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arXiv:1901.05685 · Physical Review Letters

Single-Shot Nondestructive Detection of Rydberg-Atom Ensembles by Transmission Measurement of a Microwave Cavity

Sébastien Garcia, M. Stammeier, Johannes Deiglmayr, Frédéric Merkt, Andreas J. Wallraff

Published Nov 8, 2019• 18 citations
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Abstract

We present an experimental realization of single-shot nondestructive detection of ensembles of helium Rydberg atoms. We use the dispersive frequency shift of a superconducting microwave cavity interacting with the ensemble. By probing the transmission of the cavity, we determine the number of Rydberg atoms or the populations of Rydberg quantum states when the ensemble is prepared in a superposition. At the optimal microwave probe power, determined by the critical photon number, we reach single-shot detection of the atom number with 13% relative precision for ensembles of about 500 Rydberg atoms with a measurement backaction characterized by approximately 2% population transfer.

DOI
10.1103/physrevlett.123.193201

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We present an experimental realization of single-shot nondestructive detection of ensembles of helium Rydberg atoms. We use the dispersive frequency shift of a superconducting microwave cavity interacting with the ensemble. By probing the transmission of the cavity, we determine the number of Rydberg atoms or the populations of Rydberg quantum states when the ensemble is prepared in a superposition. At the optimal microwave probe power, determined by the critical photon number, we reach single-shot detection of the atom number with 13% relative precision for ensembles of about 500 Rydberg atoms with a measurement backaction characterized by approximately 2% population transfer.
In the last decades the interest in studying interactions of Rydberg atoms or ensembles thereof with electromagnetic fields at optical and microwave frequencies has steadily increased. The physics Nobel prize of 2012, for instance, has been partially awarded to cavity quantum electrodynamics (QED) experiments with single Rydberg atoms that allow for quantum nondemolition measurements of the photon number in a superconducting microwave cavity. These experiments detect the dispersive shift of the atomic transition frequency that results from the interaction with the photons to measure the photon number nondestructively. In this thesis, we employ the reversed effect, the dispersive shift of the resonance frequency of the TE301 mode of a rectangular microwave cavity induced an ensemble of helium Rydberg atoms, to perform nondestructive measurements of the atom number. In a first experiment, we measure the dispersive interaction between a few thousand atoms in the 36s singlet state and a copper cavity with a quality factor of about Q = 5 · 10^3. We detect the dispersive shift by continuously measuring the changes in cavity transmission with a weak coherent probe tone that injects a few hundred photons into the cavity. We model the observed time dependence of the dispersive shift, which results from the interplay between the time-dependent collective interaction strength and atom-cavity detuning. Controlling the parameters of the atom cloud, we then directly measure the dependence of the dispersive shift on the atom-cavity detuning and on the number of Rydberg atoms. The results are in good agreement with the Tavis-Cummings Hamiltonian in the dispersive regime, and indicate collective coupling strengths of gN/2pi = 1 MHz, which correspond to more than N = 3 · 10^3 Rydberg atoms. This nondestructive detection of Rydberg atoms achieves a relative precision of 4.5% and an accuracy of 20 %, which are both limited by the inhomogeneity of the atom-cavity detuning over the size of the atom cloud. In a second experiment, we reduce the inhomogeneity by decreasing the size of the atomic cloud and employing the triplet Rydberg transition at n = 42, which is less sensitive to dc electric fields. The precision of the atom number detection is further enhanced by increasing the cavity quality factor to Q = 10^5 with a superconducting niobium cavity. The combined improvements result in the nondestructive detection of a few hundred Rydberg atoms with an accuracy of 3% and a precision of 1 %, which is an order of magnitude improvement over the previous experiment. Motivated by these results obtained from averaging over about 4 × 10^4 repetitions of the experiment, we consider the nondestructive detection of the atom number in single-shot measurements with a high number of photons in the cavity. By studying the dispersive shift and the probability to excite the 42p states as functions of the photon number, we find a critical photon number ncrit = 4.4×10^4 close to the estimated value and verify that the excitation probability remains below 2.2% for all photon numbers. For optimal precision, we then use a photon number close to the critical value and perform single-shot measurements of the atom number that show a linear correlation between the results of the nondestructive detection method using the dispersive shift and the destructive detection with a microchannel plate. We achieve a single-shot uncertainty of delta N = 65 in the nondestructive atom number detection, which is close to the width of a Poissonian distribution square-root(N) = 25 for the highest atom numbers N = 600. We estimate that the precision in the single-shot measurements can be further enhanced to delta N = 5 (delta N/N = 1 %) by using quantum-limited amplifiers and increasing the single-shot integration time with Stark deceleration of the Rydberg atoms. Such precise nondestructive single-shot measurements of the number of Rydberg atoms are relevant for the quantitative evaluation of scattering cross-sections in experiments with Rydberg atoms and molecules. Moreover, the demonstrated collective coupling strengths open up the prospect for hybrid cavity QED with Rydberg atoms and superconducting quantum bits in rectangular waveguide cavities.
In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispersive interaction between an ensemble of helium atoms in the 37s37s37s Rydberg state and a single resonator mode by extracting the amplitude and phase change of a weak microwave probe tone transmitted through the cavity. The results are in quantitative agreement with predictions made on the basis of the dispersive Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a hybrid between superconducting circuits and Rydberg atoms. We measure maximal collective coupling strengths of 1 MHz, corresponding to $3\ifmmode\times\else\texttimes\fi{}{10}^{3}$ Rydberg atoms coupled to the cavity. As expected, the dispersive shift is found to be inversely proportional to the atom-cavity detuning and proportional to the number of Rydberg atoms. This possibility of measuring the number of Rydberg atoms in a nondestructive manner is relevant for quantitatively evaluating scattering cross sections in experiments with Rydberg atoms.
Single-photon detection is an essential component in many experiments in quantum optics, but it remains challenging in the microwave domain.We realize a quantum nondemolition detector for propagating microwave photons and characterize its performance using a single-photon source.To this aim, we implement a cavity-assisted conditional phase gate between the incoming photon and a superconducting artificial atom.By reading out the state of this atom in a single shot, we reach an external (internal) photon-detection fidelity of 50% (71%), limited by transmission efficiency between the source and the detector (75%) and the coherence properties of the qubit.By characterizing the coherence and average number of photons in the field reflected off the detector, we demonstrate its quantum nondemolition nature.We envisage applications in generating heralded remote entanglement between qubits and for realizing logic gates between propagating microwave photons.
Single-photon detection is an essential component in many experiments in quantum optics, but it remains challenging in the microwave domain. We realize a quantum nondemolition detector for propagating microwave photons and characterize its performance using a single-photon source. To this aim, we implement a cavity-assisted conditional phase gate between the incoming photon and a superconducting artificial atom. By reading out the state of this atom in a single shot, we reach an external (internal) photon-detection fidelity of 50% (71%), limited by transmission efficiency between the source and the detector (75%) and the coherence properties of the qubit. By characterizing the coherence and average number of photons in the field reflected off the detector, we demonstrate its quantum nondemolition nature. We envisage applications in generating heralded remote entanglement between qubits and for realizing logic gates between propagating microwave photons.
We propose and analyze an experimental scheme of quantum nondemolition detection of monophotonic and vacuum states in a superconductive toroidal cavity by means of Rydberg atoms. One of the key directions of modern physics is the following challenging problem of understanding the essence of the process of quantum measurement. Of special interest in this respect are experiments with individual quantum objects. Such applications as quantum computing, quantum cryptography and quantum teleportation, which have recently been attracting increasing attention [1], have their roots in this field. Quantum measurements and particularly experiments on the interaction of individual atoms and ions with the quantum field in a cavity are usually associated with the optical domain. With the development of the Rydberg atom technique, however, impressive results have been obtained in the microwave region [2]. This technique allows preparation and nondestructive (quantum non-demolition) repetitive measurements of Fock states with a small number of quanta in a high-Q superconductive cavity [3, 4]. In 1994 V.B.Braginsky and F.Ya.Khalili [5] proposed an elegant scheme employing Rydberg atoms which allowed nondestructive detection of vacuum and monophotonic states. The idea of the experiment is to use a cavity with a geometry such that the flying atom can interact twice with the field.