High performance solid state quantum memory

Abstract

This thesis demonstrates an efficient quantum memory that for the first time preserves more quantum information than it loses. Beyond this, it describes how even higher efficiency may be realised without sacrificing the other performance characteristics that are required for a practical device. A quantum memory is a device to store and recall the quantum state of light. Such a memory will be required for most applications of the emerging field of quantum information. The closest practical application is quantum repeaters for long distance encryption. A more distant target is quantum computing. Yet demonstrations of quantum memory to date fall far short of the performance required for repeaters, and further short of that for quantum computing. For these applications, high performance will be required in terms of 3 parameters: long storage time, high efficiency, and a large mode-capacity. Prior to this work, the storage time was the only paramter to have been demonstrated near the required level[1], and that only in a classical regime. The primary result of this thesis is to demonst rate a memory with an efficiency close the required level (70% is measured), and confirm its quantum nature with noise measurements. This represents the first quantum memory with efficiency greater than 50% and the first to operate unconditionally above the "no-cloning limit". The demonstration used the same storage medium as for the previous long-term storage experiment (Pr{u00B3}{u207A}:Y{u2082}Si0{u2085} (PrYSO)), and could in principle reach the same long storage times. The technique used is an optical "gradient echo", an optical equivalent to the long studied gradient echo technique used in Nuclear Magnetic Resonance (NMR). Aside from it's practicality for demonstrations, a gradient-echo also offers a powerful handle for manipulating stored light. However it is more complex than related memory techniques because it employs a spatially-varying medium. This becomes crucial when the optical depth (and memory efficiency) is increased; propagation effects mean the light is stored very differently than in other techniques. A simple linear-response model is used to gain insight into this high optical depth regime. As a result, some new properties are identified which may increase the efficiency in practical circumstances and improve flexibility. Of particular note, it's found that the space-dependence can be used to hide one narrow transition with another. This allows the somewhat unintuitive notion that a light pulse can fully traverse a medium without exciting a transition that it is resonant with. The spectral-hiding property has promise to overcome the problem with simultaneously acheiving all three parameters required for quantum memories. The problem is related to the preparation of available rare-earth ion doped materials. To acheive high efficiency and storage times, these require spectral holeburning whose bandwidth is limited by the splittings of spin levels. Methods exist to create high bandwidth delays lines with holeburning[2], but they cannot be efficient or long-lived. By utilising the spectral-hiding property of the gradient echo, methods to overcome this limitation are described for the first time.