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Quantum memory protocols in large cold atomic ensembles

Lucile Veissier

Published Dec 5, 2013• 2 citations
OpenAlex

Abstract

Quantum memories are an essential building block for quantum information science, and in particular for the implementation of quantum communications across long distances. A quantum memory is defined as a system capable of storing and retrieving quantum states on-demand, such as quantum bits (qubits). Atomic ensembles are good candidates for this purpose because they enables strong light-matter coupling in case of a large number of atoms. Moreover, the collective effect, enhanced in the regime of large optical depth, can lead to storage efficiency close to unity. Thus, in this thesis, a large magneto-optical trap for cesium atoms is used as a atomic medium in order to implement a quantum memory protocol based on electromagnetically induced transparency (EIT). First, the EIT phenomenon is studied through a criterion for the discrimination between the EIT and the Autler-Townes splitting models. We then report on the implementation of an EIT-based memory for photonic qubits encoded in orbital angular momentum (OAM) of light. A reversible memory for Laguerre-Gaussian modes is implemented, and we demonstrate that the optical memory preserves the handedness of the helical structure at the single-photon level. Then, a full quantum state tomography of the retrieved OAM encoded qubits is performed, giving fidelities above the classical bound. This showed that our optical memory operates in the quantum regime. Finally, we present the implementation of the so-called DLCZ protocol in our ensemble of cold atoms, enabling the generation of heralded single photons. A homodyne detection setup allows us to realize the quantum tomography of the created photonic state.

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In this paper, an optical memory based on an ensemble of cold Cesium atoms interacting with light in the electromagnetically induced transparency (EIT) regime is studied in view of spatially multimode quantum storage. The interfacing protocol involves a control field, generating EIT for the very weak field that carries the quantum signal to be stored and enabling its mapping into the collective hyperfine coherence of the cold atomic ensemble. For the demonstration of multimode storage, the signal pulse at the single photon level is prepared in a mode with (OAM), while the control field is in a TEM00 mode. After the writing process, the information is kept in the memory register. In the retrieval phase, the information is read back from atoms to light and we show that these pulses at the single photon level with OAM can be stored and retrieved with a preserved spatial phase and a good efficiency.

International Journal of Quantum Information•Dec 1, 2012•0 citations
Quantum memories are an essential building block for quantum information science, and in particular for the implementation of quantum communications across long distances. A quantum memory is defined as a system capable of storing and retrieving quantum states on-demand, such as quantum bits (qubits). Atomic ensembles are good candidates for this purpose because they enables strong light-matter coupling in case of a large number of atoms. Moreover, the collective effect, enhanced in the regime of large optical depth, can lead to storage efficiency close to unity. Thus, in this thesis, a large magneto-optical trap for cesium atoms is used as a atomic medium in order to implement a quantum memory protocol based on electromagnetically induced transparency (EIT). First, the EIT phenomenon is studied through a criterion for the discrimination between the EIT and the Autler-Townes splitting models. We then report on the implementation of an EIT-based memory for photonic qubits encoded in orbital angular momentum (OAM) of light. A reversible memory for Laguerre-Gaussian modes is implemented, and we demonstrate that the optical memory preserves the handedness of the helical structure at the single-photon level. Then, a full quantum state tomography of the retrieved OAM encoded qubits is performed, giving fidelities above the classical bound. This showed that our optical memory operates in the quantum regime. Finally, we present the implementation of the so-called DLCZ protocol in our ensemble of cold atoms, enabling the generation of heralded single photons. A homodyne detection setup allows us to realize the quantum tomography of the created photonic state.
We present an experimental study of two optical quantum memory systems based on electromagnetically induced transparency (EIT) in cold cesium atoms.We explain the relevance of quantum memories for the development of large-scale quantum networks, we give a comprehensive theory of the EIT phenomenon and underline the role of relevant parameters regarding the implementation of quantum memories.The first system under study is prepared in a free-space magneto-optical trap. The main result of this thesis is the demonstration of the spatial multimode capability of this system at the quantum level. For this, we used Laguerre-Gaussian (LG) light beams, i.e. beams possessing a non-zero value of orbital angular momentum (OAM). In a first step, we showed that the orbital angular momentum of stored light pulses is preserved by the memory, deep in the single photon regime. In a second step, we encoded information in the orbital angular momentum state of a weak light pulse and defined a qubit using two LG beams of opposite helicities. We developed an original setup for the measurement of this OAM qubit and used it to characterize the action of the memory during the storage of such a light pulse. Our results show that the memory performs the quantum storage of such a qubit.The second system under study, also a cloud of cold atoms, has the specificity that the atoms are trapped optically in the vicinity of a nano-waveguide. This innovative design ensures a higher light-matter interaction and facilitates the interfacing of photons into and out of the memory. We describe the building of this setup and the first steps towards quantum memory implementations.
Quantum memories enabling the storage of an input photonic qubit and its later retrieval with a fidelity beating any classical device constitute essential components in quantum communication networks and optical quantum information processing [1]. Our main goal is to develop a new generation of memories which have a near-unity efficiency for storage-and-retrieval operations, as well as high multiplexing capabilities. In the recent years, we demonstrated for instance the implementation for quantum bits encoded in the orbital angular momentum degree of freedom, which provides an essential capability for future networks with multimode capability [2]. We also realized multiple-degree-of-freedom memory, which can find applications in classical data processing but also in quantum network scenarios where states structured in phase and polarization have been shown to provide promising attributes [3].
Quantum storage of flying optical qubits, namely coherently mapping photonic states into and out of an optically controlled memory on demand, constitutes an essential component in the optical quantum information processing science, with applications to long-distance optical communication [1]. In this context, practical protocols require sufficiently large storage efficiency to achieve a performance better than direct transmission. Theoretically, the storage and retrieval efficiency can only be improved with the increase of the optical depth (OD). Recent progresses have shown that high OD media enable to reach high-efficiency electromagnetically induced transparency (EIT)-based optical storage, albeit without the demonstration of qubit storage [2, 3]. Here we experimentally implement an optical quantum memory with single-photon-level probes, showing a EIT-based storage efficiency around 70%. Furthermore, thanks to the spatial multiplexing of the dense atomic ensemble, highly efficient storage of polarization qubit is performed.
A reversible quantum memory allowing us to store and retrieve quantum information serves as a key necessity for implementing many of novel quantum information protocols. As light serves as a reliable long-range carrier of quantum information, and atoms offer the possibility of long storage times, current attempts at creating quantum memories focus on the transfer of the quantum fluctuations of light onto atomic coherences. The work in this thesis focuses on the development of a quantum memory for squeezed light using an ensemble of cold Cesium atoms stored in a magneto-optical trap. Our two major milestones were the development of a source of nonclassical light, and the development of a suitable atomic medium for storage. We first present the results of our efforts to generate a source of squeezed vacuum states resonant with the Cesium D2 line using a PPKTP nonlinear crystal inside of an optical parametric oscillator. Additionally, we characterize these squeezed states by carrying out a quantum state tomography using an iterative maximum likelihood approach. Next we look at the development of a new experiment which would allow us to use cold Cesium atoms as a storage medium in our recently developed magneto-optical trap. As this requires an array of novel tools and experimental techniques, we will discuss the development of these elements, and how they have furthered our progress towards storing quantum states onto our Cesium atoms, and eventually entangling two atomic ensembles.
A quantum memory is a key component for quantum networks, which will enable the distribution of quantum information. Its successful development requires storage of single-photon light. Encoding photons with spatial shape through higher-dimensional states significantly increases their information-carrying capability and network capacity. However, constructing such quantum memories is challenging. Here we report the first experimental realization of a true single-photon-carrying orbital angular momentum stored via electromagnetically induced transparency in a cold atomic ensemble. Our experiments show that the non-classical pair correlation between trigger photon and retrieved photon is retained, and the spatial structure of input and retrieved photons exhibits strong similarity. More importantly, we demonstrate that single-photon coherence is preserved during storage. The ability to store spatial structure at the single-photon level opens the possibility for high-dimensional quantum memories. Photonic quantum memories are necessary for quantum information networks and can be built using cold atomic gases. In this work, Ding et al. show the first storage and retrieval of single photons carrying orbital angular momentum using electromagnetically induced transparency in a cold rubidium ensemble.
In this paper, an optical memory based on an ensemble of cold Cesium atoms interacting with light in the electromagnetically induced transparency (EIT) regime is studied in view of spatially multimode quantum storage. The interfacing protocol involves a control field, generating EIT for the very weak field that carries the quantum signal to be stored and enabling its mapping into the collective hyperfine coherence of the cold atomic ensemble. For the demonstration of multimode storage, the signal pulse at the single photon level is prepared in a mode with (OAM), while the control field is in a TEM00 mode. After the writing process, the information is kept in the memory register. In the retrieval phase, the information is read back from atoms to light and we show that these pulses at the single photon level with OAM can be stored and retrieved with a preserved spatial phase and a good efficiency.