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.
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.
Quantum communication founds on the possibility to encode quantum states into photons and allows, for instance, the provable secure distribution of encryption keys. Despite a lot of progress during the past two decades, quantum communication still faces a distance barrier, which can be overcome by quantum repeaters whose currently most challenging ingredient is a memory for quantum states of light. In addition to robustness and ease of use, the requirements for such a quantum memory include high fidelity quantum state storage and the preservation of entanglement, sufficiently long storage time with on-demand recall, high efficiency, large bandwidth, and high multimode capacity. Six years ago, some of these properties had already been demonstrated using atomic vapor. However, practical solid-state memories were virtually unexplored in view of quantum state storage. The main goal of this thesis was to realize a solid-state quantum memory for light with emphasis on meeting requirements for long distance quantum communication. First, we identified a potential material candidate, a thulium doped lithium niobate waveguide, and conducted spectroscopic investigations to assess its suitability for quantum state storage using a photon-echo type approach. Next, we demonstrated that our storage device is suitable for high fidelity quantum state storage and for the storage of photonic entanglement, and that its large spectral multimode capacity allows for on-demand selective recall in conjunction with standard spectral filters and frequency shifters. In addition, by performing two-photon interference experiments and Bell-State measurements, we found that our memory preserves not only quantum information, but, more generally, the entire photonic wave function, which further confirms its suitability for quantum repeaters as well as for linear optics quantum computers. Finally, we showed that our integrated device also allows for general temporal and spectral manipulation of individual quantum optical pulses, which paves the way towards on-chip quantum optical processors. While more work remains to be done, in particular to improve memory efficiency and storage time, the large number of achievements, together with known ways to overcome the remaining obstacles, makes us confident that a quantum memory suitable for quantum repeaters will soon be built.
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.
This dissertation is devoted to the development of quantum memories for light. Quantum memory is an important part of future long-distance quantum fiber networks and quantum processing. Quantum memory is required to be efficient, multimode, noise free, scalable, and should be able to provide long storage times for practical applications in quantum communications and beyond. Here I concentrate on solving particular problems of different quantum protocols and find ways for extending the performance of memories and adding new capabilities. I theoretically show that an array of whispering-gallery resonators is capable of being an efficient and noise-free optical memory with an adjustable storage time. The potential for on-chip realization at room temperature makes the scheme attractive for easy implementation. The effect of Raman scattering in echo memory was evaluated experimentally and theoretically. The noise performance of gradient echo memory in Λ configuration proves, that the developed theory is in a good agreement with an experiment. I proposed a mechanism for extending the bandwidth of impedance-matched memories via a white-light cavity effect. The introduced additional dispersion compensates a bandwidth decrease induced by the cavity and hence increases the spectral zone of impedance matching. Theoretically the scheme allows to increase the bandwidth of high efficient storage (>90%) several times without adding extra noise. Finally, I have proposed an architecture of quantum random-access memory for time-bin photons. The architecture consists from a memory unit and a strongly coupled three-level atom. Both of them are placed in their own cavities, which are coupled to each other. The protocol allows to achieve quantum addressing of quantum information stored in the memory with only a single control unit. This is useful for numerous tasks in quantum machine learning.
Quantum memories for light lie at the heart of long-distance provably-secure communication [1], while containing the potential to help break current encryption methods [2], and allow better measurement of quantities than ever before [3]. Demand for a functioning quantum memory is therefore at a premium. Unfortunately, the same properties of light that make it such an effective carrier of quantum information make it difficult to store. Furthermore, by the laws of quantum mechanics, storage must be achieved without measurement to preserve the quantum state. A quantum memory needs to have an efficiency approaching unity without adding noise to the state, and storage times from milliseconds to seconds. Ideally it would also have a high bandwidth and be able to store many pieces of information simultaneously. Many different techniques are currently being developed and much experimental progress has been made over the past few years, with: efficiencies approaching 90% [4]; storage times of over seconds [5]; bandwidths of gigahertz [6, 7]; and over 1000 pieces of information stored at one time [8]. These results were, however, achieved using different memory schemes in different storage media. The challenge now is to reproduce these results with one memory. This thesis focuses on extending the gradient echo memory (GEM) scheme, which shows great promise due to the high efficiencies achieved (87%) [4]. GEM has also been used to demonstrate temporal compression and stretching of pulses, as well as a capacity to arbitrarily resequence stored information [9] and the interference of initially time-separated pulses [10]. Firstly, we demonstrate the noiseless nature of GEM storage in a warm vapour cell to prove that the output from the memory is the best-possible copy of the input allowed by quantum mechanics. We show GEM’s ability to coherently and precisely spectrallymanipulate stored information by having fine control over the memory’s frequency gradient, with potential applications for dynamic conditioning of information inside quantum networks [11]. We demonstrate cross-phase modulation of a stored light pulse with an additional optical field, a process with applications in quantum computing [12]. We also carry out storage of different spatial modes and arbitrary images, demonstrating the potential for orders of magnitude improvement in storage capacity. We then switch from warm vapour cells to cold atomic ensembles to improve the storage time of GEM, seeing a maximum coherence time of 350 μs (seven times that of the warm vapour system) and achieving efficiencies of up to 80%, on a par with the highest efficiency achieved with a cold atomic ensemble [13]. In the process we developed an ultra-dense cold atomic cloud with potential applications in a range of quantum optics experiments. Cold atoms, and the small volumes they occupy, also allowed us to develop an alternative to using magnetic field gradients for our alkali-atom memories in the form of a light-field gradient. This holds promise for extremely fast gradient switching and fine control over the gradient. We also present a digital locking code with application in a range of quantum optics experiments.
Interaction of quantum radiation with multi-particle ensembles has sparked off intense research efforts during the past decade. Emblematic of this field is the quantum memory scheme, where a quantum state of light is mapped onto an ensemble of atoms and then recovered in its original shape. While opening new access to the basics of light–atom interaction, quantum memory also appears as a key element for information processing applications, such as linear optics quantum computation and long-distance quantum communication via quantum repeaters. Not surprisingly, it is far from trivial to practically recover a stored quantum state of light and, although impressive progress has already been accomplished, researchers are still struggling to reach this ambitious objective. This special issue provides an account of the state-of-the-art in a fast-moving research area that makes physicists, engineers and chemists work together at the forefront of their discipline, involving quantum fields and atoms in different media, magnetic resonance techniques and material science. Various strategies have been considered to store and retrieve quantum light. The explored designs belong to three main—while still overlapping—classes. In architectures derived from photon echo, information is mapped over the spectral components of inhomogeneously broadened absorption bands, such as those encountered in rare earth ion doped crystals and atomic gases in external gradient magnetic field. Protocols based on electromagnetic induced transparency also rely on resonant excitation and are ideally suited to the homogeneous absorption lines offered by laser cooled atomic clouds or ion Coulomb crystals. Finally off-resonance approaches are illustrated by Faraday and Raman processes. Coupling with an optical cavity may enhance the storage process, even for negligibly small atom number. Multiple scattering is also proposed as a way to enlarge the quantum interaction distance of light with matter. The quest for higher efficiency, better fidelity, broader bandwidth, multimode capacity and longer storage lifetime is pursued in all those approaches, as shown in this special issue. The improvement of quantum memory operation specifically requires in-depth study and control of numerous physical processes leading to atomic decoherence. The present issue reflects the development of rare earth ion doped matrices offering long lifetime superposition states, either as bulk crystals or as optical waveguides. The need for quantum sources and high efficiency detectors at the single photon level is also illustrated. Several papers address the networking of quantum memories either in long-haul cryptography or in the prospect of quantum processing. In this context, much attention has been paid recently to interfacing quantum light with superconducting qubits and with nitrogen-vacancy centers in diamond. Finally, the quantum interfacing of light with matter raises questions on entanglement. The last two papers are devoted to the generation of entanglement by dissipative processes. It is shown that long lifetime entanglement may be built in this way. We hope this special issue will help readers to become familiar with the exciting field of ensemble-based quantum memories and will stimulate them to bring deeper insights and new ideas to this area.
Quantum information processing promises to have transformative impacts on information and communication science and technology. Photonic implementation of quantum information processing is among successful candidates for implementation of quantum computation and is an essential part of quantum communication. Linear optical quantum computation, specifically the KLM scheme, and quantum repeaters are prominent candidates for practical photonic quantum computation and long-distance quantum communication. Quantum memories for photons are key elements for any practical implementation of these schemes. Practical quantum memories require theoretical and experimental investigations into quantum memory protocols and physical systems for implementations. The present thesis is focused on studying new approaches toward practical solid-state based quantum memories. First, I present a proposal for a new quantum memory protocol called the controllable-dipole quantum memory. It represents a protocol, in a two-level system, without any optical control that is shown to be equivalent to the Raman type-quantum memory. Then I include our studies on the quantum memory based on the refractive index modulation of the host medium. It is shown that it can resemble the gradient echo quantum memory without a spatial gradient in the external field. These two protocols can be implemented in rare-earth doped crystals. With regards to using new physical systems, I present a proposal based on nitrogen vacancy centers. This may pave the way toward micron-scale on-chip quantum memories that may contribute to the implementation of integrated quantum photonics. Finally, I studied the precision requirements for the spin echo technique. This technique is necessary to extend the storage time in solid-state quantum memories, in which the coherence times are limited by spin inhomogeneous broadening.