We propose a Bell-measurement node for quantum repeaters based on a planar semiconductor microcavity operating in the strong-coupling regime. Cavity photons hybridize with quantum-well excitons to form polaritons combining properties of photons and matter quasiparticles. A control photon loaded into one polariton mode changes the polarization response seen by a subsequently incident target photon. This conditional rotation is governed by the interplay of self-induced Larmor precession triggered by spin-dependent exciton-exciton interactions and the polarization beats caused by the splitting of transverse-electric and transverse-magnetic cavity modes. We identify conditions of the experiment that enable implementation of a controlled-Z gate and allow to distinguish all four Bell states in the ideal limit. The one-sided scattering scheme provides a lower interaction threshold than a scalar Kerr reference under the same assumptions. At a selected operating point, the bandwidth-induced identification error scales as the fourth power of the pulse bandwidth in the narrow-band limit. An additional fixed input rotation reduces this error even further. We describe the entanglement swapping between remote memories and determine the minimum quality of the elementary links needed to obtain an entangled output.
We propose a Bell-measurement node for quantum repeaters based on a planar semiconductor microcavity operating in the strong-coupling regime. Cavity photons hybridize with quantum-well excitons to form polaritons combining properties of photons and matter quasiparticles. A control photon loaded into one polariton mode changes the polarization response seen by a subsequently incident target photon. This conditional rotation is governed by the interplay of self-induced Larmor precession triggered by spin-dependent exciton-exciton interactions and the polarization beats caused by the splitting of transverse-electric and transverse-magnetic cavity modes. We identify conditions of the experiment that enable implementation of a controlled-Z gate and allow to distinguish all four Bell states in the ideal limit. The one-sided scattering scheme provides a lower interaction threshold than a scalar Kerr reference under the same assumptions. At a selected operating point, the bandwidth-induced identification error scales as the fourth power of the pulse bandwidth in the narrow-band limit. An additional fixed input rotation reduces this error even further. We describe the entanglement swapping between remote memories and determine the minimum quality of the elementary links needed to obtain an entangled output.
Summary form only given. Semiconductor microcavities can be engineered to produce strong coupling between photons and quantum well excitons leading to the formation of new eigenstates called polaritons. Due to their peculiar dispersion, enabling non-equilibrium Bose-Einstein condensation [1], and their straightforward experimental accessibility, microcavity polaritons have gained considerable interest in recent years. Polariton states can be selectively populated by choosing the excitation angle and the wavelength of the excitation laser. After a lifetime of a few picoseconds the polariton decays and a photon is emitted from the cavity, carrying the complete information on the underlying polariton state. Several theoretical works [2-3] suggest that polariton-polariton interactions can be a source of non-classical photon correlations, producing photon pairs that are entangled in either the energy, the polarization, or the momentum degree of freedom. One appeal of such a system is that many parameters such as the polariton-polariton interaction strength are tunable [4] and multipartite entangled states can be created in a single device [5]. However, due to the lack of high-quality samples, experimental realisations of these schemes have been elusive so far. In this contribution, we will discuss some of our experimental work on the statistics of polariton-polariton parametric scattering. Excited polaritons can scatter in a four-wave mixing process into signal and idler states lying on a figure-eight shaped curve in k-space, where phase-matching is fulfilled. We make use of a sample with ultra narrow exciton resonances, which allows for the generation and detection of correlated light. We present our measurements of the second order correlation functions using time-correlated photon counting. We discuss in detail, how polariton quantum correlations depend on a number of experimental parameters, such as phase space location and excitation density and find that the results can be understood in the framework of the well known Bogoliubov theory for 2D Bose gases utilised for microcavity polaritons [6]. We find bunching below the parametric threshold and a slow buildup of coherence above the threshold similar to the quantum statistics of a non-degenerate optical parametric oscillator, which is well known for its nonclassical properties [7]. We discuss in particular the influence of strong background processes on the observed photon statistics.
Entanglement generation by means of microcavity exciton-polaritons is an interesting application of the peculiar properties of these half-light/half-matter quasiparticles. In this paper, we theoretically investigate their luminescence dynamics and entanglement formation in single, and double cavities. We discuss the selection rules for polariton–polariton scattering in double cavities, and evaluate a number of possible parametric scattering schemes in terms of entanglement in the polarization degree of freedom, and identify the ones that are experimentally most promising.
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).
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).
Nonlinear optical response of cavity polaritons formed in high-g semiconductor microcavities has attracted much attention because of their potential applications to entangled-photon sources and other quantum information technologies. Theoretical ideas of entangled-photon generation, e.g., biexcitonic cavity quantum electrodynamics and polariton-polariton scattering, have been suggested, however these theories are limited in scope of target because they restrict two-exciton states to either biexciton or unbound two-exciton states. As for exciton system with small binding energy of biexciton, it is likely that the interplay between the biexciton and unbound two-exciton states have much impact on generation efficiency of entangled photons. The authors introduces a general model including both biexciton and unbound two-exciton states and discuss generation efficiency of entangled photons.
Summary form only given. The optical coherent control of quantum mechanical systems enables the direct manipulation of quantum states, and has been demonstrated on excitonic transitions in semiconductors. We have demonstrated the coherent control of the nonlinear optical response of quantum-well excitons in a resonant high-Q microcavity. There are two significant advantages to using excitons in microcavities instead of bare quantum wells: first, the multiple reflections which occur in the cavity strongly enhance the optical nonlinearity, enabling large-signal coherent control, and second, 2-color coherent control may be implemented, where a pump pulse tuned to one of the normal modes of the coupled exciton-microcavity system can control the response of a probe tuned to the other normal mode. The nonlinear response of the normal modes is due to the nonlinearity of the underlying exciton transition.