Polarization-entangled photons provide intriguing insight into the fundamentals of quantum physics, and are vital technology for large scale quantum computing with linear optics, and extending the distance over which a quantum key can be shared. However, those entangled photon sources that have been implemented so far are typically probed using time-averaged measurements, thus obscuring any quantum correlations that evolve over the lifetime of the state. Here we measure the evolution of quantum correlations over the lifetime of a photon pair state, generated by an optically excited quantum dot at ~10K. Time-dependent oscillations of the entanglement fidelity are observed, and attributed to the spin-dependent phase acquired in the intermediate, non-degenerate, exciton-photon state.
Polarization-entangled photons provide intriguing insight into the fundamentals of quantum physics, and are vital technology for large scale quantum computing with linear optics, and extending the distance over which a quantum key can be shared. However, those entangled photon sources that have been implemented so far are typically probed using time-averaged measurements, thus obscuring any quantum correlations that evolve over the lifetime of the state. Here we measure the evolution of quantum correlations over the lifetime of a photon pair state, generated by an optically excited quantum dot at ~10K. Time-dependent oscillations of the entanglement fidelity are observed, and attributed to the spin-dependent phase acquired in the intermediate, non-degenerate, exciton-photon state.
We investigate the evolution of quantum correlations over the lifetime of a multiphoton state. Measurements reveal time-dependent oscillations of the entanglement fidelity for photon pairs created by a single semiconductor quantum dot. The oscillations are attributed to the phase acquired in the intermediate, nondegenerate, exciton-photon state and are consistent with simulations. We conclude that emission of photon pairs by a typical quantum dot with finite polarization splitting is in fact entangled in a time-evolving state, and not classically correlated as previously regarded.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states. In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system. These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.
Quantum dots are considered an attractive system for applications in quantum communication and quantum logic, confirmed by experimental demonstrations of quantum dot based single photon emission devices. Another key quantum optics resource is the on-demand generation of entangled photon pairs, for which the radiative decay of the biexciton state in a quantum dot has been proposed. The realization of such a device has been prevented due to polarization splitting of the exciton fine structure, caused by anisotropies of various structural parameters of the dot. We present the results of recent experiments that manage the splitting in quantum dots, in order to allow entangled photon emission. We demonstrate that dots with splitting within the homogeneous linewidth can be realized by carefully controlling the thickness of the dot layer. Furthermore, we show that the splitting can be reduced to zero by the application of an in-plane magnetic field. Polarization dependent correlation measurements on these dots will be presented that show characteristic features of entanglement, such as polarization correlation for all linear detection bases, and circular polarization anti-correlation. Our results indicate that for the first time, we have observed triggered entangled photon pair emission from a quantum dot.
During recent years, quantum dots have become an increasingly established source of highly entangled photons 1. The main motivation for the development of this technology has resided in the expectation that a resonantly driven quantum emitter can offer a path towards on-demand photon pair generation 2. In fact, state-of-the-art sources relying on spontaneous parametric down-conversion intrinsically suffer from multipair emission at high pair generation rates, which causes a tradeoff between brightness and degree of entanglement 3. Despite the key importance of this aspect, the experimental study of how multiphoton emission affects the entanglement properties of quantum dot-based sources has received surprisingly little attention. In this paper we report the investigation of the multipair emission of the source under quasi-deterministic resonant two-photon excitation without filtering the excitation laser using polarization suppression. The focus is on measuring the real multipair emission entering in entanglement-based measurements, minimizing measurement artefacts from the setup and in particular from the excitation source. This is investigated by measuring the second-order correlation function at zero-time delay in several measurement conditions, including spectral filtering. Our work confirms that the multipair emission is provided also for entanglement-based measurement conditions and thus helps the design of efficient photon sources for quantum information and communication technologies.
The generation of entangled photons from radiative cascades has enabled milestone experiments in quantum information science with several applications in photonic quantum technologies. Significant efforts are being devoted to pushing the performances of near-deterministic entangled-photon sources based on single quantum emitters often embedded in photonic cavities, so to boost the flux of photon pairs. The general postulate is that the emitter generates photons in a nearly maximally entangled state of polarization, ready for application purposes. Here, we demonstrate that this assumption is unjustified. We show that in radiative cascades there exists an interplay between photon polarization and emission wavevector, which can be further amplified by embedding the emitters in micro-cavities. We discuss how the polarization entanglement of photon pairs from a biexciton-exciton cascade in quantum dots strongly depends on their propagation wavevector and we even observe entanglement vanishing for large emission angles. Our experimental results, backed by theoretical modeling, yield a brand-new understanding of cascaded emission for various quantum emitters. In addition, our model provides quantitative guidelines for designing optical microcavities that retain both a high degree of entanglement and collection efficiency, moving the community one step further towards an ideal source of entangled photons for quantum technologies. The assumption that light sources based on radiative cascades provide maximally polarization-entangled photons is not always correct. Here the authors show experimentally in cavity-embedded quantum dots that photon polarization correlates with its emission mode, impacting the degree of entanglement for emitted photon pairs.
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.