The results reported by Versteegh et al. 1 and Huber et al. 2 certainly constitute a step forward in pursuing quantum communication technology towards future larger scalability in arrays of sources. They realised bright entangled photon sources in the spectral region of ∼935 nm and ∼913 nm, respectively, that combine high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nano-scale. These properties are now achieved in single devices. In 1909 Guglielmo Marconi shared with Carl Ferdinand Brown the Nobel Prize in physics in recognition of their contribution to the development of wireless telegraphy, setting the start of long distance radio transmission based on discrete signal. Marconi then developed with others the radio transmission to a commercial success. Less than a century later, Bennett et al. 3 proposed to transmit not only bits but also additional information to communicate and transfer the inherent property of a quantum physical system interconnected with a sender and a receiver. According to Bennett 3 “An unknown quantum state ϕ can be disassembled into, then later reconstructed from, purely classical information and purely non-classical Einstein-Podolsky-Rosen (EPR) correlations. To do so the sender, Alice, and the receiver, Bob, must prearrange the sharing of an EPR-correlated pair of particles. Alice makes a joint measurement on her EPR particle and the unknown quantum system, and sends Bob the classical result of this measurement. Knowing this, Bob can convert the state of his EPR particle into an exact replica of the unknown state ϕ which Alice destroyed.” The work by Bennett et al. 3 immediately received huge interest 4. It set the basis of the last 20 years of a research endeavour known as quantum communication. Significant work has been carried out to prove the feasibility of this idea and now more practical solutions have been made available, as demonstrated by the emergence of recent quantum technologies. Quantum communication is strongly relying on transmission of both classical bits and quantum bit or qubits. “Classical” bits are units of information represented as zero or one, true or false. Qubits are superposition of zero and one states and can encode quantum information. Quantum teleportation provides a mechanism of transferring a qubit from one location to another relying on the quantum superposition of, for example, strongly correlated photons states, which allow by measuring one photon of the pair to simultaneously know the state of the sibling photon. This translates in a transfer of information without the limitation associated of the actual physical qubit transportation. In fact, the transfer of qubits requires that an entangled quantum state is created and the two parts of this state are shared between two locations. Teleportation requires that a quantum channel is established between the source and the destination (Alice and Bob) and a classical information link is used to transmit two classical bits with each qubit. Classical bits can only be transmitted slower than the speed of light. Conversely, quantum correlations may occur instantaneously. The quantum states of single atoms, photons and solid state qubits have been teleported so far (Bouwmeester et al. 5; Gottesman & Chuang 6; Riebe et al. 7; Chen et al. 8; Krauter et al. 9 just to name a few). Photons are regarded as the most robust source of entangled quantum states due to their weak interaction with the environment, therefore they have a long coherence time as opposite to solid state qubits. Quantum communication protocols like quantum teleportation and quantum repeaters require entangled states. These states have been practically achieved in bulk optical systems based on nonlinear optics effects in birefringent crystals since many years now (Ou & Mandel 10). These sources are not deterministic as they rely on the random decay of one higher energy pump photon in two photons by spontaneous parametric down-conversion. Also they are not ideal for integration in nanophotonics circuits and have limited scalability to achieve large numbers of quantum gates or quantum networks. Artificial atoms, like quantum dots (QDs), on the other hand constitute a deterministic source of single or entangled photon states. A QD is made of a semiconductor of lower band gap embedded in a semiconductor of higher band gap. The band gap offsets results in a three dimensional electronic confinement. An electron may then be excited optically or electrically to the conduction band, leaving a hole in the valence band. An exciton state may be formed by these electron–hole pairs trapped by the QD. Further excitation of the QD may lead to a biexciton state with two electrons and two holes. The radiative decay of the exciton leads to the emission of one photon. The biexciton-exciton radiative cascade results in the emission of two photons. The photons are not re-emitted until the exciton or biexciton are excited again. This delivers a determinist photon emission. Owing to asymmetries in the QD, there is actually a fine structure splitting (FSS) in the exciton state due to the different electron and hole spin states of the QD, leading to slightly different transition frequencies for the two photons generated with horizontal and vertical polarization. This requires temporal post selection of the emitting photons to create a perfect entangled photons state, limiting the efficiency of QDs for entanglement generation (loss of photons) and therefore their practical use in quantum communication protocols. Versteegh et al. 1 and Huber et al. 2 grew semiconductor QDs embedded in [111]-oriented tapered nanowire waveguides grown in the wurtzite phase, specifically InP nanowires containing single InAsP; the synthesis methods are slightly different, in Versteegh et al. 1 thicker QDs are grown (diameter ∼25-30 nm). Although theoretically previously predicted, entanglement was not yet been demonstrated for a nanowire QD. Versteegh et al. 1 demonstrate a bright and coherent source of strongly entangled photon pairs from a position controlled nanowire QD with a fidelity as high as 0.859 ± 0.006 and concurrence of 0.80 ± 0.02 with a FSS value of 1.2 μeV, deduced from the polarization-dependence measurement of the energy emission difference from the exciton and biexcition states. There are two main advantage in a nanowire embedded QD for photon pairs generation: the high refractive index of the nanowire allows guiding the light originated from the QD and the tapered end allows an efficient light extraction with broad spectral operation; in QD embedded in nanowires grown along [111] direction, the FSS should vanish, thus providing a nearly ideal entanglement via biexcition-exciton radiative cascade decay without the need of any post-selection. In Versteegh et al. 1 the created two-photon quantum states provide different photon cross-correlation from what observed previously from QDs, revealing that the two-photon quantum state from the nanowire is closer to (|HV> + |VH>)/ than to the commonly measured state (|HH> + |VV>)/. The entangled photon states produced are modified by the nanowire shape and small anisotropy, justifying different photons states compared to the ones measured in QDs. The photons pairs produced by the QDs experience a variation of their polarization due to birefringence while propagating along the nanowire. Figure 1 shows SEM image of (a) symmetric and (b) asymmetric nanowires. The nanowire elongation in (b) is an extreme example that leads to geometric birefringence responsible for the rotation of the quantum state originated from the QDs. In Huber et al. 2 a FSS value of 18 μeV was determined, possibly responsible for a lower fidelity of the generated entangled states. In this case they did not observe evident sign of lateral elongation in scanning electron microscopy images showing the cross-section of the hexagonal nanowires cores, a small elongation ratio was confirmed unable to induce a significant level of FSS as the one observed. By inspecting the cross-section of the nanowires dislocation of the QD insertion with respect to the nanowire axis was observed. Wave function calculations of the electrons and holes for on and off-centred QDs show that off-centred QDs exhibit orbitals with lower symmetry (C2v) compared to centred QDs (C3v). This lower symmetry can be responsible only in minor percentage for the FSS. The authors conclude that the large FSS observed can be attributed to the compositional anisotropy rather than the geometrical asymmetry. This new nano-scale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes, allowing producing arrays of such sources on a chip. A further step may certainly be identified in having an electrically rather than as optical pump of the entangled photon pairs in the nanowire. Also photons extraction efficiency of 90% is predicted by redesigning the nanowires. Some work needs still to be done however to surpass the present sources based on spontaneous parametric down-conversion. The nanowire photon states entanglement fidelity is not as high as conventional sources and the single-photon coherence of the emitted photon pairs is not yet Fourier-transform limited. By developing a thermal annealing process to get rid of the compositional inhomogeneity in the nanowires a decrease of the FSS to sub-μeV level for any desired target wavelength could be achieved. By two-photon resonant excitation techniques, cooling of the quantum dot sample to 300mK and Purcell effect radiative decay enhancement, photons indistinguashibility could be achieved and possibly surpass in the future conventional sources.
The results reported by Versteegh et al. 1 and Huber et al. 2 certainly constitute a step forward in pursuing quantum communication technology towards future larger scalability in arrays of sources. They realised bright entangled photon sources in the spectral region of ∼935 nm and ∼913 nm, respectively, that combine high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nano-scale. These properties are now achieved in single devices. In 1909 Guglielmo Marconi shared with Carl Ferdinand Brown the Nobel Prize in physics in recognition of their contribution to the development of wireless telegraphy, setting the start of long distance radio transmission based on discrete signal. Marconi then developed with others the radio transmission to a commercial success. Less than a century later, Bennett et al. 3 proposed to transmit not only bits but also additional information to communicate and transfer the inherent property of a quantum physical system interconnected with a sender and a receiver. According to Bennett 3 “An unknown quantum state ϕ can be disassembled into, then later reconstructed from, purely classical information and purely non-classical Einstein-Podolsky-Rosen (EPR) correlations. To do so the sender, Alice, and the receiver, Bob, must prearrange the sharing of an EPR-correlated pair of particles. Alice makes a joint measurement on her EPR particle and the unknown quantum system, and sends Bob the classical result of this measurement. Knowing this, Bob can convert the state of his EPR particle into an exact replica of the unknown state ϕ which Alice destroyed.” The work by Bennett et al. 3 immediately received huge interest 4. It set the basis of the last 20 years of a research endeavour known as quantum communication. Significant work has been carried out to prove the feasibility of this idea and now more practical solutions have been made available, as demonstrated by the emergence of recent quantum technologies. Quantum communication is strongly relying on transmission of both classical bits and quantum bit or qubits. “Classical” bits are units of information represented as zero or one, true or false. Qubits are superposition of zero and one states and can encode quantum information. Quantum teleportation provides a mechanism of transferring a qubit from one location to another relying on the quantum superposition of, for example, strongly correlated photons states, which allow by measuring one photon of the pair to simultaneously know the state of the sibling photon. This translates in a transfer of information without the limitation associated of the actual physical qubit transportation. In fact, the transfer of qubits requires that an entangled quantum state is created and the two parts of this state are shared between two locations. Teleportation requires that a quantum channel is established between the source and the destination (Alice and Bob) and a classical information link is used to transmit two classical bits with each qubit. Classical bits can only be transmitted slower than the speed of light. Conversely, quantum correlations may occur instantaneously. The quantum states of single atoms, photons and solid state qubits have been teleported so far (Bouwmeester et al. 5; Gottesman & Chuang 6; Riebe et al. 7; Chen et al. 8; Krauter et al. 9 just to name a few). Photons are regarded as the most robust source of entangled quantum states due to their weak interaction with the environment, therefore they have a long coherence time as opposite to solid state qubits. Quantum communication protocols like quantum teleportation and quantum repeaters require entangled states. These states have been practically achieved in bulk optical systems based on nonlinear optics effects in birefringent crystals since many years now (Ou & Mandel 10). These sources are not deterministic as they rely on the random decay of one higher energy pump photon in two photons by spontaneous parametric down-conversion. Also they are not ideal for integration in nanophotonics circuits and have limited scalability to achieve large numbers of quantum gates or quantum networks. Artificial atoms, like quantum dots (QDs), on the other hand constitute a deterministic source of single or entangled photon states. A QD is made of a semiconductor of lower band gap embedded in a semiconductor of higher band gap. The band gap offsets results in a three dimensional electronic confinement. An electron may then be excited optically or electrically to the conduction band, leaving a hole in the valence band. An exciton state may be formed by these electron–hole pairs trapped by the QD. Further excitation of the QD may lead to a biexciton state with two electrons and two holes. The radiative decay of the exciton leads to the emission of one photon. The biexciton-exciton radiative cascade results in the emission of two photons. The photons are not re-emitted until the exciton or biexciton are excited again. This delivers a determinist photon emission. Owing to asymmetries in the QD, there is actually a fine structure splitting (FSS) in the exciton state due to the different electron and hole spin states of the QD, leading to slightly different transition frequencies for the two photons generated with horizontal and vertical polarization. This requires temporal post selection of the emitting photons to create a perfect entangled photons state, limiting the efficiency of QDs for entanglement generation (loss of photons) and therefore their practical use in quantum communication protocols. Versteegh et al. 1 and Huber et al. 2 grew semiconductor QDs embedded in [111]-oriented tapered nanowire waveguides grown in the wurtzite phase, specifically InP nanowires containing single InAsP; the synthesis methods are slightly different, in Versteegh et al. 1 thicker QDs are grown (diameter ∼25-30 nm). Although theoretically previously predicted, entanglement was not yet been demonstrated for a nanowire QD. Versteegh et al. 1 demonstrate a bright and coherent source of strongly entangled photon pairs from a position controlled nanowire QD with a fidelity as high as 0.859 ± 0.006 and concurrence of 0.80 ± 0.02 with a FSS value of 1.2 μeV, deduced from the polarization-dependence measurement of the energy emission difference from the exciton and biexcition states. There are two main advantage in a nanowire embedded QD for photon pairs generation: the high refractive index of the nanowire allows guiding the light originated from the QD and the tapered end allows an efficient light extraction with broad spectral operation; in QD embedded in nanowires grown along [111] direction, the FSS should vanish, thus providing a nearly ideal entanglement via biexcition-exciton radiative cascade decay without the need of any post-selection. In Versteegh et al. 1 the created two-photon quantum states provide different photon cross-correlation from what observed previously from QDs, revealing that the two-photon quantum state from the nanowire is closer to (|HV> + |VH>)/ than to the commonly measured state (|HH> + |VV>)/. The entangled photon states produced are modified by the nanowire shape and small anisotropy, justifying different photons states compared to the ones measured in QDs. The photons pairs produced by the QDs experience a variation of their polarization due to birefringence while propagating along the nanowire. Figure 1 shows SEM image of (a) symmetric and (b) asymmetric nanowires. The nanowire elongation in (b) is an extreme example that leads to geometric birefringence responsible for the rotation of the quantum state originated from the QDs. In Huber et al. 2 a FSS value of 18 μeV was determined, possibly responsible for a lower fidelity of the generated entangled states. In this case they did not observe evident sign of lateral elongation in scanning electron microscopy images showing the cross-section of the hexagonal nanowires cores, a small elongation ratio was confirmed unable to induce a significant level of FSS as the one observed. By inspecting the cross-section of the nanowires dislocation of the QD insertion with respect to the nanowire axis was observed. Wave function calculations of the electrons and holes for on and off-centred QDs show that off-centred QDs exhibit orbitals with lower symmetry (C2v) compared to centred QDs (C3v). This lower symmetry can be responsible only in minor percentage for the FSS. The authors conclude that the large FSS observed can be attributed to the compositional anisotropy rather than the geometrical asymmetry. This new nano-scale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes, allowing producing arrays of such sources on a chip. A further step may certainly be identified in having an electrically rather than as optical pump of the entangled photon pairs in the nanowire. Also photons extraction efficiency of 90% is predicted by redesigning the nanowires. Some work needs still to be done however to surpass the present sources based on spontaneous parametric down-conversion. The nanowire photon states entanglement fidelity is not as high as conventional sources and the single-photon coherence of the emitted photon pairs is not yet Fourier-transform limited. By developing a thermal annealing process to get rid of the compositional inhomogeneity in the nanowires a decrease of the FSS to sub-μeV level for any desired target wavelength could be achieved. By two-photon resonant excitation techniques, cooling of the quantum dot sample to 300mK and Purcell effect radiative decay enhancement, photons indistinguashibility could be achieved and possibly surpass in the future conventional sources.
A quantum network is believed to be the next generation of quantum technology [1]. In the past decades, individual quantum systems—for example, quantum dot (QD) [2–4], etc.—have been studied in depth and the technologies are mature; therefore, the consideration of combining different systems to construct a quantum network (especially a hybrid one) is timely. Moreover, a quantum network is absolutely necessary in the fulfilment of a general quantum task, which needs the cooperation of many different kinds of quantum units. On a larger scale, the local quantum networks can be further linked to construct a global quantum network, namely the quantum Internet [1]. With this technology at hand, distributed quantum computation can be realized, making scalable quantum information processing possible in multiple sites over long distances. The primary challenge of the construction of a quantum network is the state distribution among different nodes. An elementary quantum network based on two identical nodes each constituted by a single atom in the optical cavity has been realized in an experiment [5]. However, here we focus on the quantum network comprising heterogeneous nodes, which is more general and can combine advantages of different quantum systems to complete more complicated quantum tasks. The construction of a hybrid quantum network is more difficult, since heterogeneous quantum systems have different properties that are not naturally matched, such as the different emission (or absorption) wavelengths and different bandwidths [8], etc. To establish the hybrid interface, QD is a good candidate. QD can act as a deterministic single [2] or entangled [3] photon source and is also a promising physical carrier for solid-state spin qubit [4]. QD has been interfaced with hot atoms [6] and single ions [7] in recent years. Almost at the same time, Li's group experimentally demonstrated the first optical connection between QD and a solid-state quantum memory [8]. Distinguishing from the former works, the all-solid-state character of this work makes the whole architecture more stable and easier to be manipulated and integrated. During this work, the sample was grown by Niu's group [8]. A special sample design moves the emission band of QDs to approximately 879.7 nm—the memory band of the solid-state memory [9]. Since the fine-tuning range is only several 0.1 nm, hundreds of QDs on the sample were detected by Li et al. and finally one of them was selected, whose emission line is located at 879.5 nm. Using a local-heating technique, the researchers precisely tuned the emission wavelength of this QD to the solid-state memory's memory band, whose bandwidth is as narrow as hundreds of MHz [9]. The quantum memory in this experiment was based on the atomic frequency comb (AFC) protocol [10]. The memory efficiency is as high as 20% when the storage time is 40 ns, and the non-collinear configuration for the pump light and the signal photons can substantially reduce the noise from the pump light. To further reduce the detection noise, a superconducting single-photon detector (SSPD), whose dark count rate can be as low as 1.5 per second, is used instead of the silicon single-photon avalanche diode. With these conditions, Li's experiment clearly shows the stored and retrieved single photons that are originally generated from the QD. The signal-to-noise ratio (SNR) is 9:1. The second-order retrieved single photons are even seen in the data, which further validates the experimental result. Another advantage of Li's work is the demonstration of multi-temporal-mode quantum memory with up to 100 narrow deterministic-single-photon pulses [8]. This result is directly associated with the construction of a quantum repeater [11]—one of the essential components for a long-distance quantum network. The primary characteristics of this experiment, namely the deterministic single-photon memory (differing from the previous works) and the multimode memory, are both critically important in the improvement of a quantum repeater's efficiency [12]. Also, Li's further work finished the quantum memory of 51 spatial modes using the orbital angular momentum (OAM) modes of herald single photons [9]. By combining both the temporal and spatial multiplexing, and the deterministic single-photon source, more efficient quantum repeaters can be constructed. We hope that the recent studies [8,9] will pave the way for the construction of quantum repeaters, and furthermore quantum networks—both in the short term and the long term—based on all-solid-state devices. As a special application of a quantum network, quantum key distribution (QKD) can also benefit from Li's work both in security and transmission distance (key rate). QKD utilizes the principles of quantum mechanics to guarantee secure communication between two authorized parties (or quantum nodes, for instance) and the famous protocols including BB84 (proposed by Bennett and Brassard in 1984) [13]), etc. However, the commonly used weak-coherent-pulse photon source (typically attenuated laser) will decrease the security of QKD because of its multiphoton probability, which means that QKD suffers from several certain attacks, such as the photon number splitting (PNS) attack. Although a decoy state has been introduced to achieve potential solutions to this problem, a substantial shrinking of the sifted key will be required and the complexity of the protocol may contain loopholes [14,15]. Therefore, the unconditional security guaranteed by quantum mechanics still requires a single-photon source, such as QD (experimental work regarding QKD on the basis of a QD single-photon source can be referred to in [14,15]), etc. The deterministic-single-photon pulse train generated by QD in Li's experiment [8] can provide a single-photon-source node (in the network) and keep the security of QKD. Moreover, because of both the elimination of multiphoton probability and the multi-temporal-mode single-photon generation, the key rate of QKD will be improved, which is associated with a longer transmission distance. The quantum repeater that benefited from Li's work will also further improve the transmission distance of QKD.
A bright photon source that combines high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nanoscale is required for implementing ambitious schemes in quantum information processing, such as that of a quantum repeater. Still, all of these properties have not yet been achieved in a single device. Semiconductor quantum dots embedded in nanowire waveguides potentially satisfy all of these requirements; however, although theoretically predicted, entanglement has not yet been demonstrated for a nanowire quantum dot. Here, we demonstrate a bright and coherent source of strongly entangled photon pairs from a position-controlled nanowire quantum dot with a fidelity as high as 0.859±0.006 and concurrence of 0.80±0.02. The two-photon quantum state is modified via the nanowire shape. Our new nanoscale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes.
Harnessing quantum mechanics to revolutionize various fields of established technology has fueled research activities in recent years. Especially the prospect of inherently secure quantum communication channels has become increasingly desirable to businesses, politicians and society as a whole to protect sensitive information. \nAt heart, quantum communication relies on the distribution of entangled quantum states that make the communication impervious to eavesdroppers. Naturally single photons or photon pairs are an excellent choice for distributing entangled states at the speed-of-light through existing fiber-networks. \nOne of the most promising quantum light sources constitute epitaxial quantum dots. The high oscillator strength renders them exceptionally bright, while still emitting nearly indistinguishable single photons with quantum efficiencies close to unity, an important prerequisite for high-fidelity photonic quantum applications. By carefully manufacturing the semiconductor heterostructure, the optical environment can be individually tailored, utilizing Purcell enhancement, by embedding the emitter in a semiconductor cavity or exploiting wave guiding properties in form of micropillars or nanowires to enhance the extraction efficiency. \nInevitable optical attenuation in fiber networks however necessitates the overall communication channel length to be divided into subsections with nodes that can temporarily store the quantum information. Naturally this requires a quantum memory which can efficiently store the quantum state for a sufficiently long time and subsequently recreate the stored photon on demand. \nIn this framework, atomic memories represent the established benchmark, unrivaled by quantum dots spin states which remain intrinsically limited by the decoherence-inducing interaction with the solid state environment. Combining the excellent single-photon source of a quantum dot with the superior coherence properties of alkali quantum memories in a hybrid system at each quantum node offers the best of both worlds, promising exponential speed-up of truly secure communication. \nThis PhD thesis focuses on the requirements imposed on the quantum dot in such a hybrid quantum network and shows how these challenges can be overcome. \nThe first part of the introduction aims to give a detailed overview on the underlying quantum communication protocol of a hybrid quantum network and how it fares against the more established DLCZ protocol. Next, single-photon sources, and quantum dots in particular, will be outlined and the growth mechanism and optical properties of epitaxial GaAs quantum dots discussed in detail. Lastly, to illustrate the framework in which a quantum dot can efficiently be paired with alkali atoms and to understand the challenges that arise, the mode of operation and attributes of the state-of-art broadband quantum memory will be summarized in chapter 2. \nThe third chapter investigates the optical properties of an epitaxial GaAs quantum dot spectrally matched to rubidium. By means of strain-tuning, the quantum dot can address all hyperfine transitions of the rubidium D2 line and a first interaction with atomic vapors is shown in a transmission measurement. In conjunction with other optical measurements, true Fourier-limited emission of single photons is demonstrated. Furthermore, we establish a possible route to overcome the bandwidth mismatch of the two systems in form of the coherent-scattering regime. \nWhile this coherent-scattering regime offers quantum dot single-photons with sub-natural bandwidths, in form of elastically scattered single photons that predominately retain the small linewidth of the excitation laser, the emission is highly probabilistic and relies on continuous-wave excitation or weak, resonant laser pulses of durations exceeding the exciton lifetime. The fourth chapter demonstrates the generation of true on-demand single photons with tailored temporal waveform envelopes between 14 and 245 ns, overcoming the temporal limitations imposed by the exciton two-level system. The photonic bandwidth is reduced by almost one order of magnitude. \nIn the following, the decay dynamics of a positively charged exciton in an GaAs quantum dot will be investigated by time-resolved photolumincescence and resonance fluorescence measurements (chapter 5). In Chapter 6 the optical properties of GaAs quantum dots in 500 nm thick membranes are characterized. Finally, an outlook into future developments and the solutions to remaining challenges will be presented. \n
The biexciton-exciton cascade in quantum dots (QDs) yields entangled photon pairs, and recent developments in engineering photonic structures around the dot for efficient light extraction and proper control of the fine-structure splitting, has led to significant breakthroughs in achieving an ideal entangled photon source -- exhibiting properties such as high extraction efficiency, low multi-photon emission and high entanglement fidelity. This thesis presents our development in enhancing the performance of entangled photon generation of QDs towards near-unity efficiency and fidelity. We study InAsP quantum dots in photonic nanowires for efficient light extraction and to reveal the effect of a quantum dot with high nuclear spin on the entanglement fidelity. \n \nRevealing the ultimate limit of entanglement in QDs is still a challenge, and it is generally believed that a sample made of atoms possessing high nuclear spins exhibits limited entanglement due to the dephasing caused by spin-spin interactions. Moreover, it is assumed that the fine-structure splitting of QDs degrades the entanglement fidelity, as it introduces a which-path information in the biexciton-exciton cascade. We have performed two-photon state tomography on InAsP QDs in InP nanowires, comprising of nuclear spin 9/2, and have realized that such a source is capable of generating entangled photon pairs with negligible dephasing during the emission lifetime, with a peak concurrence C = 77% and a count-weighted average concurrence 62%. This result was obtained by comparing the outcome of our measurements with a theoretical model that assumes no dephasing, however, takes into account the details of the detection system, i.e., dark-count rates and timing resolution, as well as, specific features of the QD, e.g., emission lifetime, fine-structure splitting, multi-photon emission probability, etc. Proper engineering of tapered nanowires allows for extraction of the emitted photons with high efficiency, which makes it possible to perform a complete two-photon state tomography and monitoring the evolution of the exciton state, as well as, the entanglement fidelity of the photon-pair, during the emission lifetime. This enabled us to reveal the effect of an imperfect detection system, as well as, multi-photon emission in recording low values for entanglement fidelity. Moreover, our calculations show that proper analysis of the time-energy uncertainty relation provides the necessary means to measure a high entanglement fidelity even in the presence of fine-structure splitting; since, a detection system with high timing resolution can overlook the which-path information, as the uncertainty in measuring the energy of the photons will be larger than the fine-structure splitting. Therefore, realizing near-unity entanglement fidelity using QDs is merely a technical challenge, i.e., resonant excitation of the QD, in order to suppress multi-photon emission, and use of a detection system with low timing jitter and dark-count rates. \n \nAs a next step to achieve this goal, we have performed the resonant two-photon excitation scheme for the first time on a QD in a photonic nanowire, which resulted in an enhancement of the performance of our source. We managed to improve the single-photon purity of the source, g^(2)(0), by two orders of magnitude; moreover, analysis of the emission spectrum reveals that this scheme increases the pair-production efficiency to values above 93% for the biexciton-exciton cascade. Taking the efficiency of our experimental setup into consideration, the results indicate an approximately eight-fold enhancement of the pair-extraction efficiency as compared to the quasi-resonant excitation scheme (12.55% vs. 1.6%). Based on these results, two-photon state tomography on our source, under resonant two-photon excitation and performed by a detection system with high timing resolution and low dark-count rates, yields near-unity entanglement fidelity. By considering the enhancement in pair-extraction efficiency that we obtained, QDs in photonic nanowires can surpass the performance of spontaneous parametric down conversion sources in terms of entanglement fidelity and pair-extraction efficiency. \n \nEven though we have shown a finite fine-structure splitting does not degrade the entanglement fidelity in QDs, for various application in quantum information removing the fine-structure splitting is beneficial. Therefore, we have proposed a universal all-optical approach for removing the fine-structure splitting. In this scheme, the energy of the photons are shifted by using a pair of quarter-wave plates and fast-rotating half-wave plates after they have been emitted; and as a result, the energy difference between the two decay paths in the biexciton-exciton cascade is erased. This method is applicable to any QD source and can be easily implemented using the currently available technology; moreover, no further sample processing will be required in order to achieve zero fine-structure splitting.
Quantum communication and quantum computation are novel methods of information transfer and information processing, all fundamentally based on the principles of quantum physics. The performances outdo their classical counterparts in many aspects [1,2]. In almost all quantum communication and quantum computation schemes, quantum entanglement [3] plays a decisive role. In essence, an entangled system can carry all information (e.g., on their polarization properties) only in their correlations, while no individual subsystem carries any information. This leads to correlations that are much stronger than classically allowed [89, 100], which is a powerful resource for information processing. It is therefore important to be able to generate, manipulate, and distribute entanglement as accurately and as efficiently as possible.
Quantum entanglement underpins the realization of quantum simulators and computers, which can be used for unbreakable cryptography and powerful computational algorithms. Entangled photons are an ideal medium for creating and manipulating quantum states due to the low noise and ease of transmission. A qubit encoded into a photon can be easily sent between different photonic elements along an optical fiber, in analogy with the transmission of classical bits along electrical wires. Furthermore, logic operations can be performed on entangled photons by exploiting the nonlinearity inherent to quantum measurements [1]. The practical implementation of complex applications requires the minimization of coupling losses, as well as stable quantum interference. Low losses and interferometric stability allow strong entanglement between output photons. One of the biggest milestones in achieving these requirements is the integration of photon sources together with optical circuits on the same photonic chip.