Realization of distributed quantum systems requires fast generation and long-term storage of quantum states. Ground atomic states enable memories with storage times in the range of a minute, however their relatively weak interactions do not allow fast creation of non-classical collective states. Rydberg atomic systems feature fast preparation of singly excited collective states and their efficient mapping into light, but storage times in these approaches have not yet exceeded a few microseconds. Here we demonstrate a system that combines fast quantum state generation and long-term storage. An initially prepared coherent state of an atomic memory is transformed into a non-classical collective atomic state by Rydberg-level interactions in less than a microsecond. By sheltering the quantum state in the ground atomic levels, the storage time is increased by almost two orders of magnitude. This advance opens a door to a number of quantum protocols for scalable generation and distribution of entanglement.
Realization of distributed quantum systems requires fast generation and long-term storage of quantum states. Ground atomic states enable memories with storage times in the range of a minute, however their relatively weak interactions do not allow fast creation of non-classical collective states. Rydberg atomic systems feature fast preparation of singly excited collective states and their efficient mapping into light, but storage times in these approaches have not yet exceeded a few microseconds. Here we demonstrate a system that combines fast quantum state generation and long-term storage. An initially prepared coherent state of an atomic memory is transformed into a non-classical collective atomic state by Rydberg-level interactions in less than a microsecond. By sheltering the quantum state in the ground atomic levels, the storage time is increased by almost two orders of magnitude. This advance opens a door to a number of quantum protocols for scalable generation and distribution of entanglement.
Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.
The advances in laser cooling and trapping techniques allow samples of ultra-cold atoms to be created and controlled for the study of quantum physics. In this thesis, we focus on using ensembles of cold atoms to investigate novel quantum effects and their applications in quantum optics. Realization of a scalable intercontinental quantum system, e.g. a global quantum internet, relies on quantum memories with long lifetimes on the order of seconds. We have reduced the differential light shifts of the atomic ground states to a sub-Hz level and achieved a lifetime of 16 seconds. Achieving fast and efficient quantum operations, on the other hand, requires strongly-interacting systems. We explore highly excited Rydberg atoms towards this goal and demonstrate Rydberg excitation blockade, many-body Rabi oscillations, trapping of Rydberg atoms, deterministic single photon source, and atom-photon entanglement. Finally, we integrate the two essential capabilities for quantum information processing, fast quantum state generation and long-term storage, by simultaneously exploiting Rydberg levels for interactions and ground atomic levels for the storage of quantum state. Our work advances the control of coherence properties and interactions in cold atomic ensembles to a new level and opens new opportunities for studies of complex quantum systems.
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.
Strong interactions can amplify quantum effects such that they become important on macroscopic scales. Controlling these coherently on a single particle level is essential for the tailored preparation of strongly correlated quantum systems and opens up new prospects for quantum technologies. Rydberg atoms offer such strong interactions which lead to extreme nonlinearities in laser coupled atomic ensembles. As a result, multiple excitation of a Micrometer sized cloud can be blocked while the light-matter coupling becomes collectively enhanced. The resulting two-level system, often called "superatom", is a valuable resource for quantum information, providing a collective Qubit. Here we report on the preparation of two orders of magnitude scalable superatoms utilizing the large interaction strength provided by Rydberg atoms combined with precise control of an ensemble of ultracold atoms in an optical lattice. The latter is achieved with sub shot noise precision by local manipulation of a two-dimensional Mott insulator. We microscopically confirm the superatom picture by in-situ detection of the Rydberg excitations and observe the characteristic square root scaling of the optical coupling with the number of atoms. Furthermore, we verify the presence of entanglement in the prepared states and demonstrate the coherent manipulation of the superatom. Finally, we investigate the breakdown of the superatom picture when two Rydberg excitations are present in the system, which leads to dephasing and a loss of coherence.
Photons are very good candidates to carry quantum information due to their negligible interactions. However, photon-photon interactions are needed to process photonic quantum information. This is very useful for quantum repeater applications [1], in which the success probability of entanglement swapping can be highly increased if the Bell-state measurement is performed deterministically. In the case of photons, it requires mapping them in a non-linear medium at the single-photon level. This strong non-linearity has been demonstrated by using Rydberg atoms, which are atoms excited to states with a high principal quantum number. However, previous experiments with Rydberg atoms have been implemented with weak coherent input states of light [2, 3]. Hence, the mapping of true single photons to a medium with strong single-photon non-linearity is, so far, an outstanding goal. In this work, we have demonstrated the storage and retrieval of a paired single photon on a highly non-linear medium based on atoms excited to Rydberg levels. More recently, we have also enhanced the non-linearity of our ensemble to the single photon-level, as witnessed by the antibunching of a stored and retrieved incoming coherent field [3]. This will allow the storage of a single photon in a medium with single-photon non-linearity.
Summary form only given. Strong interaction between two single photons [1], is a long standing and important goal in quantum photonics. This would enable a new regime of nonlinear optics and unlock several applications in quantum information science, including photon-photon gates and deterministic Bell-state measurements for quantum networking. In the context of quantum networks [2], a particularly important case is to achieve strong interaction between single photons from independent photon pairs that can be stored in quantum memories. So far, most of the efforts towards nonlinear effects at the single-photon level have been realized using weak classical input light.In this contribution, we demonstrate the storage and retrieval of a paired single photon emitted by an ensemble quantum memory in a strongly nonlinear medium based on highly excited Rydberg atomic excitations [3,4]. We demonstrate that non-classical correlations between the two photons persist after the retrieval from the Rydberg ensemble. We show that our Rydberg cold atomic ensemble displays optical nonlinearity with input pulses containing few tens of photons. Our results [5], mark an important step towards deterministic photonphoton interactions, and may enable deterministic Bell-state measurement with multimode quantum memories.