The quantum information community is entering a period in which scientists and engineers are transitioning from ‘understanding’ to ‘control’ by harnessing aspects of quantum mechanics to realize capabilities in computing, sensing, and communication that are not possible in the classical world. Although many of these quantum technologies are still in their very early stages, it is clear that our future world will include many quantum devices. The existence of multiple devices will, in turn, necessitate the transfer of quantum information between devices—if isolated devices are useful, it follows that connected devices will be even more useful. Therefore, it is imperative to develop the resources needed for connecting quantum devices at different locations. One such resource is quantum entanglement, a physical phenomenon in which two or more quantum systems cannot be described independently, even when separated by large distances. Entanglement is an important resource for quantum computing and is critical for overcoming loss constraints that limit the transfer of quantum states across large distances. The latter can be accomplished via ‘quantum repeaters,’ which propose to connect multiple shorter entangled photon links to form a longer entangled photon link. Much of our understanding of entanglement comes from experiments involving entangled photon pairs, which are still the best candidates for the exchange of quantum information across any appreciable distance. Since the 1990s, spontaneous parametric downconversion (SPDC) has been established in the academic and research community as a reliable source of entangled photon pairs. During this time, source brightness and stability has improved considerably over the first demonstrations. Yet, entangled photon sources are still built primarily by graduate students, and there is significant variability among sources. Moreover, SPDC entangled photon sources lack flexibility, typically designed to output a single type of entangled state. In this paper we report on the efforts to develop a ‘user-programmable’ entangled photon source capable of producing any type of two-photon entangled state. The goal is a source capable of emitting photon pairs with user-defined polarization states. That is, the user will be able to adjust the probability amplitudes {𝛼,𝛽,𝛾,𝛿} in the general expression for a two-photon polarization state, |𝜓⟩=𝛼|𝐻𝐻⟩+𝛽|𝐻𝑉⟩+𝛾|𝑉𝐻⟩+𝛿|𝑉𝑉⟩, thereby accessing any point in the two-photon polarization entanglement Hilbert space. This requires a mapping between the experimentally controllable parameters and the probability amplitudes listed above. We will describe this mapping for our particular experimental setup and will share results showing the states that our system is able to access.
The quantum information community is entering a period in which scientists and engineers are transitioning from ‘understanding’ to ‘control’ by harnessing aspects of quantum mechanics to realize capabilities in computing, sensing, and communication that are not possible in the classical world. Although many of these quantum technologies are still in their very early stages, it is clear that our future world will include many quantum devices. The existence of multiple devices will, in turn, necessitate the transfer of quantum information between devices—if isolated devices are useful, it follows that connected devices will be even more useful. Therefore, it is imperative to develop the resources needed for connecting quantum devices at different locations. One such resource is quantum entanglement, a physical phenomenon in which two or more quantum systems cannot be described independently, even when separated by large distances. Entanglement is an important resource for quantum computing and is critical for overcoming loss constraints that limit the transfer of quantum states across large distances. The latter can be accomplished via ‘quantum repeaters,’ which propose to connect multiple shorter entangled photon links to form a longer entangled photon link. Much of our understanding of entanglement comes from experiments involving entangled photon pairs, which are still the best candidates for the exchange of quantum information across any appreciable distance. Since the 1990s, spontaneous parametric downconversion (SPDC) has been established in the academic and research community as a reliable source of entangled photon pairs. During this time, source brightness and stability has improved considerably over the first demonstrations. Yet, entangled photon sources are still built primarily by graduate students, and there is significant variability among sources. Moreover, SPDC entangled photon sources lack flexibility, typically designed to output a single type of entangled state. In this paper we report on the efforts to develop a ‘user-programmable’ entangled photon source capable of producing any type of two-photon entangled state. The goal is a source capable of emitting photon pairs with user-defined polarization states. That is, the user will be able to adjust the probability amplitudes {𝛼,𝛽,𝛾,𝛿} in the general expression for a two-photon polarization state, |𝜓⟩=𝛼|𝐻𝐻⟩+𝛽|𝐻𝑉⟩+𝛾|𝑉𝐻⟩+𝛿|𝑉𝑉⟩, thereby accessing any point in the two-photon polarization entanglement Hilbert space. This requires a mapping between the experimentally controllable parameters and the probability amplitudes listed above. We will describe this mapping for our particular experimental setup and will share results showing the states that our system is able to access.
Efficient Generation of Entangled Photon-Pairs The first experiments with correlated photons have been performed in the context of EPR-Bell experiments on the realistic and local properties of quantum mechanics. The source used there produced pairs of polarization entangled photons from a 2-photon decay of Calcium atoms. The technical requirements of these experiments were high (vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of the source was quite low. An important step forward was the introduction of spontaneous parametric down conversion (SPDC), which has become the most common source in quantum optics for generating correlated or entangled photon pairs. In this process photons of an intense pump laser convert to photon pairs in an optical nonlinear crystal. Conservation of energy and momentum leads to strong correlations between the generated photons. With this kind of two-photon source it was possible to realize or improve many experiments on the foundations of quantum mechanics addressing the EPR-Paradoxon and in the new field of quantum information. But again, more advanced experiments and applications suffer from the limited ef- ficiency of the fluorescence process. Many photon pairs are lost by spatial and spectral filtering, which is necessary to achieve polarization entanglement and long coherence times. Different techniques have been implemented to increase the number of photon pairs using two-crystal arrangements, focusing techniques or periodically poled crystals. Most of these methods have the disadvantage that no entangled photons have been observed. It is the aim of this work to increase the yield and to improve the mode definition of entangled photon pairs generated by resonant enhancement of the pump mode and the fluorescence modes. As a first step a linear cavity for the pump mode was realized. Since the conversion probability is proportional to the pump power it was possible to increase the photon pair count rate by factor of 7 over the previous source. Besides the possibility of further improvement on already established pair correlation experiments, such an enhancement allows to build a compact source for photon pairs, in which an expensive argon-ion laser is replaced by a cheap diode laser. Among other applications such sources are of strong interest for quantum cryptography. 3 In many quantum information experiments optical fibers are use to carry the photons over long distance. Therefore, light from the parametric down-conversion source has to be efficiently coupled into fibers. In the second part we report on a new method to optimize collection efficiency by matching the angular distribution of the parametric fluorescence to the spatial mode of an optical fiber. By using this technique, we detected 366500 polarization-entangled photon pairs per second in the near-infrared region in single-mode optical fibers for 465 mW pump power (at 351.1 nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs was verified by measuring polarization correlations of more than 96% in a HV-basis and in a ±45◦-basis. To our knowledge, such enormous count rates of highly entangled photon pairs have not been reached yet with any other technique. In the third part of this thesis we investigated the process of parametric downconversion in a cavity which is resonant to certain longitudinal down-conversion modes only. The idea of placing the parametric down-conversion source inside a cavity is not new. Such a device is usually referred to as a single or double resonant optical parametric oscillator (OPO) and is mainly used to generate squeezed quantum states. In that kind of application the system is operating close to but still under the threshold of oscillation. In our application the situation is quite different. The system is operating far below threshold so that mainly spontaneous emission occurs. In that mode correlations between single photons can still be observed. But bouncing the light back and forth inside the cavity increases the interaction length and hence enhances the signal levels of the down-conversion fields. Further, by resonating two certain modes only, the bandwidth is reduced by orders of magnitude and the coherence time is found to be inverse proportional to the bandwidth. A similar experiment has already been realized with a type-I parametric down-converter in the resonator. We have tried to realize a compact double resonant OPO far below threshold with a type-II parametric down-converter in a high-finesse cavity to realize a bright source of entangled photon pairs with extremely narrow bandwidth.
During the last decade, quantum entanglement has paved the way out to of the lab modern applications such as quantum computation and communication. Today, small scale quantum networks exist already, but they are limited to a few 100 km distance, due to intrinsic fiber transmission losses and non perfect detectors. These networks are typically established using photon pair sources based on spontaneous parametric down conversion (SPDC). Widely used entanglement observables are time-bin and polarization, where the latter one is undoubtedly simpler to analyze due to lack of interferometric devices. It was shown recently, that introducing quantum memories (QM) between the communication parties could increase both communication speed and distance. Such memories already exist, but their narrow absorption bandwidth does not match the broadband emission spectrum of SPDC sources. Our report is twofold. First, we introduce a new polarization entanglement scheme based on a birefringent delay line (BDL). A pair of incoming photons is projected on a polarizing beam splitter (PBS), where vertically polarized photons travel along a polarization maintaining fiber loop accumulating a delay δ compared to horizontally polarized photons, which exit the BDL directly upon reflection at the PBS. Via post selection of only simultaneously arriving photon pair events, we create entangled states of the form α|H>_1|H>_2 + e^(iφ)β|V>_1 |V>_2. The probability amplitudes α and β are simply tuned by controlling the input pair's polarization and the phase φ can be varied by piezo fiber stretcher (PZT) in the delay line. Therefore, any superposition of the polarization entangled Bell states can be created. The scheme may also be used for entanglement distillation of non maximally entangled states, e.g. emitted by quantum dots. We will introduce all requirements needed to create entanglement from such a BDL. Second, we show how to apply this scheme to produce polarization entanglement from one of the most efficient photon pair generators, namely a periodically poled Lithium Niobate waveguide structure (PPLN/W), where the quasi phase matching is optimized for the type-0 SPDC interaction. The pair's wavelength is chosen in the standardized ITU-21 channel of the low loss telecom C-band and a fiber Bragg grating is applied to reduce the initial spectral bandwidth from 5 THz to 540 MHz, which is suitable for some solid-state quantum memories. Using the BDL, we create the maximally entangled states, with near perfect quality, which is proven by a violation of the Bell's inequalities by more than 29 standard deviations. We achieve a source spectral brightness of 211 narrowband polarization entangled pairs created per second, mW of pump power, and MHz of spectral bandwidth, coupled into a single mode fiber. This is on the same level as the best time-bin entanglement sources reported to date. The source is very practical due to the use of standard telecom fiber components. Moreover the choice for polarization instead of time-bin entanglement allows for simple state analysis using only rotating PBSs. This is a non negligible advantage, especially for narrowband photons with long coherence times, where time-bin analysis requires interferometer path length differences on the order of a few meters stabilized to a few tens of nanometers. Non unit visibilities are solely explained by the nonlinear character of our photon pair source, giving rise to double pair contributions. Applying this scheme to deterministic photon pair sources should result in perfect quality entanglement. We believe that our scheme should play an important role for future quantum networking applications.
This thesis deals with the production of entangled photons using spontaneous parametric down-conversion (SPDC). We start with a short overview of some important theoretical concepts. First we provide a brief reminder of the theory of entanglement. We then discuss how the state of quantum systems can be determined using quantum state tomography. We also explain SPDC, the physical process which we use to produce entangled photons. Finally, we give an overview of the methods which have been used to produce entangled photons in the past, both for two- and three-photon entanglement. \n \nThe first experiment is the design of an efficient source of entangled photon pairs based on a polarizing Sagnac interferometer configuration. With this configuration, we can use quasi-phasematched materials which allow for higher efficiencies than standard bulk nonlinear materials. The source is pumped by a low-power continuous-wave laser diode, and produces degenerate photon pairs at 809nm. It has a spectral brightness of 87,500 pairs/(s mW nm), and the fidelity of the produced quantum states with a Bell state is 98.9%. The source is used for experiments in quantum key distribution, cluster state quantum computing, remote state preparation, state discrimination, and entanglement-enhanced classical communication. \n \nThe second experiment discussed in this thesis is the generation of photon triplets using cascaded SPDC. In this experiment, a primary SPDC source is pumped with a low-power, continuous-wave laser diode, producing photon pairs. Single photons from these pairs serve as the pump for a second down-conversion, resulting in photon triplets. This is the first demonstration of the direct production of photon triplets, and the first observation of SPDC at the single photon level. This method could potentially be used to produce entangled photon triplets without post-selection, and as a source of triggered Bell pairs.
In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1-3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.