Society as we know it today would not have been possible without the explosive and astonishing development of telecommunications systems, and optical fibers have been one of the pillars of these technologies.

Despite the enormous amount of data being transmitted over optical networks today, the trend is that the demand for higher bandwidths will also increase. Given this context, a central element in the design of telecommunications networks will be data security, since information can often be confidential or private.

Quantum information emerges as a solution to encrypt data by quantum key distribution (QKD) between two users. This technique uses the properties of nature as the fundamentals of operation rather than relying on mathematical constructs to provide data protection. A popular alternative to performing QKD is to use the relative phase between two individual photon paths for information encoding. However, this method was not practical over long distances. The time-bin- based scheme was a solution to the previous problem given its practical nature, however, it introduces intrinsic losses due to its design, which increases with the dimension of the encoded quantum system.

In this thesis we have designed and tested a fiber-optic platform using spatial-division- multiplexing techniques. The use of few-mode fibers and photonic lanterns are the cornerstone of our proposal, which also allow us to support orbital angular momentum (OAM) modes. The platform builds on the core ideas of the phase-coded quantum communication system and also takes advantage of the benefits proposed by the time-bin scheme. We have experimentally tested our proposal by successfully transmitting phase-coded single-photon states over 500 m few-mode fiber, demonstrating the feasibility of our scheme. We demonstrated the successful creation of OAM states, their propagation and their successful detection in an all in-fiber scheme. Our platform eliminates the post-selection losses of time-bin quantum communication systems and ensures compatibility with next-generation optical networks and opens up new possibilities for quantum communication.

The protection of confidential data is a fundamental need in the society in which we live. This task becomes more relevant when observing that every day, data traffic increases exponentially, as well as the number of attacks on the telecommunication infra-structure. From the natural sciences, it has been strongly argued that quantum communication has great potential to solve this problem, to such an extent that various governmental and industrial entities believe the protection provided by quantum communications will be an important layer in the field of information security in the next decades. However, integrating quantum technologies both in current optical networks and in industrial systems is not a trivial task, taking into account that a large part of current quantum optical systems are based on bulk optical devices, which could become an important limitation. Throughout this thesis we present an all-in-fiber optical platform that allows a wide range of tasks that aim to take a step forward in terms of generation and detection of photonic states. Among the main features, the generation and detection of photonic quantum states carrying orbital angular momentum stand out.   

The platform can also be configured for the generation of random numbers from quantum mechanical measurements, a central aspect in future information tasks.  

Our scheme is based on the use of new space-division-multiplexing (SDM) technologies such as few-mode-fibers and photonic lanterns. Furthermore, our platform can also be scaled to high dimensions, it operates in 1550 nm (telecommunications band) and all the components used for its implementation are commercially available. The results presented in this thesis can be a solid alternative to guarantee the compatibility of new SDM technologies in emerging experiments on optical networks and open up new possibilities for quantum communication. 

Skydd av konfidentiell information är ett grundläggande behov i det samhälle vi lever i. Denna uppgift blir mer relevant när man observerar att datatrafiken ökar exponentiellt varje dag, liksom antalet attacker på telekommunikationsinfrastrukturen. Från naturvetenskapen har det starkt hävdats att kvantkommunikation har en stor potential att lösa detta problem, i en sådan utsträckning att olika statliga och industriella enheter tror att skyddet som tillhandahålls av kvantkommunikation kommer att vara en viktig del av informationssäkerhet de kommande decennierna.

Att implementera kvantteknik både i nuvarande optiska nätverk och i industriella system är dock inte en trivial uppgift, med tanke på att en stor del av nuvarande kvantoptiska system inte är i optiska fiber, vilket kan bli en begränsning. Genom hela denna avhandling presenterar vi en plattform som är helt konstruerad med optiska fiber som tillåter ett brett utbud av uppgifter som syftar till att ta ett steg framåt när det gäller integration med nu-varande optiska nätverk. Bland huvuddragen i denna idé utmärker den skapandet och detekteringen av fotoniska kvanttillstånd i  ”Orbital Angular Momentum”, såväl som allmänna överlagringar  av rumsliga optiska moder som kan transmitteras inom en optisk fiber. Plattformen kan också konfigureras för generering av slumptal från kvantmekaniska mätningar, vilket är en central aspekt i framtida hantering av information.  

Vår plattform är baserad på användningen av nya tekniker för ”Space Division Multiplexing” (på Engelska) såsom ”few-mode-fibers” och ”photonic lanterns”. Dessutom kan vår plattform skalas till högre dimensioner, den fungerar i 1550 nm (telekommunikationsbandet) och alla komponenter som används för dess implementering är kommersiellt tillgängliga, därför kan resultaten som presenteras i denna avhandling vara ett robust alternativ för att säkerställa kompatibiliteten av nya framväxande teknologier och experiment med nästa generations optiska nätverk och öppnar nya möjligheter för kvantkommunikation.