The transmission of quantum information over long distances will allow new forms of data security, based on quantum cryptography. These new technologies rely for security on the quantum "uncertainty principle" and on the long distance transmission of "quantum entanglement". A new type of tele-communications device called the "quantum repeater" can allow the faithful transmission of quantum information over worldwide distances, in spite of the inevitably severe losses while propagating along optical fibers. In a quantum repeater, information is stored in the quantum state of a semiconductor electron spin, while complementary entangled information is transmitted as a photon down the optical fiber. This long-range entanglement permits the execution of the teleportation algorithm, which accurately transmits a quantum state over long distances. The basis for an opto-electronic quantum repeater is an entanglement preserving InP photodetector with special selection rules, in which polarization information from a photon is transferred to spin polarization information of a photo-electron, and vice versa. Nevertheless, the algorithm requires that the photo-electron be transferred to a group IV semiconductor for long time storage since there is rapid loss of quantum information in III-V semiconductors. This paper reviews the experimental status of semiconductor quantum repeaters, including the spin resonance transistor logic gates, and the experimental detection of single photons in a manner that preserves their spin information.
The transmission of quantum information over long distances will allow new forms of data security, based on quantum cryptography. These new technologies rely for security on the quantum "uncertainty principle" and on the long distance transmission of "quantum entanglement". A new type of tele-communications device called the "quantum repeater" can allow the faithful transmission of quantum information over worldwide distances, in spite of the inevitably severe losses while propagating along optical fibers. In a quantum repeater, information is stored in the quantum state of a semiconductor electron spin, while complementary entangled information is transmitted as a photon down the optical fiber. This long-range entanglement permits the execution of the teleportation algorithm, which accurately transmits a quantum state over long distances. The basis for an opto-electronic quantum repeater is an entanglement preserving InP photodetector with special selection rules, in which polarization information from a photon is transferred to spin polarization information of a photo-electron, and vice versa. Nevertheless, the algorithm requires that the photo-electron be transferred to a group IV semiconductor for long time storage since there is rapid loss of quantum information in III-V semiconductors. This paper reviews the experimental status of semiconductor quantum repeaters, including the spin resonance transistor logic gates, and the experimental detection of single photons in a manner that preserves their spin information.
The transmission of quantum information over long distances will allow new forms of data security, based on quantum cryptography. These new technologies rely for security on the quantum "uncertainty principle" and on the long distance transmission of "quantum entanglement." A new type of telecommunications device called the "quantum repeater" can allow the faithful transmission of quantum information over worldwide distances, in spite of the inevitably severe losses while propagating along optical fibers. In a quantum repeater, information is stored in the quantum state of a semiconductor electron spin, while complementary entangled information is transmitted as a photon down the optical fiber. This long-range entanglement permits the execution of the teleportation algorithm, which accurately transmits a quantum state over long distances. The quantum repeater is an excellent stepping stone to larger quantum information processors, since teleportation requires only three quantum logic gates. This paper reviews the experimental status of semiconductor quantum repeaters, including the spin resonance transistor logic gates, and the experimental detection of single photons in a manner that preserves their spin information.
Long-distance quantum communication requires transmitting, buffering, and processing quantum information via nodes called quantum repeaters. Unfortunately, current technology is suited for either communication or storage, but not both simultaneously. To address this challenge, the authors propose an all-semiconductor hybrid quantum device that capacitively couples an optically active quantum-dot molecule to a gated-dot array. This optoelectronic device, which could be orders of magnitude faster than today's repeaters, emits a photon whose polarization is entangled with the electron spins in the array.
In the last few years there has been a growing interest in the possible applications of quantum properties of different physical systems in well established technologies. This was motivated by the miniaturization of electronic devices. For very small systems, the laws of quantum mechanics start to govern their behavior and it is then to expect that for instance electronic components reach their "quantum limit". A good example of the introduction of quantum properties into conventional technologies is the case of the electron spin, which offers an alternative to electron-charge for the storage and transport of information in electronics. The idea of using electron spins in electronics gave rise to the new and exciting field of spintronics and was encouraged by experiments that showed coherent spin transport over long distances (over hundred micrometers) in semiconductors and long electron spin dephasing times. In addition to its applications in conventional technologies, the use of the electron spin as information-carrier has been suggested for the new field of quantum information processing. An important advantage of the electron spin with respect to other quantum information-carriers proposed is that it can profit from the well developed semiconductor industry and its related technologies. For the long-range transport of information, however, the highly interacting environment in solid-state devices represents a difficulty to the use of electron spins. In contrast, photons are very weekly coupled to their environment, and therefore more suitable for the transfer of information over long distances (on the order of kilometers), required, for instance, in quantum communication. Furthermore, the technology needed for the transport of photons over such long distances is well developed. Thus, an efficient transfer of quantum information between electron spins and photons is highly desirable. \n \nIn this dissertation, we show that the transfer of entanglement, a quantum correlation, from electron spins to the polarization of photons is not only theoretically possible, but also feasible under realistic experimental conditions. We study the transport of an entangled pair of electron spins through Fermi leads which split them spatially until they reach spin-LEDs, semiconductor structures containing quantum dots. In these structures, the electrons optically recombine with holes, producing then photons. We analyze the polarization state of the out-coming photons and give the conditions for which the entanglement of the electron spins is fully recovered in the photon-polarizations. We further show that it is possible to produce four-photon entanglement. \n \nWe also include in this work the study of coherent spin-transfer between quantum dots layers bridged by molecules. This study was motivated by an experiment showing that the molecules act not only as binders to construct the multilayer structures, but also as "wires" for the information through electron spins in the quantum dots. We show that a two-site Hamiltonian captures some of the essential features of the experiment. We calculate the dependence of the experimentally observed Faraday rotation (FR) signal as a function of probe energy on microscopic parameters such as spin transfer probabilities. The Faraday angle is related to the difference in the dielectric response for different polarizations of the light. We calculate the dielectric response functions of coupled quantum dots and derive an analytical expression for the FR angle in terms of electron transfer probabilities and Heisenberg exchange splittings.
Recently, a quantum media converter from a photon qubit to an electron spin qubit was proposed for quantum repeaters. Quantum information can take several different forms and it is preferable to be able to convert among different forms. One form is the photon polarization and another is the electron spin polarization. Photons are the most convenient medium for sharing quantum information between distant locations. However, it is necessary to realize a quantum repeater in order to send the information securely over a very long distance overcoming the photon loss. A quantum repeater requires quantum information storage and the electron spin is a promising candidate of such a quantum memory because of its long decoherence time. The most important ingredient of the quantum repeater is the entanglement swapping through the Bell (correlation) measurement. We propose an optical means to carry out the correlation measurement. The advantage of this method is the nondestructive character which keeps the quantum state unchanged.
A fascinating dream in quantum information science is to build quantum computers with super-performance and quantum networks with unconditional security [1]. Quantum networks utilize static (matter) quantum bits (qubits) to store and process quantum information at local nodes, and photons as flying qubits for long-distance quantum state transmission between different nodes. To realize a quantum network, it is crucial to achieve light-matter entanglement and reversible quantum-state transfer between light and matter, i.e., the light-matter quantum interface, and the quantum repeater for large-scale quantum communications [2]. Recent experiments have shown that both electrons and holes confined in semiconductor quantum dots (QDs) have long spin relaxation time (T1e, T1h ~ ms) and long spin coherence time (T2e ~ microseconds, T2h > 100 ns). Moreover, fast spin cooling and ultra-fast spin manipulation as well as spin echoes to preserve the spin coherence have also been demonstrated. Undoubtedly these rapid progresses imply that the QD spin is a good candidate for matter qubit in quantum information processing. Furthermore, QD-based single photon sources have been also developed. Therefore, semiconductor QDs offer a good platform for solid-state quantum networking and quantum computation. Here we present two types of conditional quantum gates, i.e., the photon-spin entangling gates [3-4] using a single QD spin in a single-sided or double-sided optical microcavity. Both gates are universal and deterministic (if they are optimized). The spin-selective coherent photon-spin interaction enhanced by the cavity QED lead to giant circular birefringence, which allows us to build these gates. We will show the versatile spin-cavity systems can be applied in all aspects of quantum information science and technology, not only for large-scale quantum communication networks (solid-state quantum repeaters) [5], but also for scalable quantum computing with either photons or spins as qubits. We will also discuss other applications using these gates, such as photon-number resolving detection, quantum feed-back control, loss-resistant quantum metrology, and loophole-free Bell test [6], and other devices based on optical nonlinearity at single-photon levels in these gate structures. All these schemes are feasible with current semiconductor technology [7], and we have recently seen conditional phase shifts in uncharged QD-cavity systems [8]. References [1] H.J. Kimble, Nature (London) 453, 1023 (2008). [2] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998). [3] C.Y. Hu et al., Phys. Rev. B 78, 085307 (2008); ibid. 78, 125318 (2008). [4] C.Y. Hu et al., Phys. Rev. B 80, 205326 (2009). [5] C.Y. Hu and J.G. Rarity, Phys. Rev. B 83, 115303 (2011). [6] N. Brunner et al., Arxiv: quant-phys 1303.6522 (2013). [7] S. Reitzenstein et al., Appl. Phys. Lett. 90, 251109 (2007). [8] A.B. Young et al., Phys. Rev. A 84, 011803 (2011).
A major goal in the field of quantum communication is to achieve long-distance (>100 km) transmission of quantum information, which would allow for the formation of a global quantum network. Devices called 'quantum repeaters' will enable delicate quantum states to be transmitted over long distances without succumbing to the signal losses inherent in the use of optical fibres. This thesis presents my work on a new type of hybrid quantum repeater design, which will combine both the photonic and spin qubit platforms to achieve more robust and efficient quantum communication. The focus of this work is on the optical aspects of this 'photon-to-spin' system, specifically the development of a method for delivering fibre-coupled single photons through an optical fibre to a lateral quantum dot device in a dilution refrigerator while preserving their polarisation states.