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Single photon entanglement: from foundations to applications

Natalia Bruno

Published Jan 1, 2015• 0 citations
Publisher

Abstract

Optical quantum systems are ideal for testing quantum mechanical laws, but they also find many uses into possible applications to our everyday life related to quantum information science, such as quantum key distribution (QKD) or, in general, quantum communication. Nonlinear and quantum optics and their applications both to quantum communication and to tests of fundamental physics are described in this Thesis. In particular, we have developed and characterised heralded single photon sources at telecommunication wavelengths (1550 nm), based on nonlinear interactions of the second order between electric fields in dielectric materials. This is the starting point for the preparation of single photon entangled states, that we used as a resource for heralded amplification of photonic qubits, with a direct application in Device Independent Quantum Key Distribution (DIQKD) protocols, but also for testing fundamental questions in quantum physics, demonstrating the presence of entanglement in a state that counts a macroscopic number of particles, 500 photons.

DOI
10.13097/archive-ouverte/unige:73247

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Optical quantum systems are ideal for testing quantum mechanical laws, but they also find many uses into possible applications to our everyday life related to quantum information science, such as quantum key distribution (QKD) or, in general, quantum communication. Nonlinear and quantum optics and their applications both to quantum communication and to tests of fundamental physics are described in this Thesis. In particular, we have developed and characterised heralded single photon sources at telecommunication wavelengths (1550 nm), based on nonlinear interactions of the second order between electric fields in dielectric materials. This is the starting point for the preparation of single photon entangled states, that we used as a resource for heralded amplification of photonic qubits, with a direct application in Device Independent Quantum Key Distribution (DIQKD) protocols, but also for testing fundamental questions in quantum physics, demonstrating the presence of entanglement in a state that counts a macroscopic number of particles, 500 photons.
For any communication, the conveyed information must be carried by some physical system. If this system is a quantum system rather than a classical one, its behavior will be governed by the laws of quantum mechanics. Hence, the properties of quantum mechanics, such as superpositions and entanglement, are accessible, opening up new possibilities for transferring information. The exploration of these possibilities constitutes the field of quantum communication. The key ingredient in quantum communication is the qubit, a bit that can be in any superposition of 0 and 1, and that is carried by a quantum state. One possible physical realization of these quantum states is to use single photons. Hence, to explore the possibilities of optical quantum communication, photonic quantum states must be generated, transmitted, characterized, and detected with high precision. This thesis begins with the first of these steps: the implementation of single-photon sources generating photonic qubits. The sources are based on photon-pair generation in nonlinear crystals, and designed to be compatible with fiber optical communication systems. To ensure such a compatibility and to create a high-quality source, a theoretical analysis is made, optimizing the coupling of the photons into optical fibers. Based on the theoretical analysis, a heralded single-photon source and a two-crystal source of entangled photons-pairs are experimentally implemented. The source of entangled photons is further developed into a compact source with a narrow bandwidth compatible with standard telecommunication wavelength-division multiplexers, and even further developed to a more stable one-crystal source. The sources are to be used for quantum communication in general and quantum cryptography in particular. Specifically, a heralded single-photon source is implemented and then used for a full test of a decoy-state quantum cryptography protocol.
At the heart of quantum physics lies the principle of superposition, and at the heart of information theory lies the bit. Perhaps the most useful property of quantum systems is that they can be loaded with information bits, so-called qubits, that are indefinitely both 0 and 1 until a measurement is made. Another consequence is that several qubits can become entangled, which is manifested by the non-classical correlations between such quantum systems when measured in all possible bases. Within the rapidly progressing fields of quantum information and quantum communication these quantum effects are utilized to perform tasks such as quantum computing and quantum cryptography. In this thesis we present experimental and theoretical work using single photon sources to prepare ``flying'' photonic qubits. We describe work using mainly quasi-phase-matched nonlinear crystals to generate beams of entangled photon pairs, that are either encoded in polarization at near-visible wavelengths, or in time at optical fiber telecommunication wavelengths (1550 nm). The optical fiber is the medium used for transporting the qubits over a long distance, and it is therefore essential to couple the photons well into the fibers. By focusing the beams optimally, we have investigated how this problem can meet the requirement of creating photons of a narrow frequency bandwidth and a high photon flux. Furthermore, we have generated truly single photons that are heralded by an electrical signal. As a result of modifying the statistics of such sources we have been able to show the effect of photon antibunching. In two separate works, we have implemented a quantum key distribution system based on faint laser pulses at the telecom wavelength of 1550 nm, as well as protocols based on entanglement for performing authentication of key distribution in quantum cryptography.
I will discuss a couple of recent experiments that are at opposite extremes of quantum photonics. We have demonstrated an integrated nonlinear optics scheme – based on sum-frequency generation - capable of photon-photon interactions. This approach has the potential for significant impact in quantum communication, e.g. for heralding entanglement over large distances and providing a significant step towards device-independent quantum key distribution. It also has the added advantage that it is incredibly flexible in terms of the wavelengths and bandwidths that can be incorporated, thus providing a potential solution for a wide range of experiments and applications beyond quantum communication. In a more fundamental experiment, we have tested a way of characterising macroscopic entanglement – the approach firstly amplifies a “microscopic” - single photon - entangled state, to the level of ~500 photons, before reducing this state back to the microscopic regime where a well known entanglement witness can be used to characterise the macroscopic state.
The creation and manipulation of biphotons is important for many applications in quantum optics and quantum information. Topics that benefit from efficient biphoton sources range from the most fundamental quantum science experiments to the highly applied fields of quantum communication and quantum computation. Biphoton sources have long been hailed as one of the leading methods for creating entangled photon pairs for tests of Bell's inequality, creating heralded photon pairs that are used in on-demand single-photon sources and heralded measurement techniques, and for quantum communication protocols to name a few. Specifically for quantum communication, biphoton sources are commonly used for cutting edge quantum key distribution (QKD) protocols. In the first part of the thesis, I focus on realizing an efficient biphoton source that produces high yield photon pairs. More specifically, I develop an optimized biphoton source using the nonlinear optical process of spontaneous parametric down-conversion in a second-order nonlinear crystal. I develop a formalism for predicting the two important metrics of a biphoton source: the heralding efficiency and joint count rate. I show how, from a large parameter space, one can tailor the phase matching of the nonlinear interaction to create a high quality biphoton source that produces both high heralding efficiency and high joint count rate. I achieve heralding efficiencies of 86$\\pm555\\%$ and joint count rates of 2.58$\\pm0.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.70.6 kHz per mW pump power. I show that using a collinear nondegenerate geometry allows for heralding efficiencies of up to 99.70.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.7\\%$ assuming no loss in the system. I verify the theoretical model with experimental results and find good agreement. In the second part of the thesis I turn to manipulating the single photons born from the biphoton source for applications in creating single-photon spectrometers and time-frequency QKD systems. The security of QKD is only guaranteed if the two parties have access to a set of states called mutually unbiased states. I create a set of these states in time and their conjugate states in the frequency basis and show that I can manipulate single photon correlations in time and frequency so that an eavesdropper can be detected if she localizes a photon to a 1 ns time interval. Additionally, in these experiments, I stretch a single photon wavepacket of 5-ps-width to a wavepacket of 8.3-ns-width and subsequently recompress it to at least the resolution of the detectors (sim\\simsim 300 ps). This demonstrates a stretch factor of >1600 for a single-photon pulse using a group velocity dispersive material. To my knowledge, this is the largest reported stretch factor for a single-photon wavepacket produced by a biphoton source. The ability to stretch and recompress a single photon by this amount has applications in creating high-resolution, high-efficiency, single-photon spectrometers as well as advancing time-frequency QKD systems and other temporal pulse shaping applications.
The scope of this thesis is to explore different ways of bringing the phenomenon of quantum entanglement to our classical world. A first manner is to make entanglement an element of everyone's cultural background. I show that nontrivial quantum nonlocality tests can be performed with do-it-yourself optical sources. Furthermore, entanglement can be brought to the real world as a technological resource for improving everyday activities, like communication. Sources of entangled photons, suited to long distance quantum communication, need to be developed. I present a source of entangled photons realized via Spontaneous Parametric Down Conversion in an integrated cavity-waveguide. I show how this kind of sources can be engineered. I also report the realization of low-loss sources for the implementation of an experiment of faithful entanglement swapping adopting a nonlinear interaction between two single photons. Last, one can also attempt to physically generate and observe entanglement at macroscopic scales. I analyze the possibility of cloning a photon of an entangled pair and detecting the amplified state with human eyes. Bringing entanglement to scales one can see allows one to investigate ways of measuring light in the quantum-to-classical regime. I report the experimental demonstration of a method, based on optimal quantum cloning, for measuring low optical radiances in an absolute way. Moreover, I present the realization of photon number resolving detectors at infrared wavelengths and room temperature operation.
We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.