This paper presents our experimental work on preparing and manipulating quantum entangled states in optical quantum information processing systems. The main contents are: (1) Using continuous wave(CW) laser beam to pump nonlinear crystals we successfully prepared two-photon polarization entangled states with high brightness and entanglement degree, and it is tunable in entanglement degree and we use this source to produce one kind of important mixed states——Werner states with tunable entanglement fraction in it. (2) We realized the 1→2 qubits Buek-Hillery cloning machine with linear optical devices and showed that the fidelities between the two output qubits and the original qubit are both 5/6 for arbitrary input pure states. (3) We presented a method to test the maximal violation of the CHSH inequality for non-maximally entangled pure states and experimentally realized it with our entangled state source. (4) We perform an all-or-nothing-type Kochen-Specker experiment to verify whether non-contextual hidden variables (NCHV) or quantum mechanics is right. The results strongly agree with quantum mechanics.
This paper presents our experimental work on preparing and manipulating quantum entangled states in optical quantum information processing systems. The main contents are: (1) Using continuous wave(CW) laser beam to pump nonlinear crystals we successfully prepared two-photon polarization entangled states with high brightness and entanglement degree, and it is tunable in entanglement degree and we use this source to produce one kind of important mixed states——Werner states with tunable entanglement fraction in it. (2) We realized the 1→2 qubits Buek-Hillery cloning machine with linear optical devices and showed that the fidelities between the two output qubits and the original qubit are both 5/6 for arbitrary input pure states. (3) We presented a method to test the maximal violation of the CHSH inequality for non-maximally entangled pure states and experimentally realized it with our entangled state source. (4) We perform an all-or-nothing-type Kochen-Specker experiment to verify whether non-contextual hidden variables (NCHV) or quantum mechanics is right. The results strongly agree with quantum mechanics.
A parametric source of polarization-entangled photon pairs with striking spatial characteristics is reported. The distribution of the output electromagnetic k modes excited by spontaneous parametric down-conversion and coupled to the output detectors can be very broad. Using these states realized over a full entanglement ring output distribution, the nonlocal properties of the generated entanglement have been tested by standard Bell measurements and by Ou-Mandel interferometry. A ``mode-patchwork'' technique based on the quantum superposition principle is adopted to synthesize in a straightforward and reliable way any kind of mixed state, of large conceptual and technological interest in modern quantum information. Tunable Werner states and maximally entangled mixed states have indeed been created by this technique and investigated by quantum tomography. A study of the entropic and nonlocal properties of these states has been undertaken experimentally and theoretically, by a unifying variational approach.
The nascent field of quantum information offers the promise of dramatic speed increases for certain types of computation and logical protocols which are classically impossible. The phenomenon of nonlocality is fundamental to this study, and polarization-entangled photons o er a clean, bright, and stable system for its experimental investigation. This thesis investigates, both experimentally and theoretically, the creation, manipulation, and measurement of polarization-entangled photon pairs. Our entangled photon pairs are created through spontaneous parametric down-conversion within a pair of adjacent, orthogonally oriented nonlinear crystals. The quality and brightness of this source is then dramatically improved through the use of birefringent crystals which compensate for an angle-dependent phase shift. The use of these compensation crystals has allowed measured count rates of two million pairs per second with 97.7% fidelity (with a maximally entangled state) or alternately, ten thousand pairs per second with 99.5% fidelity. By manipulating these entangled states, it is possible to study both how they change and how they can be used. We discuss the theory of state manipulation and the experimental implementation of extremely precise single-qubit operations. To study the operations themselves, we use quantum process tomography to characterize them and have successfully implemented the first experimental realization of ancilla-assisted process tomography. To study decoherence, we implemented the first experimental decoherence-free subspace. By using techniques developed during these investigations, we are able to transform our source of entangled photons into a source of any two-photon polarization state, mixed or pure. Developing new techniques for state creation and manipulation is possible because of simultaneous development in state measurement. In addition to detailing both a theoretical analysis and experimental instructions for state tomography, we experimentally and theoretically compared state tomography with tests of local realism (Bell inequalities) and entanglement witnesses (an entanglement detection technique). In the process, we have measured the largest violations of local realism to date, both statistical (over 2400-sigma) and absolute (S = 2:826 ± 0:005 -- within 0:2% of a maximal violation).
Entanglement is a key resource in many quantum information schemes and in the last years the research on multi-qubit entanglement has drawn lots of attention. In this thesis the experimental generation and characterisation of multi-qubit entanglement is presented. Specifically we have prepared entangled states of up to six qubits. The qubits were implemented in the polarisation degree of freedom of single photons. We emphasise that one type of states that we produce are rotationally invariant states, remaining unchanged under simultaneous identical unitary transformations of all their individual constituents. Such states can be applied to e.g. decoherence-free encoding, quantum communication without sharing a common reference frame, quantum telecloning, secret sharing and remote state preparation schemes. They also have properties which are interesting in studies of foundations of quantum mechanics. In the experimental implementation we use a single source of entangled photon pairs, based on parametric down-conversion, and extract the first, second and third order events. Our experimental setup is completely free from interferometric overlaps, making it robust and contributing to a high fidelity of the generated states. To our knowledge, the achieved fidelity is the highest that has been observed for six-qubit entangled states and our measurement results are in very good agreement with predictions of quantum theory. We have also performed another novel test of the foundations of quantum mechanics. It is based on an inequality that is fulfilled by any non-contextual hidden variable theory, but can be violated by quantum mechanics. This test is similar to Bell inequality tests, which rule out local hidden variable theories as possible completions of quantum mechanics. Here, however, we show that non-contextual hidden variable theories cannot explain certain experimental results, which are consistent with quantum mechanics. Hence, neither of these theories can be used to make quantum mechanics complete.
This thesis describes experimental generation, manipulation and measurement of quantum information using photon pairs emitted in bulk crystals. Multi-photon sources engineered during the course of this thesis have proven to be ideal for original contributions in the field of optical quantum information. \n \nIn the first part of this dissertation, we study nonlocality, bound entanglement and measurement-based quantum computing using entangled resources produced by our source. First, we produced and characterised three-photon GHZ polarisation states. We then experimentally violate the long-standing Svetlichny's inequality with a value of 4.51, which is greater than the classical bound by 3.6 standard deviations. Our results agree with the predictions of quantum mechanics, rule out nonlocal hidden-variable theories and certify the genuine tripartite entanglement achievable by our source. Second, with four-photon polarisation states, we demonstrate bound entanglement in Smolin states and realize all of their conceptually important characteristics. Our results highlight the difficulties to achieve the critical condition of undistillability without completely losing entanglement. We conclude the first part by simulating, for the first time, valence-bond solid states and use them as a resource for measurement-based quantum computing. Affleck-Kennedy-Lieb-Tasaki states are produced with 87% fidelity and single-qubit quantum logic gates reach an average fidelity of 92% over all input states and rotations. \n \nIn the second part of this dissertation, we explore controlled waveform manipulation at the single-photon level. Specifically, we shrink the spectral bandwidth of a single photon from 1740 GHz to 43 GHz and demonstrate tunability over a range 70 times that bandwidth. The results are a considerable addition to the field of quantum frequency conversion and have genuine potential for technological applications.
Maximally entangled states, Werner state and maximally entangled mixed states (MEMS) have been created and fully characterized by a novel high brilliance universal source of entangled photon pairs with striking spatial characteristics. Mixed states of any structure, spanning a 2 x 2 Hilbert space may be created by this source. The non local properties of the generated entanglement have been tested by standard Bell measurements. Tunable Werner states and Maximally Entangled Mixed States (MEMS) have been created by an original patchwork technique and investigated by quantum tomography. The entropic and nonlocal properties of these states have been also undertaken.
Abstract Using the type-I SPDC process in BBO nonlinear crystal (NLC), we generate a polarization-entangled state near to the maximally-entangled Bell-state with high-visibility (high-brightness) 98.50 ±1.33 % (87.71 ± 4.45 %) for HV (DA) basis. We calculate the CHSH version of the Bell inequality, as a nonlocal realism test, and find a strong violation from the classical physics or any hidden variable theory (HVT), S= 2.71±0.10. Via measuring the coincidence count (CC) rate in the SPDC process, we obtain the quantum efficiency of single-photon detectors (SPDs) around (25.5 ±3.4)% , which is in good agreement to their manufacturer company. As expected, we verify the linear dependency of the CC rate vs. pump power of input CW-laser, which may yield to find the effective second-order susceptibility crystal. Using the theory of the measurement of qubits, includes a tomographic reconstruction of quantum states due to the linear set of 16 polarization-measurement, together with a maximum-likelihood-technique (MLT), which is based on the numerical optimization, we calculate the physical non-negative definite density matrices, which implies on the non-separability and entanglement of prepared state. By having the maximum likelihood density operator, we calculate precisely the entanglement measures such as Concurrence, entanglement of formation, tangle, logarithmic negativity, and different entanglement entropies such as linear entropy, Von-Neumann entropy, and Renyi 2-entropy. Finally, this high-brightness and low-rate entangled photons source can be used for short-range quantum measurements in the Lab.