It is known that photons are the ideal carriers of quantum information and ensemble of atoms can be acted as the long lived storage and processing units. In the coherently prepared atomic ensemble with the light resonantly interacting with the atoms, various schemes have been proposed to implement quantum state transfer and storage, quantum phase gate, generation of entangled photons, and the long-distance quantum communication [1]. We first proposed the dark-state polaritons in a five-level M-type atomic ensemble in the electro magnetically induced transparency (EIT) [2], Under conditions of the low excitation limit and adiabatic following, the quantum field can be mapped onto the atomic polarization states, forming the shape preserving quantum states of polaritons. Two channels for robust quantum memory are provided. Then, we investigated the nonlinear optical response of a five-level tripod atomic system in the EIT, and proposed a three-qubit phase gate protocol based on the fifth-order nonlinearity [3]. The cross phase modulation among three weak fields induced by the fifth-order nonlinearity in EIT can produce the phase shift of order n, which can be used to realize the three-qubit polarization phase gate. Finally, we proposed a cavity-QED scheme that can effectively generate EPR polarization-entangled photon pairs [4], A four-level tripod atom successively couples to two high-Q optical cavities possessing polarization degeneracy, assisted by a classical π-polarized pump field. The stimulated Raman adiabatic passage process in the atom-cavity system is used to produce the polarization-entangled photon pairs. The proposal is particularly robust against atomic spontaneous decay. The above protocols should have potential applications in quantum information processing.
It is known that photons are the ideal carriers of quantum information and ensemble of atoms can be acted as the long lived storage and processing units. In the coherently prepared atomic ensemble with the light resonantly interacting with the atoms, various schemes have been proposed to implement quantum state transfer and storage, quantum phase gate, generation of entangled photons, and the long-distance quantum communication [1]. We first proposed the dark-state polaritons in a five-level M-type atomic ensemble in the electro magnetically induced transparency (EIT) [2], Under conditions of the low excitation limit and adiabatic following, the quantum field can be mapped onto the atomic polarization states, forming the shape preserving quantum states of polaritons. Two channels for robust quantum memory are provided. Then, we investigated the nonlinear optical response of a five-level tripod atomic system in the EIT, and proposed a three-qubit phase gate protocol based on the fifth-order nonlinearity [3]. The cross phase modulation among three weak fields induced by the fifth-order nonlinearity in EIT can produce the phase shift of order n, which can be used to realize the three-qubit polarization phase gate. Finally, we proposed a cavity-QED scheme that can effectively generate EPR polarization-entangled photon pairs [4], A four-level tripod atom successively couples to two high-Q optical cavities possessing polarization degeneracy, assisted by a classical π-polarized pump field. The stimulated Raman adiabatic passage process in the atom-cavity system is used to produce the polarization-entangled photon pairs. The proposal is particularly robust against atomic spontaneous decay. The above protocols should have potential applications in quantum information processing.
Dark-state polaritons in a five-level M-type atomic ensemble are proposed in the electromagnetically induced transparency. Under conditions of the low excitation limit and adiabatic following, the quantum field can be mapped onto the atomic polarization states, forming the shape preserving quantum states of polaritons. Two channels for robust quantum memory are provided, which enables the potential applications in quantum-information processing.
We first investigate the nonlinear optical response of a five-level tripod atomic system in electromagnetically induced transparency and then propose a 3-qubit phase gate protocol based on the fifth-order nonlinearity. The cross-phase modulation among the three weak fields induced by the fifth-order nonlinearity in electromagnetically induced transparency can produce the phase shift of order π, which can be used to realize the 3-qubit polarization phase gate. This protocol should have potential applications in quantum information processing.
In this chapter, we present a proposal for the realization of an entangling gate for photons. The protocol works deterministically and is experimentally feasible under realistic conditions. We start outlining the general problem of quantum information processing with photons and describe the current status in this field. Then we introduce the proposed scheme and present its main features. This part is followed by a review of the concepts and procedures employed in our proposal. Finally, we explain the quantum gate, in detail, in the last section of this chapter, where we also briefly discuss the performance of the gate in the presence of imperfections. 26.1.1 Quantum information processing with light Photons play a key role in applications of quantum information because they are ideally suited to transmit quantum states between distant sites. This feature makes them indispensable for the realization of quantum communication protocols and the construction of quantum networks. While being a good flying carrier of information, photons are naturally less adequate for storage than the long-lived matter degrees of freedom. For this reason, matter systems are employed as quantum memories. In order to combine both elements in a quantum network (i.e., photons as flying qubits and atoms as memory devices), light-matter interface schemes have been developed to transfer quantum states of light onto an atomic system. Some of these schemes are based on quantum-nondemolition interactions, electromagnetically induced transparency, and Raman processes. Moreover, Raman processes have been used to entangle two distant atomic ensembles, which is an important step to realize quantum repeaters and, thus to solve the problem of losses and decoherence that exists in a photonic channel. Apart from the transmission and storage of information, in a quantum network it is necessary to process quantum states. However, manipulation of quantum states of light is still challenging, since it requires the ability to create entanglement between photons. This task is difficult because photons are noninteracting particles, in principle. One possibility to entangle photons is to employ materials that possess optical nonlinearities, but so far, there are no materials available whose nonlinearities are strong enough to allow for short gate times. An alternative approach was put forward by Knill et al., which requires only linear optical operations and measurements. However, this scheme is probabilistic and not very efficient in practice. In this chapter, we present a scheme for the realization of a deterministic entangling gate for photons by using an atom lattice. An atom lattice consists of an ensemble of cold atoms loaded in the periodic optical potential created by a standing wave. In order to use the lattice as a quantum register for quantum computation, it is necessary to prepare it in a Mott insulating phase, in which the number of atoms in each site of the potential (i.e., in each of its minima) is approximately constant, and set equal to 1.
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 optics provides tools for the accurate control of light using atoms and also for manipulating atomic states using light. These techniques are being used for many applications, including quantum information processing and the generation of exotic quantum states. This thesis contains proposals for implementing a photonic quantum memory and a photon-photon gate, which are essential elements of photonic quantum information processing. Furthermore it proposes a scheme for the creation of many-body entangled states. First a proposal for a new quantum memory protocol, called the atomic frequency sweep quantum memory, is presented. A two-level polariton model is derived that explains the coherent storage and retrieval of light through the manipulation of the atomic resonance frequency. This is followed by a scheme for a deterministic photonic controlled-PHASE gate based on the strong interaction between two stationary collective Rydberg excitations in an atomic ensemble. Distortion effects caused by nonuniform interaction are quantified and compensation techniques for these effects are proposed. Finally a proposal is presented that uses Rydberg dressing for the generation of energy cat states in an atomic medium, i.e. superposition states of all the atoms being in the ground or excited state, where these two states are connected by an optical transition and thus have a significant difference in energy. Considering the fragility of the state, the effects of many different imperfections and decoherence sources are quantified. The resulting cat state would allow testing of energy decoherence models with greatly improved sensitivity.
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.