Coherence and Rydberg Blockade of Atomic Ensemble Qubits

Abstract

We demonstrate |W state encoding of multi-atom ensemble qubits. Using optically trapped Rb atoms the T2 coherence time is 2.6(3) ms forN = 7.6 atoms and scales approximately inversely with the number of atoms. Strong Rydberg blockade between two ensemble qubits is demonstrated with a fidelity of 0.89(1) and a fidelity of ∼ 1.0 when postselected on control ensemble excitation. These results are a significant step towards deterministic entanglement of atomic ensembles.PACS numbers: 42.50.Dv, 32.80.Rm Qubits encoded in hyperfine states of neutral atoms are a promising approach for scalable implementation of quantum information processing [1]. While a qubit can be encoded in a pair of ground states of a single atom, it is also possible to encode a qubit, or even multiple qubits, in an N atom ensemble by using Rydberg blockade to enforce single excitation of one of the qubit states [2,3]. Ensemble qubits have several interesting features in comparison to single atom qubits. Using an array of traps it is simpler to prepare many ensemble qubits with N ≥ 1 for each ensemble, than it is to prepare an array with exactly one atom in each trap which remains an outstanding challenge [4][5][6]. In addition, a |W state ensemble qubit encoding is maximally robust against loss of a single atom [7], which can be remedied with error correction protocols [8], while atom loss is a critical error for single atom qubits. Furthermore an ensemble encoding facilitates strong coupling between atoms and light, an essential ingredient for quantum networking protocols [9] and atomic control of photonic interactions in Rydberg blockaded ensembles [10]. As the atomlight coupling strength grows with the number of atoms, recent experiments[10], [11] and theory proposals [12] are based on ensembles with N > 100. We are focused here on studying the physics of ensembles for computational qubits and therefore work with smaller ensembles with up to N ∼ 10 atoms.In this letter we demonstrate and study the coherence and interactions of atomic ensemble qubits. We measure the T 2 coherence time of ensemble qubits achieving a ratio of coherence time to single qubit π rotation time of ∼ 2600. We furthermore proceed to demonstrate strong Rydberg blockade between two, spatially separated ensemble qubits. Together with the recent demonstration of entanglement between a Rydberg excited ensemble and a propagating photon[13] these results establish a path towards both local and remote entanglement of arrays of ensemble qubits, which will enable enhanced quantum repeater architectures [14].

Effects of molecular resonances on Rydberg blockade

“…We also neglected the s-d excitation channels (allowed in the excitation scheme [10]) when computing molecular Rabi frequencies. Moreover, the experiment [10] (and similar experiments [5,11,17,18]) are affected by a multitude of other decoherence effects. At this point it may be desirable to design experiments that could disentangle various decoherence mechanisms, and the molecular losses in particular.…”

“…This mechanism enables performing quantum logic operations between atom pairs and manipulate collective many-body states of an Natom ensemble [1]. Such collective states efficiently couple to laser fields with the coupling enhanced by a factor of √ N ; see experiments [10,11]. While a number of advanced protocols involving Rydberg blockade are being explored, an outstanding challenge is to identify and realize conditions for high-fidelity atomic and optical state control via Rydberg blockade.…”

A high repetition rate experimental setup for quantum non-linear optics with cold Rydberg atoms

“…In particular, we are interested in generating well-defined, reproducible microwave fields as a means to control interactions between Rydberg excitations. Similar overall requirements also apply for applications of Rydberg atoms outside quantum non-linear optics, such as single ion [37] and atom sources [38], and collectively encoded ensemble qubits [39,40].…”

Grover search algorithm with Rydberg-blockaded atoms: quantum Monte Carlo simulations

“…The interactioninduced level shifts suppress resonant optical excitation of Rydberg states of more than one atom within a certain blockade distance from each other [1,2]. This blockade effect can then be used to implement quantum logic gate operations between closely spaced atoms [3][4][5][6][7][8], or to realize atomic ensemble qubits with Rydberg superatoms which can accommodate at most one collective Rydberg excitation at a time [9][10][11][12][13].…”