The ladder configuration of atomic levels provides a source for telecom photons (signal) from the upper atomic transition. For rubidium and caesium atoms, the signal field has the range around 1.3–1.5 μm that can be coupled to an optical fibre and transmitted to a remote location. Cascade emission may result in pairs of photons, the signal entangled with the subsequently emitted infrared photon (idler) from the lower atomic transition. This correlated two-photon source is potentially useful in the DLCZ (Duan–Lukin–Cirac–Zoller) protocol for the quantum repeater. We implement the cascade emission to construct a modified DLCZ quantum repeater and investigate the role of the time-frequency entanglement in the protocol. The dependence of the protocol on the resolving and non-resolving photon-number detectors is also studied. We find that the frequency entanglement deteriorates the performance but the harmful effect can be diminished by using shorter pump pulses to generate the cascade emission. An optimal cascade-emission-based DLCZ scheme is realized by applying a pure two-photon source in addition to using detectors of perfect quantum efficiency.
The ladder configuration of atomic levels provides a source for telecom photons (signal) from the upper atomic transition. For rubidium and caesium atoms, the signal field has the range around 1.3–1.5 μm that can be coupled to an optical fibre and transmitted to a remote location. Cascade emission may result in pairs of photons, the signal entangled with the subsequently emitted infrared photon (idler) from the lower atomic transition. This correlated two-photon source is potentially useful in the DLCZ (Duan–Lukin–Cirac–Zoller) protocol for the quantum repeater. We implement the cascade emission to construct a modified DLCZ quantum repeater and investigate the role of the time-frequency entanglement in the protocol. The dependence of the protocol on the resolving and non-resolving photon-number detectors is also studied. We find that the frequency entanglement deteriorates the performance but the harmful effect can be diminished by using shorter pump pulses to generate the cascade emission. An optimal cascade-emission-based DLCZ scheme is realized by applying a pure two-photon source in addition to using detectors of perfect quantum efficiency.
In this thesis, we investigate the quantum mechanical interaction of light with matter in the form of a gas of ultracold atoms: the atomic ensemble. We present a theoretical analysis of two problems, which involve the interaction of quantized electromagnetic fields (called signal and idler) with the atomic ensemble (i) cascade two-photon emission in an atomic ladder configuration, and (ii) photon frequency conversion in an atomic diamond configuration. The motivation of these studies comes from potential applications in long-distance quantum communication where it is desirable to generate quantum correlations between telecommunication wavelength light fields and ground level atomic coherences. We develop a theory of correlated signal-idler pair correlation. The analysis is complicated by the possible generation of multiple excitations in the atomic ensemble. An analytical treatment is given in the limit of a single excitation assuming adiabatic laser excitations. The analysis predicts superradiant timescales in the idler emission in agreement with experimental observation. To relax the restriction of a single excitation, we develop a different theory of cascade emission, which is solved by numerical simulation of classical stochastic differential equation using the theory of open quantum systems. The simulations are in good qualitative agreement with the analytical theory of superradiant timescales. We provide a quantum theory of near-infrared to telecom wavelength conversion in the diamond configuration. The system provides a crucial part of a quantum-repeater memory element, which enables a "stored" near-infrared photon to be converted to a telecom wavelength for transmission without the destruction of light-atom quantum correlation.
The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for long-distance quantum communication with atomic ensembles is analyzed. Its fidelity and throughput in entanglement distribution, entanglement swapping, and quantum teleportation is derived within a framework that accounts for multiple excitations in the ensembles as well as loss and asymmetries in the channel. The DLCZ performance metrics that are obtained are compared to the corresponding results for the trapped-atom quantum communication architecture that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT and NU). Both systems are found to be capable of high-fidelity entanglement distribution. However, the DLCZ scheme only provides conditional teleportation and repeater operation, whereas the MIT-NU architecture affords full Bell-state measurements on its trapped atoms. Moreover, it is shown that achieving unity conditional fidelity in DLCZ teleportation and repeater operation requires ideal photon-number resolving detectors. The maximum conditional fidelities for DLCZ teleportation and repeater operation that can be realized with nonresolving detectors are 1/2 and 2/3, respectively.
The cascade-emitted biphotons generated from alkali metal atomic ensembles are an excellent entanglement resource which enables long-distance quantum communication. The communication of quantum information between distant locations can be realized by utilizing the low-loss telecom bandwidth in the upper transition of the cascaded photons in a fiber-based quantum network. Meanwhile, the infrared photon from the lower transition of this highly directional and frequency-correlated biphoton can be created using the four-wave mixing process and can be stored locally as a collective spin wave. Here, we theoretically investigate the frequency entanglement of this biphoton and propose two approaches to remove the two photons' mutual correlations in frequency spaces. The first approach applies an optical cavity which modulates the biphoton spectrum into a more symmetric and narrow spectral function by multiplexing multiple atomic ensembles with phase compensation. The purity of the single photons reaches up to 0.999, and the entanglement entropy S of the biphoton is reduced to 0.006, which is 200 times smaller than the one without multiplexing. The other approach employs a symmetric pumping of the laser fields in two atomic ensembles, which leads to a moderate reduction of $S\ensuremath{\sim}0.3$ when nondiscrimination detection devices are used for both photons. An extremely low frequency entanglement implies an almost indistinguishable single-photon source, which offers a potential resource for photonic quantum emulation and computation.
With the accelerating developments in the field of quantum communication, there will be a need for sources of highly quantum single-photons. In addition to communication applications, the purity, brightness, and fidelity of a given single-photon source are intrinsically linked to the success rate and scalability of a range of photonic technologies in quantum computing, metrology, memory, and cryptography. Further, quantum repeater architectures for maintaining quantum information over long distances will become crucial. In these schemes, end-to-end entanglement is generated by establishing entanglement between the ends of several elementary segments, followed by successive entanglement-swapping operations to increase the entanglement distance until it spans the transmission channel. The most widely studied proposal for the practical implementation of a quantum repeater is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. This consists of off-resonant Raman excitation by a write pulse, which induces a spin-wave excitation in an atomic ensemble and emits a Stokes photon. At a later time, this spin-wave may be read with a high-power resonant pulse, resulting in the emission of an anti-Stokes photon into a well-defined spatial mode. The Stokes photon may be used for entanglement with another ensemble, while the anti-Stokes may be read out for an entanglement-swapping operation. Notably, in the DLCZ protocol, the atomic ensemble serves as both the quantum memory and a heralded single-photon source. Using atomic ensembles for single-photon sources enables the production of highly pure, indistinguishable photons with minimal spectral broadening; however, the generation rate remains a challenge. In the case of DLCZ, the Raman-scattering step is highly probabilistic, reducing the anti-Stokes generation rate considerably. Furthermore, DLCZ requires operation in the low-excitation-probability regime to avoid multiphoton noise contributions. Additionally, the profile of the induced spin wave is suboptimal for readout. To compensate for these drawbacks, in this thesis we report the experimental realization of a variant of DLCZ that leverages collectively enhanced superradiant (SR) emission. Superradiant emission is characterized by intense bursts of photons emitted in a shorter amount of time than independent atomic state decay. By switching to a resonant excitation and optimizing the write profile, the induced spin-wave is primed for readout, thus increasing retrieval efficiency and the level of quantum correlation. We report data for both the DLCZ and SR schemes under similar optical conditions and compare the two. At high write power, the SR protocol exhibits conditional readout probabilities as high as $\sim$1.2\%, compared to $\sim$0.6\% for the DLCZ protocol under the same conditions. We also analyze the temporal distribution of anti-Stokes emission and find a degree of maintenance of the spin-wave profile, even in the anti-Stokes direction.
Abstract We theoretically investigate the spectral property of a biphoton state from multiplexed thermal atomic ensembles. This biphoton state originates from the cascade emissions, which can be generated by two weak pump fields under four-wave mixing condition. Under this condition, a signal photon from the upper transition, chosen in a telecommunication bandwidth, can be generated along with a correlated idler photon from the lower infrared transition. We can spectrally shape the biphoton state by multiplexing the atomic ensembles with frequency-shifted emissions, where the entropy of entanglement can be analyzed via Schmidt decompositions. We find that this spectral entanglement increases when more vapor cells are multiplexed with correlated or anti-correlated signal and idler fields. The eigenvalues in Schmidt bases approach degenerate under this multiplexing scheme, and corresponding Schmidt numbers can be larger than the number of the multiplexed vapor cells, showing the enlarged entropy of entanglement and excess correlated modes in continuous frequency spaces. We also investigate the lowest entropy of entanglement allowed in the multiplexing scheme, which is preferential for generating a pure single photon source. This shows the potentiality to spectrally shape the biphoton source, where high-capacity spectral modes can be applied in long-distance quantum communication and multimode quantum information processing.
Abstract We theoretically investigate the spectral property of biphoton state from the cascade emissions from a Doppler-broadened atomic ensemble. This biphoton state is spontaneously created in the four-wave-mixing process. The upper transition of the emissions lies in telecom bandwidth, which prevails in fiber-based quantum communication for low-loss transmission. We obtain the spectral function of the biphoton state in terms of superradiant decay rates of the lower transition, excitation pulse durations, and temperature of the medium. We quantify their frequency entanglement by Schmidt decomposition and find that more entangled source can be generated with longer excitation pulses, enhanced decay rates, and significant Doppler broadening. A minimally entangled biphoton source can also be located at some optimal temperature of the atoms. By tuning these parameters, this biphoton source allows spectral shaping of continuous frequency entanglement, which is useful in multimode long-distance quantum communication.