This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.
This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.
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.
The emergence of quantum physics from the page to the lab and the world at large is an exciting development of recent years. The prospects of absolutely secure communication and efficient simulation of physical systems have spurred great human effort into understanding these possibilities and turning them into realities. Photons are the most easily manipulated quantum particles and are a promising candidate for implementing these technologies. Limitations of photons include the difficulty of keeping objects that move at the speed of light, and producing strong interactions between particles that do not normally interact. The work presented in this thesis is motivated by the possibility of overcoming these limitations. The ability to faithfully store and reproduce a quantum state is essential for many quantum information technologies. Quantum memories for light have been developed over the last two decades to provide this ability. The group at the Australian National University developed the gradient echo memory (GEM): A quantum state of light can be controllably stored and released from an atomic ensemble by the use of additional optical fields and magnetic field gradients. This scheme was previously shown to preserve the quantum characteristics of the light. We used the GEM scheme with a cold rubidium ensemble to create the first optical memory that simultaneously beat the no-cloning limit, a benchmark for many of the technologies relying on quantum memories, and the loss rate for a delay line composed of optical fibre. We also created an analogue to a pulsed optical resonator using GEM with a warm rubidium vapour. This was done by replacing the circulating optical field of a resonator with light stored in the memory, and replacing the coupling of light into and out of that circulating mode with storage and recall from the memory. The bandwidth and repetition rate of this resonator were rapidly tunable as they were controlled by external optical and magnetic fields. We worked on implementing GEM with strings of thousands of atoms strongly coupled to the evanescent field of an optical nanofibre. This raised new possibilities for creating a true random access memory that would allow a more flexible use of the multi-mode capacity of GEM. We developed the theory for a novel type of stationary light in the gradient echo memory. Our stationary light scheme relies on the destructive interference of counter-propagating optical fields throughout the memory. The optical intensity scales with optical depth, as with other forms of stationary light. However, as the destructive interference could be set up over a much greater distance, more of the optical depth is available for generating stationary light. Finally, we studied how a control-phase gate for single-photon optical states could be implemented using a nonlinear interaction with stationary light. The stationary light generated by one state modulates the phase of another state stored in the memory. The second state modifies the stationary light, also producing a back-action on the first state and generating the required cross-phase shift.
Photonic quantum information processing has emerged as a powerful platform for realising quantum-enhanced technologies. In order to be scalable, many of these technologies depend on the availability of a suitable quantum memory for the coherent storage and on-demand retrieval of photonic quantum states. In this thesis, I investigate broadband light storage in a room-temperature Raman memory, implemented both in free space and, for the first time, inside a low-finesse optical cavity designed for low-noise operation. The ability of the Raman memory to preserve phase coherence was tested by storing coherent polarisation states in two spatially separate atomic ensembles. Polarisation storage with a fidelity of up to 97 ± 1% was demonstrated by performing full process tomography on the system. The Raman memory was then interfaced for the first time with a spontaneous parametric downconversion (SPDC) source of heralded, GHz-bandwidth single photons. The memory performance was characterised by measuring the second-order autocorrelation of the retrieved fields. While the SPDC input photon statistics showed a clear influence on the statistics of the retrieved field, four-wave mixing (FWM) noise, stimulated by spontaneous Raman scattering, prevented the preservation of non-classical photon statistics during read-out. Suppressing this source of noise represents the last remaining challenge for realising a broadband single-photon Raman memory suitable for quantum information applications. To this end, I demonstrate a novel cavity implementation of the Raman memory which reduces the FWM contribution relative to the signal field by re-distributing the density of states into which the noise photons can be scattered. Cavity-enhanced memory operation was investigated using weak coherent input states, showing a significant improvement of the signal-to-noise ratio compared to the free-space memory implementation. This proof-of-principle demonstration suggests that cavity Raman memories may offer a practical route towards low-noise, high-bandwidth quantum storage at room temperature.
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.
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.
An optical quantum memory, a device which can store and retrieve on demand an arbitrary quantum state at the single photon level, has been identified as a significant cornerstone of photonic quantum technologies. The development of such a memory would allow the creation and development of a large number of quantum technologies. These would range from the ability to synchronise processing steps in an optical quantum computer, to the construction of a quantum repeater which would allow faithful transmission of quantum states over arbitrarily long distances. In this thesis I investigate broadband light storage in a room-temperature Raman memory for purposes of quantum information processing with photons. During the course of this thesis, a novel scheme to suppress four-wave mixing noise is simulated, based on equations derived to describe the physics of the memory interaction. The experimental arrangement to test the suppression method is described and an original method using photon statistics is demonstrated. The results of these experiments prove the ability of the memory to retrieve and maintain the non-classicality of the state, given a single photon input. Following the affirmation that, using this noise suppression technique, the memory can function at the single-photon level, I go on to consider how the temporal-spectral properties of a pulsed mode of light may be manipulated using the memory. There are two primary reasons for this investigation: 1) Increasing the efficiency of the read-in process by carving the optimum mode of the memory 2) Using the memory as a device operating in a high dimensional quantum information basis. The restrictions of such applications are considered and a method to test the efficacy of transformations in this basis is devised and experimentally tested. I also perform demonstrations of light storage using conventional temporal modes using a novel method of velocity selection within warm vapour. Finally I detail further experiments that are achievable with the Raman quantum memory given the progress that has been achieved during my DPhil research. The results show that the Raman memory is a viable candidate for a practical route towards low-noise quantum storage and for manipulation of temporal modes of light.