Browsing by Author "Tittel, Wolfgang"
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Item Open Access Analysis of a Deterministic Entangled Photon Pair Source using Single Photons(2016) Goswami, Abhirup; Simon, Christoph; Tittel, Wolfgang; Barclay, PaulQuantum repeaters are the most promising approach for distributing entanglement over long distances. Recent approaches to develop quantum repeaters involve the use of deterministic entangled photon pair sources. On the other hand heralded entangled photon pair sources have been proposed independently both with parametric down conversion source and single photon sources. We modify the latter scheme to implement it as an on demand entangled photon pair source and analyse its performance considering inefficient detectors, inefficient quantum memories and inefficient single photon sources. We conclude that with the current state of art, generation of high fidelity deterministic entangled photon pairs is possible with moderate efficiency. We then compare the results to the deterministic entangled photon pair source obtainable using parametric down conversion source and conclude that the single photon scheme described in this thesis is more practical for such an implementation.Item Open Access Bell state measurements for quantum communication(2017-12-20) Valivarthi, Venkata Ramana Raju; Tittel, Wolfgang; Barclay, Paul; Hobill, David; Shields, Andrew; Safavi-Naeini, ReyhanehOver the last few decades, quantum key distribution (QKD) has gained a lot of attention due to its promise of establishing secret keys between authenticated users even in the presence of an eavesdropper who is only confined by the laws of nature. Secure key established by QKD in conjunction with one-time pad (OTP) encryption thus promises to end the long standing battle between code-makers and code-breakers. Spurred by its great promise, QKD has been the first quantum information technology to be commercialized and QKD systems are available from a number of vendors.However, these systems are still vulnerable to side-channel attacks as the components used in these systems don’t necessarily conform to the idealistic assumptions made in security proofs of QKD. Of the many components, single photon detectors have been identified as the most vulnerable component allowing, for instance, so-called ‘blinding attacks.’ In light of this, measurement device independent quantum key distribution (MDIQKD) protocol was proposed as a means to make QKD inherently immune to all possible detector side channel attacks, due to the particular property of the so-called Bell state measurement. The aim of this thesis is to develop techniques that will allow developing a cost- effective MDIQKD system that is suitable for quantum networks and to use these techniques to perform quantum teleportation on a metropolitan scale for the first time. More precisely this thesis describes the assessment of performance of MDIQKD using differ- ent hardware; the development of cost-effective MDIQKD system for quantum networks; the building of a practical quantum random generator (QRNG) suitable for high speed QKD systems; the demonstration of quantum teleportation on a metropolitan scale and the realization of an efficient Bell state analyzer for time-bin qubits that improves the efficiency of all quantum information processing tasks including MDIQKD and quantum teleportation. The above demonstrations constitute an important step towards realizing practical quantum internet.Item Open Access Broadband quantum interface using a thulium-doped waveguide(2010) Sinclair, Neil; Tittel, WolfgangItem Open Access Broadband Waveguide Quantum Memory for Quantum Communication(2013-01-15) Saglamyurek, Erhan; Tittel, WolfgangQuantum 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.Item Open Access Cavity-Enhanced Waveguide Quantum Memory(2015-02-06) Mallahzadeh, Hassan; Tittel, WolfgangAn optical quantum memory is a device capable of storing the quantum state of a photon and subsequently recalling it faithfully. The efficiency of a memory, which is the focus of this work, is defined as the probability of successful storage and subsequent retrieval of the information. This thesis reports an experiment performed towards making a high efficiency quantum memory by using a cavity enhanced Atomic Frequency Comb (AFC) protocol, implemented in a waveguide in a rare-earth ion doped crystal. Our measurements identified the obstacles that have to be overcome to achieve that aim. Exploiting the cavity dynamics, we managed to estimate the essential parameters in implementation of the scheme. The investigations give a valuable insight into the protocol and the ways to exploit its full potential. Furthermore, the analysis of the experimental observations leads to proposals that may serve as alternatives to current ways of implementing the protocol.Item Open Access Device-dependent and device-independent quantum key distribution without a shared reference frame(Institute of Physics, 2014-04-03) Slater, Joshua A; Branciard, Cyril; Brunner, Nicolas; Tittel, WolfgangItem Open Access Engineered impurity-doped materials for Quantum Information Processing applications - nano-structures and disordered materials(2017) Lutz, Thomas; Tittel, Wolfgang; Barclay, Paul E.; Moazzen-Ahmadi, Nasser; Simon, Christoph; Heyne, Belinda Josiane M.; Loncar, Marko; Pierce, Greg DavidIn this thesis we explore various ways to extend population lifetimes and coherence times of solid-state emitters. We focus on rare-earth-ion doped host materials and silicon vacancy centers in diamond, both of which are used for applications in quantum information processing and quantum communications. Enhanced lifetimes and coherence times would improve the performance of these applications. One approach investigates the possibility to suppress lattice vibrations that cause decoherence and population relaxation by engineering the phonon density of states through nano-structuring of the emitter's host material. Towards this end we study different materials and methods to obtain the desired nano-materials. Using various optical spectroscopic methods, we show that population dynamics can indeed be influenced by modifying the structure of the surrounding host material. However, we also find that the employed fabrication and synthesis methods often induce crystal damage that, in turn, degrades spectroscopic properties. As a second approach, we study rare-earth-ions in disordered host materials. Detailed spectroscopic characterizations are presented and we show with the example of an erbium doped fiber that such materials can indeed feature better properties, specifically longer population lifetimes, than the commonly used bulk crystals. We found optimal operation parameters for the erbium doped fiber which made it possible to use this medium for successful proof of principle experiments demonstrating a multimode quantum memory that operates within the convenient telecom band (around 1550 nm wavelength). Besides increasing the fundamental knowledge, the results of the studies presented in this thesis are highly relevant for the fields of quantum communications and quantum information processing since nano-structured materials are beginning to be investigated for on-chip implementations of various applications such as quantum memories and quantum gates. In addition, we found that population dynamics driven by detrimental lattice vibrations can indeed be modified in small powder materials and thus, with improved fabrication techniques, the complete suppression of lattice vibrations should be possible, benefiting a plethora of applications.Item Open Access Entaglement swapping with imperfect sources and detectors(2009) Howard, Regina B.; Sanders, Barry C.; Tittel, WolfgangItem Open Access Fiber-taper collected photoluminescence characterization of diamond microdisks(2018-05-15) Masuda, Tamiko; Barclay, Paul E.; Tittel, Wolfgang; Simon, Ch. H.A key architectural element of future quantum photonic networks is an efficient light-matter interface to connect electronic and photonic qubit systems. Nanophotonic resonators can be fabricated on-chip to provide such interfaces for atomic-like defect centers in diamond, which are leading qubit candidates. Fabrication advancements have recently lead to the construction of high quality diamond microdisk resonators, which show potential to reach enhancements with Purcell factor CNV ∼ 50. Here, a room-temperature experimental apparatus integrating free space and visible wavelength fiber-taper measurement capabilities is built to characterize diamond microdisk resonators. Using this setup, microdisk wisphering gallery modes with quality factors at visible wavelengths resonant with defect centers as high as Q ∼ 1 × 105, are observed for the first time. Spectral filtering effects of the taper on the microdisk are analysed to reveal that coupling to these disks may be limited by phase matching requirements. By thinning these disks it should be possible to improve coupling while lowering mode volumes, as desired to optimize Purcell factors.Item Open Access High-rate single-photon detection(2012) Healey, Christopher; Tittel, WolfgangItem Open Access Long-distance practical quantum key distribution by entanglement swapping(Optical Society of America, 2011-02-14) Scherer, Artur; Sanders, Barry C.; Tittel, WolfgangItem Open Access Low-density parity-check codes for quantum key distribution(2009) Chan, Philip; Jullien, Graham; Tittel, WolfgangThe use of low-density parity-check codes for error correction in a quantum key diwtribution protocol is examined. These code are suitable for high speed implementation in hardware because of the potential for parallel computation. With this in mind, the design of parity check matrice is examined and several modification to an existing matrix design metholology are proposed. The matrices designed are characterized, and it is shown that, with minor modification fixed-point arithmetic can be used without degrading the decoding performance. A scalable VHDL implementation of the decoder has been designed. While a parallel decoder with large block length was not possible with the current design, the achievable secret key rate is an improvement over a software implementation by a factor of ten. There are a number of avenues that may be pursued to improve the hardware, and it is recommended that they are investigated in future work.Item Open Access Measurement-device-independent quantum key distribution for metropolitan network(2019-12) Umesh, Prathwiraj; Thompson, Robert; Barclay, Paul E.; Tittel, Wolfgang; Davidsen, JorenSince its initial proposal in 2012, measurement-device-independent quantum key distribution (MDIQKD) has inspired rapid experimental progress as it is invulnerable to all detector side-channel attacks and ideally suitable for quantum key distribution (QKD) networks with star-topology, such as metropolitan networks. The main goal of this thesis is to develop MDIQKD systems for building a cost-effective metropolitan quantum network. Towards this end, we experimentally demonstrate the coexistence of MDIQKD with classical communication on the same fibre. This eliminates the use of dark fibre for quantum communication and minimises implementation costs by utilising the existing fibre infrastructure. Additionally, we move the quantum channel from the third telecommunication window (1530-1565 nm) to second (1260-1360 nm) to ensure MDIQKD can co-exist with classical communication rates of over 10-terabits per second. Furthermore, we enhance the performance of the MDIQKD systems by increasing the repetition rate from 20MHz to 200MHz and improve flexibility and reliability to facilitate the deployment of a metropolitan quantum network.Item Open Access Measuring decay rate of spontaneous emission from an ensemble of cold atoms by homodyne detection(2018-03-06) Jalnapurkar, Shreyas; Lvovsky, Alexander; Simon, Christoph; Tittel, WolfgangNanofibers are a valuable tool for interfacing photons with atomic media through its guided mode which propagates evanescent to the fiber. However, the nanofiber modifies the natural decay rate of the atomic dipoles due to the Purcell effect. It is vital to study these atom-fiber interactions to characterize the system. This thesis delineates an experimental technique to obtain the spontaneous emission rate of a cloud of Rubidium-87 atoms in the presence of a nanofiber. The atoms are prepared in a magneto optical trap and the emissions are monitored by balanced homodyne detection. The temporal autocorrelation function of the homodyne’s photocurrent reproduces the exponential profile of spontaneous emission. The measured time constant of the profile is 26.3 ns which is almost twice that of free space emission. The modified decay rate depends on the atom’s orientation and distance from the nanofiber surface. We theoretically estimate the modified decay rate and model the expected decay profile for our ensemble of atoms which agrees well with our measurements. To our knowledge, this is the first experimental demonstration of spontaneous emission being measured by homodyne detection.Item Open Access Nanophotonic Optomechanical Devices for Torque Magnetometry(2016) Wu, Marcelo; Barclay, Paul E.; Simon, Christoph; Tittel, Wolfgang; Plume, René; Kim, Seonghwan; Fuchs, GregTorque magnetometry is a powerful and sensitive method for studying intricate mesoscopic magnetic events inside magnetic materials using nanomechanical resonators. Over the years, the field of cavity optomechanics has demonstrated ever increasing sensitivity, with measurements limited by the quantum motion of a device possible in state-of-the-art devices. In this thesis, a nanophotonic cavity is integrated into a nanomechanical resonator for optomechanical detection of torque driven by the interaction of a permalloy island with applied magnetic fields. This marks the first time were a nanocavity optomechanical sensor is applied to a nanoscale condensed matter system. This cavity optomechanics platform enabled torque magnetometry measurements to be performed with sufficient sensitivity for detection of Barkhausen features that were previously undetected in ambient conditions. The device was used to demonstrate a new form of nanomechanical radio-frequency susceptometry where enhanced magnetic susceptibility associated with single pinning and depinning events of a magnetic vortex core were observed. This optomechanical device increased torque magnetometer sensitivity by over an order of magnitude. The torque sensitivity of the device derives from the optimization of the optomechanical interactions in a photonic crystal split-beam cavity. Two types of dissipative optomechanical couplings were observed as a result of the mechanical motion modulating the intra-cavity photon lifetime and the cavity input-output coupling rate. Interference between dissipative and dispersive optomechanical mechanisms enhance detection sensitivity and generate mechanical-mode-dependent optomechanical wavelength response. Dissipative coupling of up to 500 MHz/nm and dispersive coupling of 2 GHz/nm, enables measurement of sub-pg torsional and cantilever-like mechanical resonances with a thermally-limited torque detection sensitivity of 1.2 ×10−20 Nm/sqrt(Hz) in ambient conditions. Tuning of both dissipative and dispersive optomechanical couplings is also demonstrated through renormalization of the cavity field mediated by its evanescent interaction with a fiber taper near-field probe. Strategic fiber taper placement allows for reconfiguration of the dominant optomechanical transduction mechanism and spatially selective optical readout of mechanical resonances such as out-of-plane cantilever modes suitable for sensing applications.Item Open Access Optical Quantum Memory and Signal Processing Using a Rare-earth-ion-doped Waveguide(2016) Sinclair, Neil; Tittel, Wolfgang; Simon, Christoph; Barclay, Paul; Cramb, David; Englund, DirkAdvanced applications of quantum information science, such as long-distance quantum communication based on quantum repeaters, require photons to be interfaced with different devices. These devices, such as single photon detectors, quantum memories, quantum signal processors, and linear optical elements, may be integrated on a single chip to enable efficient, robust and scalable implementations. With aim of constructing a quantum repeater, and towards realizing integrated optical quantum memories and signal processors in general, we experimentally develop quantum light-matter interfaces using a cryogenically-cooled rare-earth-ion-doped waveguide, namely thulium-doped lithium niobate (Tm3+:LiNbO3). As the basis for our work, we describe a quantum repeater architecture that uses spectral multiplexing and quantum memories that operate with a fixed storage time. Our repeater architecture promises efficient operation, is compatible with the properties of cryogenically-cooled rare-earth-ion-doped crystals, and simplifies the demands of quantum memories compared to other architectures. To demonstrate the feasibility of our repeater design, we utilize our light-matter interface as a quantum memory to perform several experimental demonstrations. These include high-fidelity storage and retrieval of 26 spectrally-multiplexed quantum bits encoded into single-photon-level laser pulses, high-fidelity storage and retrieval of single and entangled photons derived from a photon-pair source, and high-visibility two-photon interference between weak laser pulses that are stored in one or two, separate, quantum memories. The unique properties of our light-matter interface allow us to demonstrate additional protocols that further highlight its promise for integrated quantum signal processing. By combining our Tm3+:LiNbO3 waveguide with a LiNbO3 waveguide phase modulator, we demonstrate operations such as sequencing, filtering, interference, and compression of laser pulses attenuated to the single-photon level. In another experiment, we measure the cross-phase modulation between strong pulses, by which we demonstrate photon-photon interactions that point towards non-destructive measurements of quantum bits. Since the performance of all aforementioned protocols, and future light-matter applications, hinge on the basic spectroscopic properties of our waveguide, we measure these properties at temperatures as low as 800 millikelvin. Our findings, discussed over several attached publications, usher the development of integrated quantum memories and processors for multi-component optical quantum information applications such as repeaters and networks.Item Open Access Photon echo quantum memory and state transformation(2009) Delfan Abazari, Ahdiyeh; Tittel, WolfgangQuantum memory, as an essential part of a quantum repeater, is the key element for extending quantum communication beyond its current distance limit of around 100 km. In addition to memories, quantum communication tasks require state manipulation and measurement. This is generally accomplished by means of interferometric optical setups, which often suffer from the requirement of phase stabilization. In this thesis we investigate a novel, particularly robust and versatile quantum state transformation approach based on photon-echo type atom-light interaction that allows combining storage with controlled transformation of quantum states. The possibility of modifying a 100% efficient quantum memory protocol to perform this operation is studied and an experimental test-bed based on the traditional stimulated photon echo process is proposed. The method is demonstrated through simulations and experimental studies of nonorthogonal state discrimination, which is of special interest for quantum information processing from the fundamental as well as practical point of view.Item Open Access Photon pair technologies for quantum communication(2009) Slater, Joshua A.; Tittel, WolfgangItem Open Access Photonic Entanglement for a Quantum Repeater(2014-07-09) Jin, Jeongwan; Tittel, WolfgangQuantum Key Distribution (QKD) has opened a new avenue for secure communication, by allowing one to distribute a random secret key between two users that are connected through a public channel without revealing information to unauthorized parties. If a message is encrypted with the one time pad (a well-known crytography algorithm) using secret keys created by QKD, then the ciphertext is information-theoretically secure and thus unbreakable for adversaries. Despite its unprecedented security, loss in the physical channel has prevented QKD from being used over distances beyond a few hundred kilometers. Fortunately, quantum repeaters have opened a path for long-distance QKD by providing a means to establish entanglement between distant users. The goal of this thesis has been to develop a source of entangled photon pairs that is suitable for quantum repeaters and then use it to test some of the fundamental building blocks of a quantum repeater : the heralded creation of entangled photons by means of entanglement swapping with properties that allow interfacing with optical quantum memories; the reversible mapping of quantum states from members of entangled photon pairs in and out of solid-state quantum memories; two-photon interference and a Bell state measurement with photons recalled from separate quantum memories. The demonstration of these key ingredients of a quantum repeater constitutes a significant step towards the establishment of entanglement over hundreds of kilometer distance, and hence long-distance QKD.Item Open Access Probing the Completeness of Quantum Theory with Entangled Photons(2013-01-15) Stuart, Terence Everett; Tittel, WolfgangQuantum theory provides a way to describe the behaviour of matter and energy on the nano-scale, but its predictions are sometimes very surprising. Intuition, based on experience from our daily lives, suggests there are a set of assumptions that should be applicable even when describing quantum systems. In this thesis we conduct a sequence of experiments that test some alternative theories to quantum theory and, in doing so, show that we must abandon some assumptions we made based on intuition if we are to accurately describe nature. In this thesis we also conduct an experiment that bounds the maximum predictive power that theories may have if they are to describe nature at the quantum level accurately. These experiments were conducted using a novel source of polarization entangled photon pairs, whose construction and characterization are also discussed.