Quantum optical storage and processing using raman gradient echo memory




Hosseini, Mahdi

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The non-interacting and high-speed nature of light makes it an ideal carrier of information that is essential for transmission of quantum information. Indeed, many proposals and demonstrations of quantum cryptography rely on the use of fibre-optic networks. Construction of a memory that can store light and preserve its quantum properties will be useful in a range of quantum information systems such as secure quantum communication and quantum computation. This is why a quantum memory for light is a remarkable objective. The key to quantum memory is to store the probability amplitude of the possible outcomes of measurement but without measurement. An important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information, since it guarantees that nobody can access the information by copying it. This benchmark is important because any kind of deterministic amplification of quantum information is fundamentally impossible. On-demand retrieval of information and ability to controllably manipulate the quantum information are also important for quantum applications. When light is absorbed by atoms, it is actually possible to reverse the absorption process. In our memory system: light is absorbed by an ensemble of atoms and, using careful conditioning and control, we can cause the stored light to be regenerated and released at a later time. This is done by applying a gradient of magnetic field along the atomic ensemble that is the basis for our optical memory. To recall the light we flip the sign of the gradient field. This kind of reversible absorption is called photon echo, hence the name of our scheme: The Gradient Echo Memory (GEM). This simple protocol is used in our experiment and can be applied to a range of different atomic systems. We have extended the GEM protocol and experimentally implemented a memory using three-level atoms. We used an off-the-shelf Rb vapour cell operating above room temperature as the memory medium. In this realisation, we broke the efficiency record with 87% recall of the input light pulse. Moreover, through complete state tomography of coherent states we have demonstrated the ability of our memory to noiselessly store quantum states of light. We have also demonstrated that the memory can store a string of pulses and then recall the pulses ondemand in arbitrary order allowing re-sequencing of the stored information. Furthermore, we have shown that pulses could be time-compressed, time-stretched or split into multiple smaller pulses and selectively recalled in several pieces. This technique enables the construction of an optical random-access memory for quantum information. Moreover, the scheme to manipulate the spectral properties of optical data, stored inside the memory, has been introduced. We have also investigated the possibility of obtaining large nonlinear phase shifts between single photons inside the memory. Such strong interactions can be used for the implementation of universal quantum gates.






Thesis (PhD)

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