Collectively Encoded Rydberg Qubit

We demonstrate a collectively-encoded qubit based on a single Rydberg excitation stored in an ensemble of N entangled atoms. Qubit rotations are performed by applying microwave fields that drive excitations between Rydberg states. Coherent read-out is performed by mapping the excitation into a single photon. Ramsey interferometry is used to probe the coherence of the qubit, and to test the robustness to external perturbations. We show that qubit coherence is preserved even as we lose atoms from the polariton mode, preserving Ramsey fringe visibility. We show that dephasing due to electric field noise scales as the fourth power of field amplitude. These results show that robust quantum information processing can be achieved via collective encoding using Rydberg polaritons, and hence this system could provide an attractive alternative coding strategy for quantum computation and networking. Quantum technology is increasingly expanding our capabilities in computing, sensing, metrology, and communications. Atomic systems, including those exploiting highly-excited Rydberg states are particularly attractive for quantum applications [1-6], as they offer a unique combination of precision [7], high-fidelity entanglement generation [8-12], scaling to 3D [13, 14], direct photonic read-out [15, 16] and strong photon-photon interactions [17-21]. Recently, remarkable progress has been made using individual Rydberg atoms for quantum simulation [22-26]. In parallel and across the full spectrum of quantum computing platforms, there has been considerable recent interest in the use of collective encoding strategies exploiting different spatial modes [27-29], internal states [30, 31], grid states [32, 33], and Schrodinger cat states [34]. In this paper, we demonstrate a new collective-coding scheme based on Rydberg polaritons [2, 4, 35]. The novel feature of our scheme is that the qubit is stored as a superposition of Rydberg polariton modes. One advantage of this scheme is that quantum information is distributed over many atoms as opposed to single atom encoding schemes. An additional advantage is that the po-lariton phase [36] enables direct photonic state read out in a well-defined spatial mode [18]. Also, the collective character of both qubit states causes the Rabi frequency for qubit rotations to be independent of the number of atoms. Large transition dipole moments between highly-excited Rydberg states (e.g. the radial matrix element for the |r = |60S 1/2 to |r = |60P 3/2 transition is 3684 Debye [38]) provide for fast coherent control and SWAP operations [39] on time scales of order nanosec-onds. Our scheme is scalable to many collective qubits using ensemble arrays [40], and could provide an alternative hybrid strategy for quantum networking exploiting microwave interactions [41, 42].