Nonclassical states of light are an important resource in today's quantum communication and metrology protocols. Quantum up-conversion of nonclassical states is a promising approach to overcome frequency differences between disparate subsystems within a quantum information network. Here, we present the generation of heralded narrowband single photons at 1550 nm via cavity enhanced spontaneous parametric down-conversion (SPDC) and their subsequent up-conversion to 532 nm. Quantum non-Gaussianity (QNG), which is an important feature for applications in quantum information science, was experimentally certified for the first time in frequency up-converted states.
Nonclassical states of light are an important resource in today's quantum communication and metrology protocols. Quantum up-conversion of nonclassical states is a promising approach to overcome frequency differences between disparate subsystems within a quantum information network. Here, we present the generation of heralded narrowband single photons at 1550 nm via cavity enhanced spontaneous parametric down-conversion (SPDC) and their subsequent up-conversion to 532 nm. Quantum non-Gaussianity (QNG), which is an important feature for applications in quantum information science, was experimentally certified for the first time in frequency up-converted states.
The bandwidth and the frequency correlations of quantum light can be considered as a resource for the implementation of new quantum information algorithms, and it should enable the applicability of quantum techniques not yet implemented. For that purpose, the control of the frequency correlations, and the bandwidth, of single and paired photons is an essential ingredient, since the optimum bandwidth, as well as the most appropriate type of frequency correlations for a specific quantum application, depend on the specific quantum information application under consideration. Here we elucidate and implement new strategies to tailor the frequency properties of quantum light. Such strategies, which are based on the use of non collinear spontaneous parametric down conversion (SPDC) configurations, include the generation of narrow and enhanced bandwidth quantum light, the control of the frequency correlations of paired photon, and the generation of heralded single photons with a high degree of purity from pairs of uncorrelated photons.
This thesis presents experimental progress towards highly efficient generation of cavity- enhanced, narrow-band multiphotons using spontaneous parametric downconversion. It also suggests theoretical proposals for new applications of multiphoton sources in quantum information technology. Photons prepared in Fock states, with a well-defined number of particles, are the essence of the quantum nature of light, and their unique nonclassical properties help us to develop quantum information technology. The generation of Fock states has been most commonly achieved by using spontaneous parametric down-conversion (SPDC) through the χ(2) nonlinearity in bulk crystals. During the SPDC process the nonlinear crystal emits photon-number correlated modes, and a photon-number- resolving measurement in one mode heralds the preparation of the other mode in a Fock state. The fidelity and the success rate of Fock state generation is limited by the multimode nature of SPDC emission. In this thesis, I will investigate the use of cavity-enhanced SPDC modes for higher fidelity photon pair generation, in which the well defined mode of the cavity is enhanced and results in the narrow-band source of the heralded single-photons with up to 80% heralding efficiency. The heralding efficiency is defined as the ratio between number of coincidences and single events on the heralding mode. The photon-number-resolving ability of high-quantum-efficiency transition edge sensors is used for the heralding and detection. We further discuss the feasibility of Fock state generation with higher photon numbers upon improving the data acquisition and analysis techniques. We also demonstrate and implement an interferometry scheme that exploit the phase of the photon and is capable of reading boundless bits of digital information using a single photon. This thesis also introduces new theoretical proposals for using Fock states to obtain quantum advantages for various quantum information applications, here to increase the efficiency in reading out information stored in a classical digital memory and to discriminate optical phases with lower error rates than is feasible with classical protocols.
Quantum non-Gaussianity (QNG) corresponds to an essential resource in many prospective applications of quantum technologies. QNG states of light have been demonstrated in a broad range of experimental platforms with various complementary advantages [1]. However, in order to allow for their direct and efficient utilization for interaction with the target quantum system, their degrees of freedom have to be well-defined and controllable. This imposes a requirement on the generation of QNG light in a single optical mode. While the intrinsic single-photon sources including trapped ions, atoms, or molecules, naturally offer a close-to a single mode emission, their technologically more accessible counterparts including majority of the solid-state emitters and sources based on heralded generation of nonclassical states in various parametric nonlinear processes typically provide a multi-mode output. Although several demonstrations of a single-mode operation of nonclassical light sources based on these platforms have been recently demonstrated, an unambiguous observation of a single-mode QNG light has never been presented.
Quantum frequency conversion (QFC) which converts the frequencies of photons while preserving the quantum state is an essential technology for realizing the quantum internet and quantum interconnect. For the QFC based on the frequency downconversion from visible to the telecom wavelengths around 1500 nm, it is widely known that noise photons produced by the strong pump light used for QFC contaminate the frequency-converted photon, which degrades the quality of the quantum property of the photon after QFC. In conventional QFC experiments, noise photons are removed using external narrowband frequency filter systems. In contrast, in this study, we implement a compact QFC device integrating the cavity structure only for the converted mode. While the cavity structure can enhance not only the desired QFC efficiency but also the noise photon generation rate, we show that the cavity-enhanced QFC followed by a relatively wide bandpass filter achieves the signal-to-noise ratio comparable to the QFCs with external narrowband filters. We experimentally demonstrate the cavity-enhanced QFC using a single photon at 780 nm to 1540 nm, in which the non-classical photon statistics is clearly observed after QFC.
We present a theoretical model of photon counting measurements on conditionally generated narrowband single-photon states that are produced via cavity-enhanced spontaneous parametric down-conversion, then frequency up-converted from the telecom wavelength of 1550 nm to the visible wavelength of 532 nm. The highly nonclassical character of the up-converted states is certified by a quantum non-Gaussianity witness that is determined from coincidence measurements with single-photon detectors in a Hanbury-Brown--Twiss configuration. We find our model in good agreement with the experimental data, and we investigate a useful effect caused by the dead time of the trigger detector, whose clicks herald conditional preparation of the single-photon state. Due to the dead time, a click of the trigger detector excludes the possibility of a trigger event at a certain preceding time interval, during which the measured idler beam is thus projected onto a vacuum state. Due to quantum correlations between signal and idler beams, this reduces the multiphoton contributions in the conditionally generated state of the signal beam and accordingly increases the value of the quantum non-Gaussianity witness. We also show that spurious heralding detections due to after-pulsing can be suppressed by accepting a click of the trigger detector only if its distance from a previous click of this detector exceeds a certain suitably chosen threshold.
Cavity-enhanced spontaneous parametric down-conversion (SPDC) can be used to implement heralded single-photon sources with narrow bandwidth down to tens of megahertz, thus compatible with atom-based quantum memories. In this work, we propose and study the cavities that are either singly resonant at the frequency of only one of the generated photons or doubly resonant at the frequency of one generated photon and for the pump field. We derive analytical expressions for the generation rate and the spectral brightness as a function of the main structure parameters. Our analysis shows that by exploiting counterpropagating SPDC inside a cavity with properly designed distributed Bragg reflectors, pure narrow-band heralded single photons can be generated. In particular, when a pulsed pump illuminates a 1-mm-long singly resonant cavity, the bandwidth of the resonant and the nonresonant photons will be 120 MHz and 5.7 GHz, respectively, with heralded single-photon purity as high as 0.993. Due to its small absorption probability inside the cavity, the heralding efficiency can approach unity if the nonresonant partner is used as a heralded single photon. Finally, we also show that continuous-wave excitation of a 1-cm-long cavity resonant for the pump and copropagating fields can provide photon pairs in a narrow bandwidth of 10 MHz with spectral brightness as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $({\text{s}\phantom{\rule{4.pt}{0ex}}\text{mW}\phantom{\rule{4.pt}{0ex}}\text{MHz})}^{\ensuremath{-}1}$. This source can efficiently couple with quantum memories and can find applications in quantum information processing and communication.