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Cavity QED with a Rydberg superatom: coherent control, single-shot detection, and conditional optical phase flips

Julien Vaneecloo, Sébastien Garcia, Alexei Ourjoumtsev

Published May 31, 2022• 0 citations
Publisher

Abstract

We demonstrate a new approach for creating deterministic photon-photon interactions for optical quantum engineering, based on a single-ended medium-finesse optical cavity containing a mesoscopic atomic ensemble. This ensemble is made transparent by a laser beam mapping intracavity photons into Rydberg polaritons. The transparency vanishes when the cloud, acting as a single collective two-level "superatom" with an enhanced coupling to light, is driven from the ground to a Rydberg state. We observe collectively-enhanced Rabi oscillations between these states, optically discriminate them in a single shot with a 95% effciency, and show that a change between the two internal states of the superatom induces a π phase rotation on the light reflected off of the cavity. These ingredients form a complete set of tools for implementing deterministic photonic entangling gates and for generating highly non-classical light without the need for a low-volume high-finesse cavity.

DOI
10.1117/12.2621484

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Journal of Physics Conference Series•Mar 14, 2012•3 citations
We demonstrate a new approach for creating deterministic photon-photon interactions for optical quantum engineering, based on a single-ended medium-finesse optical cavity containing a mesoscopic atomic ensemble. This ensemble is made transparent by a laser beam mapping intracavity photons into Rydberg polaritons. The transparency vanishes when the cloud, acting as a single collective two-level "superatom" with an enhanced coupling to light, is driven from the ground to a Rydberg state. We observe collectively-enhanced Rabi oscillations between these states, optically discriminate them in a single shot with a 95% effciency, and show that a change between the two internal states of the superatom induces a π phase rotation on the light reflected off of the cavity. These ingredients form a complete set of tools for implementing deterministic photonic entangling gates and for generating highly non-classical light without the need for a low-volume high-finesse cavity.
We demonstrate a new versatile building block for optical quantum technologies, enabling deterministic quantum engineering of light by combining the advantages of two complementary approaches: cavity quantum electrodynamics and interacting atomic ensembles.Our system is based on an intracavity Rydberg-blockaded atomic ensemble acting as a single two-level superatom.We coherently control its state and optically detect it in a single shot with 95% efficiency.Crucially, we demonstrate a superatom-statedependent π phase rotation on the light reflected from the cavity.Together with the state manipulation and detection, it is a key ingredient for implementing deterministic photonic entangling gates and for generating highly nonclassical light states.
With Rydberg dipole interactions, a mesoscopic atomic ensemble may behave like a two-level single atom, resulting in the so-called picture of superatom. It is in potential a strong candidate as a qubit in quantum information science, especially for efficient coupling with single photons via collective enhancement that is essential for building quantum internet to connect remote quantum computers. Previously, preliminary studies have been carried out in demonstrating basic concept of Rydberg superatom, a single-photon source, and entanglement with a single photon, etc. While a crucial element of single-shot qubit measurement is still missing. Here we realize the deterministic measurement of a superatom qubit via photon burst in a single shot. We make use of a low-finesse ring cavity to enhance the atom-photon interaction and obtain an in-fiber retrieval efficiency of 44%. Harnessing dipole interaction between two Rydberg levels, we may either create a sequence of multiple single photons or nothing, conditioned on the initial qubit state. We achieve a single-shot measurement fidelity of 93.2% in 4.8 us. Our work complements the experimental toolbox of harnessing Rydberg superatom for quantum information applications.
With Rydberg dipole interactions, a mesoscopic atomic ensemble becomes a superatom that behaves like a single atom but couples collectively with photons. It is potentially a strong candidate as a qubit in quantum information science, especially for quantum networks. In this paper, we report a cavity-enhanced single-photon interface for a Rydberg superatom and demonstrate deterministic qubit readout via a photon burst. We make use of a low-finesse ring cavity to enhance the atom–photon interaction and obtain in-fiber single-photon generation efficiency of 44%. Harnessing the dipole interaction between two Rydberg levels, we may create either a sequence of multiple single photons or nothing, conditioned on an initial qubit state. We achieve a measurement fidelity of 93.0% in 4.8 µs. Our work complements the experimental toolbox of harnessing Rydberg superatoms for quantum information applications.
Integrating Rydberg-state control of atom-atom interactions with cavity control of atom-light interactions enables the combination of local entangling operations with photon-mediated long-range coupling, measurement, and communication. Such integration is challenging because electric fields generated by charges on cavity optics and by cavity-stabilizing actuators may disturb Rydberg-state coherence. Here, we couple a Rydberg tweezer array to a high-cooperativity optical cavity, suppressing the electric fields produced by cavity length actuation so that the residual field variation of about 3 mV/cm3~\mathrm{mV/cm}3 mV/cm rms does not limit Rydberg coherence. We demonstrate two capabilities that neither system provides alone. First, we use Purcell-enhanced fluorescence to read out atoms directly in the ground-Rydberg basis within 7.5 μs7.5~μ\mathrm{s}7.5 μs, well within the Rydberg-state lifetime, with ground- and Rydberg-state detection infidelities of 3%3\%3% and 5%5\%5%, respectively, and with low atom loss. Second, we use Rydberg-Rydberg interactions to control collective light-atom scattering into the cavity mode. Specifically, we drive blockaded Rabi oscillations between a state with two ground-state atoms, which scatter collectively with a superradiant or subradiant rate controlled by the atomic positions, and an entangled state with a single Rydberg excitation, which scatters like a single atom. These capabilities open routes toward networked Rydberg processors, mid-circuit measurements in analog quantum simulators, and many-body quantum optics with entangled emitters coupled collectively to light.
Engineering quantum states of free-propagating light is of paramount importance for quantum technologies. Coherent states ubiquitous in classical and quantum communications, squeezed states used in quantum sensing, and even highly-entangled cluster states studied in the context of quantum computing can be produced deterministically, but they obey quasi-classical optical field statistics described by Gaussian, positive Wigner functions. Fully harnessing the potential of many quantum engineering protocols requires using Wigner-negative states, so far produced using intrinsically probabilistic methods. We achieved the first fully deterministic preparation of Wigner-negative free-propagating states of light by mapping the internal state of an intracavity Rydberg superatom onto an optical qubit encoded as a superposition of 0 and 1 photons [1]. This approach allows us to reach a 60% photon generation efficiency in a well-controlled spatio-temporal mode, while maintaining a strong photon antibunching. By changing the qubit rotation angle, we observe an evolution from quadrature squeezing to Wigner negativity. Our experiment sets this new technique as a viable method to deterministically generate highly non-classical photonic resources, lifting several major roadblocks in optical quantum engineering.
We discuss electromagnetically induced transparency in a cold ensemble of atoms optically excited to the Rydberg states. Strong dipole-dipole or van der Waals interactions between the atomic Rydberg states translate into large nonlocal nonlinearities for a propagating probe field. In the case of weak quantum fields, this can be used to attain a conditional phase shift of π between pairs of single photons realizing a deterministic photonic phase gate. For stronger fields, long-range interactions between the atoms constrain the medium to behave as a collection of superatoms, each comprising a blockade volume that can accommodate at most one Rydberg excitation. The propagation of a probe field is affected by its two-photon correlations within the blockade distance, which are strongly damped due to low saturation threshold of the superatoms. Our calculations reproduce and interpret the results of recent experiments.