Cleverly utilized, quantum theory can tremendously enhance a broad range of optical technologies from secure communication and distributed computing to super-resolved imaging. In particular, pairs of photons can be easily produced in quantum entangled states of their polarizations – Bell states. Such states yield a non-local property. Measuring polarization of one photon gives a random result, but then the other is always found in the orthogonal polarization. While of many applications, entangled photonic states are also very fragile. After transmission through 100 km of telecom (1500 nm) fiber only ca. 1% of the photons survive and an entangled state cannot be directly amplified. As a solution quantum repeaters (QR) have been proposed and demonstrated [1] .
Cleverly utilized, quantum theory can tremendously enhance a broad range of optical technologies from secure communication and distributed computing to super-resolved imaging. In particular, pairs of photons can be easily produced in quantum entangled states of their polarizations – Bell states. Such states yield a non-local property. Measuring polarization of one photon gives a random result, but then the other is always found in the orthogonal polarization. While of many applications, entangled photonic states are also very fragile. After transmission through 100 km of telecom (1500 nm) fiber only ca. 1% of the photons survive and an entangled state cannot be directly amplified. As a solution quantum repeaters (QR) have been proposed and demonstrated [1] .
"Quantum communication is the art of transferring a quantum state from one place to another". The capability of distributing quantum states allows, for instance, provably secure communication and, in the near future, will enable the interconnection of quantum computers to establish a quantum internet. A convenient way to send quantum states from one place to another is to encode them into photons and distribute them using optical bres. The distance over which this can be achieved is still limited to a few hundred of kilometers. However, it can be extended in principle to arbitrary long distances by means of quantum repeaters. The realization of such quantum repeaters will require the engineering and interfacing of heterogeneous elements such as entangled photon-pair sources, quantum memories and entanglement swapping operations. The main goal of this thesis is to develop and study the compatibility of the main elements required to build a frequency multiplexed quantum repeater. This architecture requires a frequency multimode entangled photon pair source which, for example, could be implemented exploiting the process that involves the interaction of a strong light pulse with a non-linear crystal. Such a process is versatile and allows easy tailoring of the photon-pair spectral properties in order to match the requirements of the frequency multiplexed quantum repeater architecture. First, as a precursor of such a photon pair source, we developed a novel heralded single photon source that involves spectrally resolved single photon detection and on-demand frequency shifting. These capabilities are also essential tools in a frequency multiplexed quantum repeater. Next, we demonstrated the compatibility of such a sources with frequency multimode photonic quantum memories by performing spectrally multiplexed storage of single photons. We also tested the capability of high- delity storage of photons encoding quantum information in their polarization degree of freedom. Finally, we showed that entanglement swapping operations, i.e. Bell-state measurements, can be performed using photon pair sources compatible with quantum memories, quantum repeater architectures, and real-world scenarios. While a lot of e ort remains to be done, in the work presented in this thesis we demonstrated that all the key elements required to build a frequency multiplexed quantum repeater can be built and eventually be interfaced. The main obstacles towards that goal and possible ways to overcome them are also discussed.
The development of the quantum computer challenges the security of classical communication. This is due to the security of classical encryption being based on the computational complexity of an algorithm, for example, the difficulty of factoring a number into its prime product. Quantum communication uses quantum mechanics to manipulate quantum states and exchange information between two locations. Quantum communication has proven to be unconditionally secure. However, the fact that the maximum transmission distance is largely limited by attenuation in the distribution link remains one of the biggest challenges for practical quantum communication. Quantum repeaters can greatly extend this communication transmission distance. To realize quantum repeaters, one requires quantum memories, entanglement swapping operations, and photon pair-sources. In addition, quantum repeaters greatly benefit - in terms of the achievable entanglement distribution rate - if they can incorporate some form of multiplexing. Multiplexing is possible in any of the available photonic modes, which for transmission in single-mode fibers amounts to polarization, spectral, and temporal modes. The polarization degree of freedom is undesirable because it only has two orthogonal modes. The choice of the spectral or temporal modes for multiplexing then depends on what is most compatible with other components in the setup, particularly the quantum memory, which is the most demanding component. Based on prior demonstrations in the lab of spectral multiplexing in rare-earth ion doped crystals, the purpose of my project has been to build a spectrally multi-mode photon pair-source. Our implementation of the spectrally multi-mode photon pair sources relies on a nonlinear optics process that involves the interaction of strong light and a non-linear crystal. To achieve the spectral selection of specific modes, the non-linear crystal is combined with a suitable cavity.
Quantum Key Distribution (QKD) has opened a new avenue for secure communication, by allowing one to distribute a random secret key between two users that are connected through a public channel without revealing information to unauthorized parties. If a message is encrypted with the one time pad (a well-known crytography algorithm) using secret keys created by QKD, then the ciphertext is information-theoretically secure and thus unbreakable for adversaries. Despite its unprecedented security, loss in the physical channel has prevented QKD from being used over distances beyond a few hundred kilometers. Fortunately, quantum repeaters have opened a path for long-distance QKD by providing a means to establish entanglement between distant users. The goal of this thesis has been to develop a source of entangled photon pairs that is suitable for quantum repeaters and then use it to test some of the fundamental building blocks of a quantum repeater : the heralded creation of entangled photons by means of entanglement swapping with properties that allow interfacing with optical quantum memories; the reversible mapping of quantum states from members of entangled photon pairs in and out of solid-state quantum memories; two-photon interference and a Bell state measurement with photons recalled from separate quantum memories. The demonstration of these key ingredients of a quantum repeater constitutes a significant step towards the establishment of entanglement over hundreds of kilometer distance, and hence long-distance QKD.
Entanglement-based quantum cryptography has the appealing advantage of intuitively more evident security. While originally, weak laser pulse schemes were implemented earlier as technologically simpler, it is now possible to build entanglement-based quantum key distribution systems on a technically equally advanced level. The existing polarization-based systems as developed in Vienna now cover distances of the order of 100 km in fiber and of 144 km in free space. In a recent fiber experiment, an asymmetric source is used such that one photon at the 1.550 nm telecom wavelength is transmitted to Bob, while the other photon at 810 nm is locally measured by Alice. It turns out that polarization entanglement is rather robust, certainly over distances of 100 km in fibers. In a recent long-distance free-space experiment, one photon was sent over 144 km from the Canary Island of La Palma to the island of Tenerife, while again the other photon was measured locally. The receiving station uses the OGS telescope operated by the European Space Agency ESA. This experiment opens up the possibility for future quantum key distribution using satellites.
The quantum internet, when finally deployed, will enable a plethora of new applications such as theoretically proven secure communication and networked quantum computing, much the same as its classical counterpart whose development began back in the 1990’s. The task of creating a globe-spanning quantum network, however, is proving a rather difficult task due to the detrimental effect of loss on photon transmission. This problem can in theory be solved using so-called quantum repeaters. In one of the most well-known configurations, the long-distance span is divided into smaller segments—so called elementary links—at whose ends pairs of entangled photons are generated. One photon per pair is stored in a quantum memory, and the second member is transmitted via optical fiber to a remote measurement stations positioned at the centre of the segment. There, a joint measurement of the two photons, one from each end, then heralds the distribution of entanglement between the two quantum memories, i.e. heralds entanglement over the elementary link. In the final step, all neighbouring links are combined via a second joint measurement, and end-to-end entanglement is created...