The efficiency of an ensemble-based optical quantum memory depends critically on the strength of the atom-light coupling. An optical cavity is an effective method to enhance atom-light coupling strength, with the drawback that cavities can be difficult to integrate into a memory setup. In this work we show coherent enhancement of atom-light coupling via an interference effect. The light to be absorbed into the atomic ensemble is split and used to drive the atoms from opposite ends of the ensemble. We compare this method theoretically to a cavity-enhanced scheme and present experimental results for our coherent enhancement in cold $^{87}\mathrm{Rb}$ atoms that show an efficiency of $72\ifmmode\pm\else\textpm\fi{}5%$ and a storage lifetime of $110\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{s}$.
The efficiency of an ensemble-based optical quantum memory depends critically on the strength of the atom-light coupling. An optical cavity is an effective method to enhance atom-light coupling strength, with the drawback that cavities can be difficult to integrate into a memory setup. In this work we show coherent enhancement of atom-light coupling via an interference effect. The light to be absorbed into the atomic ensemble is split and used to drive the atoms from opposite ends of the ensemble. We compare this method theoretically to a cavity-enhanced scheme and present experimental results for our coherent enhancement in cold 87Rb atoms that show an efficiency of 72\ifmmode±\else\textpm\fi5 and a storage lifetime of 110\ifmmode±\else\textpm\fi10\ensuremathμs.
A cold atomic ensemble suits well for optical quantum memories, and its entanglement with a single photon forms the building block for quantum networks that give promise for many revolutionary applications. Efficiency and lifetime are among the most important figures of merit for a memory. In this Letter, we report the realization of entanglement between an atomic ensemble and a single photon with subsecond lifetime and high efficiency. We engineer dual control modes in a ring cavity to create entanglement and make use of three-dimensional optical lattice to prolong memory lifetime. The memory efficiency is 38% for 0.1 s storage. We verify the atom-photon entanglement after 1 s storage by testing the Bell inequality with a result of S=2.36±0.14.
The storage and retrieval efficiency (SRE) and lifetime of optical quantum memories are two key performance indicators for scaling up quantum information processing. Here, we experimentally demonstrate a cavity-enhanced long-lived optical memory for two polarizations in a cold atomic ensemble. Using electromagnetically induced-transparency (EIT) dynamics, we demonstrate the storages of left-circularly and right-circularly polarized signal light pulses in the atoms, respectively. By making the signal and control beams collinearly pass through the atoms and storing the two polarizations of the signal light as two magnetic-field-insensitive spin waves, we achieve a long-lived (3.5 ms) memory. By placing a low-finesse optical ring cavity around the cold atoms, the coupling between the signal light and the atoms is enhanced, which leads to an increase in SRE. The presented cavity-enhanced storage shows that the SRE is ∼30%, corresponding to an intrinsic SRE of ∼45%.
We propose a method to implement a quantum memory for light based on ensembles of two-level atoms. Our protocol is based on controlled reversible inhomogeneous broadening (CRIB), where an external field first dephases the atomic polarization and thereby stores an incoming light pulse into collective states of the atomic ensemble, and later a reversal of the applied field leads to a rephasing of the atomic polarization and a reemission of the light. As opposed to previous proposals for CRIB-based quantum memories, we propose only applying the broadening for a short period after most of the pulse has already been absorbed by the ensemble. We show that with this procedure there exist certain modes of the incoming light field that can be stored with an efficiency approaching 100% in the limit of high optical depth and long coherence time of the atoms. These results demonstrate that it is possible to operate an efficient quantum memory without any optical control fields.
Optical quantum memory plays an important role in scaling-up linear optical quantum computations and longdistance quantum communication. For effectively realizing such tasks, a long-lived and highly-efficient quantum memory is needed. The dynamic electromagnetically-induced-transparency (EIT) process can be used for completing an absorptive storage scheme in an atomic ensemble. In such a process, the quantum states of coming single photons can be coherently transformed into spin waves associated with coherences between atomic ground levels via switching off controlling light beam. For storing a single-mode optical signal, a pair of ground levels is involved. While for storing an optical polarization qubit, i.e., two orthogonal polarization modes, the coherence between two pairs of ground levels will be involved. Also, to obtain a high efficiency in the EIT optical storage, the optical-depth of the cold ensemble should be high. For prolonging the coherent time of the spin waves stored in atomic ensemble, decoherence between spin waves due to atomic motion and non-uniform Zeeman shift of ground levels should be effectively suppressed. Recently, a long-lived and highly-efficient optical quantum memory for single-mode storage in a high-optical-depth cold atom ensemble has been experimentally demonstrated via the gradient echo memory scheme (2016 Optical 3 100). While, for the optical polarization qubit storage, a long lifetime (in ms) and high-fidelity EIT storage experiment has been achieved by our group, but the storage efficiency in the experiment is very low (8%) due to lower optical depth of the cold ensemble (2013 Phys. Rev. Lett. 111 240503). The storage efficiency in long-lived storage of two orthogonal polarization modes still needs further improving. Here in this paper we demonstrate an experiment of long-lived and highly-efficient storage of two optical orthogonal polarization modes in a high optical-depth cold atomic ensemble via dynamic EIT process. For achieving a long lifetime in the storage experiment, we follow the two steps, which are used in our previous work (2013 Phys. Rev. Lett. 111 240503). 1) We make the signal and writing-reading light beams collinearly pass through the cold-atom cloud along the z direction to suppress the decoherence between the spin waves due to atomic motion. 2) We apply a moderate magnetic field (13.5 G) to the cold-atom ensemble to lift Zeeman degeneracy. So, the magnetic-field-sensitive transitions are removed from EIT system and the two optical orthogonal polarization modes are stored as two magnetic-field-insensitive spin waves. In contrast to our previous experiment, we finish the storage in the high optical-depth cold atomic ensemble. To obtain such a high optical-depth cold atomic ensemble, we expand the diameters of the trapping laser beams and use a pair of rectangular magnetic coils in a magnetic optical trap (MOT) to prepare a cigar-shaped cold atomic ensemble. The MOT magnetic field is further compressed, and then the optical-depth of the cold atomic ensemble increases up to ~11 in the present experiment, which allows us to achieve a storage efficiency of 30%, which exceeds the previous value (8%). At an MOT repetition rate of 10 Hz, the measured zero-delay storage efficiencies for the two orthogonal polarization modes are symmetric, which go up to ~30%. The 1/e-folding lifetimes of the two orthogonal polarization modes rise up to 3 ms. We also measure the dependence of the zero-delay retrieval efficiency on the MOT repetition rate F and find that the storage efficiency is still more than 20% when the repetition rate F is 50 Hz. The present results will allow one to achieve a long lifetime and highly-efficient quantum memory for photonic polarization qubit and then find applications in scaling-up linear-optical quantum computations and long-distance quantum communication.
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.
Summary form only given. Long-term storage of quantum information is a long standing goal since decades. It is particularly motivated by the fact that quantum network implementations strongly rely on quantum memories for light [1]. Long storage time and high efficiency are the main two figures of merit for a qubit memory. However, although quantum memories have been implemented in a large variety of physical systems, it turns out to be challenging to simultaneously fulfill those two main requirements.In our setup, a single 87Rb atom is trapped in a two-dimensional optical lattice inside a high-finesse cavity. Based on seminal work [2], the photonic polarization qubit a|L) + ß|R) described in the left/right polarization basis is mapped onto the atomic state |F = 2,= -1) + ß|F = 2, mf = +1) via a stimulated Raman adiabatic passage (STIRAP) as depicted in step I) of Fig. 1: The readout efficiency of this quantum memory protocol is basically constant for our typical coherence time. This is in contrast to systems based on atomic ensemble where decoherence affects the collective excitation reemission. However, fluctuations of the magnetic field deteriorate the coherence of the superposition. Indeed, the two levels we use are split by the Zeeman effect and thus the free evolution of the system will accumulate a random phase increasing with time. Typically, the coherence information of the qubit is lost after 200μs.