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A Quantum Light Source for Light-matter Interaction

Xing Xing

Published Aug 13, 2013• 4 citations
OpenAlex

Abstract

I present in this thesis the design, implementation and measurement results of a narrowband quantum light source based on cavity-enhanced Parametric Down-Conversion\n(PDC). Spontaneous Parametric Down-Conversion (SPDC) is the workhorse in the field of optical quantum information and quantum computation, yet it is not suitable for applications where deterministic nonlinearities are required due to its low spectral brightness. By\nplacing the nonlinear crystal inside a cavity, the spectrum of down-conversion is actively modified, such that all the non-resonant modes of down-conversion experience destructive interference, while the resonant mode sees constructive interference, resulting in great enhancement in spectral brightness. I design and construct such a cavity-enhanced down-conversion source with record high spectral brightness, making it possible to use cold atoms as the interaction medium to achieve large nonlinearity between photons. The frequency of the photons is tunable and their coherence time is measured to be on the order of 10 nanoseconds, matching the lifetime of the excited state of typical alkali atoms. I characterize extensively the output of the source by measuring the second-order correlation function, quantifying two-photon indistinguishability, performing quantum state tomography of entangled states, and showing different statistics of the source.\n\nThe unprecedented long coherence time of the photon pairs has also made possible the encoding of quantum information in the time domain of the photons. I present a theoretical proposal of multi-dimensional quantum information with such long-coherence-time photons and analyze its performance with realistic parameter settings. I implement this proposal with the quantum light source I have built, and show for the first time that a qutrit can be encoded in the time domain of the single photons. I demonstrate the coherence is preserved for the qutrit state, thus ruling out any classical probabilistic explanation of the experimental data. Such an encoding scheme provides an easy access to multi-dimensional systems and can be used as a versatile platform for many quantum information and quantum computation tasks.

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Journal of Research of the National Institute of Standards and Technology•Aug 15, 2019•66 citations
I present in this thesis the design, implementation and measurement results of a narrowband quantum light source based on cavity-enhanced Parametric Down-Conversion\n(PDC). Spontaneous Parametric Down-Conversion (SPDC) is the workhorse in the field of optical quantum information and quantum computation, yet it is not suitable for applications where deterministic nonlinearities are required due to its low spectral brightness. By\nplacing the nonlinear crystal inside a cavity, the spectrum of down-conversion is actively modified, such that all the non-resonant modes of down-conversion experience destructive interference, while the resonant mode sees constructive interference, resulting in great enhancement in spectral brightness. I design and construct such a cavity-enhanced down-conversion source with record high spectral brightness, making it possible to use cold atoms as the interaction medium to achieve large nonlinearity between photons. The frequency of the photons is tunable and their coherence time is measured to be on the order of 10 nanoseconds, matching the lifetime of the excited state of typical alkali atoms. I characterize extensively the output of the source by measuring the second-order correlation function, quantifying two-photon indistinguishability, performing quantum state tomography of entangled states, and showing different statistics of the source.\n\nThe unprecedented long coherence time of the photon pairs has also made possible the encoding of quantum information in the time domain of the photons. I present a theoretical proposal of multi-dimensional quantum information with such long-coherence-time photons and analyze its performance with realistic parameter settings. I implement this proposal with the quantum light source I have built, and show for the first time that a qutrit can be encoded in the time domain of the single photons. I demonstrate the coherence is preserved for the qutrit state, thus ruling out any classical probabilistic explanation of the experimental data. Such an encoding scheme provides an easy access to multi-dimensional systems and can be used as a versatile platform for many quantum information and quantum computation tasks.
Over the past decade, much effort has been made in identifying and characterizing systems that can form a building block of quantum networks, among which semiconductor quantum dots (QD) and spontaneous parametric down-conversion (SPDC) source are two of the most promising candidates. The work presented in this thesis will be centered on investigating and engineering the mentioned systems for generating customizable single photons. A type-II SPDC source can generate a highly flexible pair of entangled photons that can be used to interface disparate quantum systems. In this thesis, we have successfully implemented a cavity-SPDC source that emits polarization correlated photons at 942 nm with a lifetime of 950-1050ps that mode matches closely with InAs/GaAs QD photons. The source emits 80 photon pairs per second per mW pump power within the 150MHz bandwidth. Though the detection of idler photons, the source is capable of emitting heralded photons with g2≤0.5 for up to 40 mW pump power. For a low pump power of ~5 mW, the heralded g2 is 0.06, indicating that the system is an excellent heralded single photon source. By directly exciting a single QD with cavity-SPDC photons, we have demonstrated a heralded-absorption of SPDC photons by QD, resulting in the coupling of the two systems. Due to the large pump bandwidth, the emitted source is highly multimode in nature, requiring us to post-filter the downconverted field, resulting in a lower photon pair emission rate. We propose placing an intra-cavity etalon to suppress the multi-mode emissions and increase the photon count rate. Understanding and experimentally implementing two-photon interference (HOM) measurements will be crucial for building a scalable quantum network. A detailed theoretical description of HOM measurements is given and is experimentally demonstrated using photons emitted by QD. Through HOM measurements we demonstrated that the QD sample in the study is capable of emitting indistinguishable photons, with the visibility exceeding 95%. As an alternative approach to modifying the spectral mode of single photons, we performed phase modulation of photons emitted by a QD to generate additional sidebands that are separated by several GHz. By performing HOM measurements, we have shown that the central component and the sidebands are in the superposition states and the spectrally modified photons have a well-preserved indistinguishability. Such spectrally engineered photons can be used for phase-encoded cryptography applications. These experimental results should lay the foundations towards building a scalable hybrid quantum network.
Efficiently generating photon pairs with high heralding efficiency and high single photon purity that are bandwidth matched to quantum emitters, quantum memories, and other matter-based qubits is critical for quantum networking applications. However, nonlinear optics-based sources require substantial spectral engineering to overcome the orders of magnitude bandwidth mismatch between those sources and qubit systems. A popular solution is cavity-enhanced spontaneous parametric down conversion (SPDC) where the cavity sets the photon bandwidth and simultaneously enhances the spectral brightness of the SPDC. Bulk, free-space configurations are generally required to achieve the MHz-scale bandwidths required to interface with most qubit systems. Replicating these in scalable integrated photonic architectures is an ongoing challenge due to the much higher propagation losses that limit the size and linewidth of chip-based resonators. We show here how an intra-cavity slow light medium, acting as an ultra-narrow filter, would enable narrowband photon pair generation in broadband cavities with high single photon purity and without compromising the heralding efficiency. We show that such metrics can be readily realized in erbium doped thin-film lithium niobate microrings using realistic design parameters.
The promises of quantum computing is to enable faster and more powerful computation than would ever be possible with a classical processor based on binary logic. Quantum linear optics is one of many promising platforms to build a universal quantum computers by leveraging the interaction between indistinguishable photons. A common technique to prepare such states is photon heralding, where many identical photons can be generated and used to encode and process information. Photon pairs are typically generated from a second-order nonlinear interaction known as spontaneous parametric downconversion (SPDC). In all existing platforms, materials are used as both a source of optical nonlinearity, and a phase-matching medium, often resulting in narrow-band and non-reconfigurable operation. 2D materials promise to change this, thanks to their high nonlinearity, inherent broadband phase-matching, and highly configurable electro-optical properties. To date SPDC has only been reported in structures with many millions of atoms, stimulating experimental efforts to validate its scaling laws in structures only a few atoms thick. In this thesis we investigate SPDC in group IV transition-metal dichalcogenides (TMDCs), and describe efforts towards the experimental observation of non-resonant SPDC from a diffraction-limited area. Because of the intimate connection between the classical second-harmonic generation (SHG) and the quantum SPDC, the efficiency of one process provides insights on the other. This guides the design of single-photon coincidence measurements required to demonstrate the strong temporal correlations of these entangled states. Measurements are hindered by the presence of a broadband, temporally uncorrelated background, attributed to photoluminescence. This work improves the understanding of the nonlinear quantum optical potential of these crystals, and provides a performance benchmark for these ultra-thin materials.
This thesis describes work on single-photon pair creation, entanglement and entanglement measurement; single-photon spectral narrowing for atomic interaction; and single-photon detection and spectral analysis. The systems developed exploit the interactions of light beams in non-linear crystal materials. Greatly non-degenerate time-bin entangled photon pair sources are implemented based on the quantum process of spontaneous-parametric-down-conversion. The non-degenerate wavelengths correspond to an atomic transition (895 nm) and a telecommunications band (1310 nm). Interference-fringe-visibilities of over 78% are achieved, verifying entanglement. An efficient and low-noise single-photon up-conversion detector based on sum-frequency-generation is implemented. Photons at 1310 nm are converted to the visible region and efficiently detected using a visible single-photon detector. An overall detection efficiency of over 32% is achieved with a dark-count noise rate of 2500 s-1. The detector is further developed to achieve an inter-symbol-interference free detection rate at twice the limit of the type of visible detector used. The detector is adapted into an up-conversion spectrometer for wavelengths near 1310 nm. A sensitivity of -126 dBm is achieved, corresponding to photon fluxes of approximately 500 s-1. The up-conversion spectrometer is further adapted for correlated biphoton spectroscopy by incorporating a frequency entangled and time correlated source. In this scheme, the spectral function near 895 nm of a remote object can be reproduced by locally measuring the 1310 nm with the up-conversion spectrometer and monitoring the coincidence detection rates. To achieve linewidths feasible for atomic interaction, a very narrow linewidth non-degenerate single-photon pair source is implemented based on spontaneous-parametric-down-conversion embedded in a singly-resonant cavity which is locked to and resonating at the frequency corresponding to a target atomic transition. Linewidths as narrow as 28 MHz are achieved. The brightness of the source within the modes is increased by two-orders compared to the single-pass implementations.
This thesis presents experimental progress towards highly efficient generation of cavity- enhanced, narrow-band multiphotons using spontaneous parametric downconversion. It also suggests theoretical proposals for new applications of multiphoton sources in quantum information technology. Photons prepared in Fock states, with a well-defined number of particles, are the essence of the quantum nature of light, and their unique nonclassical properties help us to develop quantum information technology. The generation of Fock states has been most commonly achieved by using spontaneous parametric down-conversion (SPDC) through the χ(2) nonlinearity in bulk crystals. During the SPDC process the nonlinear crystal emits photon-number correlated modes, and a photon-number- resolving measurement in one mode heralds the preparation of the other mode in a Fock state. The fidelity and the success rate of Fock state generation is limited by the multimode nature of SPDC emission. In this thesis, I will investigate the use of cavity-enhanced SPDC modes for higher fidelity photon pair generation, in which the well defined mode of the cavity is enhanced and results in the narrow-band source of the heralded single-photons with up to 80% heralding efficiency. The heralding efficiency is defined as the ratio between number of coincidences and single events on the heralding mode. The photon-number-resolving ability of high-quantum-efficiency transition edge sensors is used for the heralding and detection. We further discuss the feasibility of Fock state generation with higher photon numbers upon improving the data acquisition and analysis techniques. We also demonstrate and implement an interferometry scheme that exploit the phase of the photon and is capable of reading boundless bits of digital information using a single photon. This thesis also introduces new theoretical proposals for using Fock states to obtain quantum advantages for various quantum information applications, here to increase the efficiency in reading out information stored in a classical digital memory and to discriminate optical phases with lower error rates than is feasible with classical protocols.
Spontaneous parametric down-conversion (SPDC) in a nonlinear crystal has been a workhorse for the generation of entangled and correlated single-photon pairs used for quantum communications applications for nearly three decades. However, as a naturally broadband process, the ability of SPDC to interface with the very narrow energy transitions in atomic ensembles for implementing quantum memories, which are needed for quantum repeaters to extend the reach of quantum communications, was initially limited. To overcome this limitation, the process was enhanced by placing the nonlinear crystal inside a resonating cavity. This modified process has some important advantages, including narrowing the spectral linewidth of generated photons into brighter resonant modes of the cavity, and the ability to lock the desired mode of the cavity to the targeted transition frequency of the atomic ensemble. This paper presents an overview of the principle of cavity-enhanced SPDC, a review of works to date using this technique, and an example of one of these implementations.