared from an optical tweezer array. We measure the kHz-scale differential level shift on the circular Rydberg qubit via beat-node Ramsey interferometry comprising spin echo. Observing this coupling requires coherent interrogation of the Rydberg states for more than 100 μs, which is assisted by tweezer trapping and circular state lifetime enhancement in a black-body radiation suppressing capacitor. Further, we find no noticeable loss of qubit coherence under continuous photon scattering on the ion core, paving the way for laser cooling and imaging of Rydberg atoms. Our results demonstrate access to weak electron-electron interactions in Rydberg atoms and expand the quantum simulation toolbox for optical control of highly excited circular state qubits via ionic core manipulation.
In recent years, advances in optical tweezers arrays have enabled the trapping and control of individual atoms. This makes it possible to simulate condensed matter systems by arranging atoms in an array that mimics the crystalline structure of real materials. This approach, known as quantum simulation, was first envisioned by R. Feynman and is now being implemented by several research groups worldwide. In order to induce interactions between atoms, they are prepared in highly excited atomic states, known as Rydberg states. In particular, circular Rydberg states, which possess maximal angular momentum, exhibit lifetimes of several milliseconds, much longer than those of other Rydberg states. This property makes them particularly interesting for the development of new quantum technologies. In this work, we present the first building blocks of a quantum simulator based on circular Rydberg states of strontium atoms. This includes the development of a new cryogenic apparatus in which individual strontium atoms are trapped in optical tweezers. Using an alkaline-earth element allows us to exploit its two valence electrons for different tasks. One electron is excited to the circular state with principal quantum number n = 52, while the ionic core behaves as a Sr+ ion, which can be directly trapped by the tweezers. Moreover, we observe coherent oscillations between singlet and triplet spin states, due to the spin-orbit coupling of the electron in the circular state. These oscillations allow us to measure, with high precision, QED corrections to the Landé g5S1/2 factor of the ground state of Sr+.
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).