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.
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.
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).
New technologies based on the properties of quantum mechanics promise to revolutionise the way information is processed by outperforming what is possible using classical devices. Examples include massively parallel processing using quantum computers, verifiably secure communication using quantum cryptography, and measurement with sensitivity beyond classical limitation with quantum metrology. Realising the full potential of these technologies necessitates the ability to communicate quantum information over large distances, a key requirement for future quantum networks. However, developing practical implementations of long-distance quantum communication is challenging as it necessitates three major ingredients: light-matter interfaces, elementary quantum operations, and quantum memories. This thesis describes work that has been undertaken to address these requirements using semiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded inside conventional diode structures constitute high-fidelity controllable interfaces between optical qubits and solid-state qubits. Indeed, the polarisation state of a photon was transferred into the spin state of an electron-hole pair and eventually restored through radiative recombination of the electron and the hole with a fidelity up to 95%. Moreover, spins were manipulated using subnanosecond modulation of a vertical electric field applied to the quantum dots. By controlling this electrical modulation, we demonstrated elementary phase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiative lifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control the spatial separation of the electron and the hole and therefore prevent their recombination. Comprehensive studies of electric field eff ects upon the photoluminescence of quantum dot molecules lead to a clear understanding and a good control over their physical properties. Single photons were stored in individual quantum dot molecules up to 1μs and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons was transferred into the spin state of electron-hole pairs with a fidelity above 90%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment. Our work on single quantum dots could be extended in the near future to allow for two-qubits quantum operations by con fining a second electron-hole pair to be electrically manipulated. Storage of a superposition of spin states in a quantum dot molecule should also be possible if the spin states are made degenerate, which is feasible using the electric fi eld dependence of the energy splitting between the spin states discussed in this thesis. We believe that combining both approaches will lead to the development of a controllable multi-qubit quantum memory for polarised light, a building block for long distance quantum communication based on semiconductor nanotechnology.
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.
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.
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 ultimate goal of quantum information science is to build quantum networks for distributed quantum computing or for the secure sharing of information between spatially remote parties [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 ~ s, 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. 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 some of the key applications [3-5], including: (i) single-shot quantum non-demolition measurement of spin; (ii) single-photon based spin manipulations and spin echoes to preserve the spin coherence; (iii) photon-spin, spin-spin and photon-photon entanglement generation with high fidelity and high efficiency; (iv) deterministic photon-spin interface and perfect spin memory; (v) complete and loss-resistant Bell-state analyzer, which could increase the distance of state-of-the-art quantum communications by one order of magnitude (from current 100 km to over 1000 km) [5]; (vi) the full quantum repeater combining quantum memory, entanglement swapping and purifications for arbitrary long distance communications [5]; (vii) spin-controlled single photon sources. All these schemes are feasible with current semiconductor technology [6], and we have recently seen conditional phase shifts in uncharged QD-cavity systems [7]. 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, but also for scalable quantum computing with either photons or spins as qubits. 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, ArXiv: quant-ph 1005.5545 (2010). [6] S. Reitzenstein et al., Appl. Phys. Lett. 90, 251109 (2007). [7] A.B. Young et al., ArXiv: quant-ph 1011.0384 (2010).