Quantum interference in a single quantum dot

处理量子信息的能力有可能彻底改变通信和计算领域。光子与环境的相互作用最小，使它们成为量子信息的强大载体。然而，处理这些信息仍然非常具有挑战性。也许最可行的方法是将光子的信息转移到原子状态。因此，量子光学，在量子水平上研究光与物质相互作用，在量子信息处理（QIP）的新兴领域中占据核心地位。使用原子气体的量子光学已经产生了惊人的结果，但是在这样的系统中，扩展到多个处理元件并结合到设备中是困难的。因此，QIP激发的一个新范例是量子光学在固态介质中的应用。然而，固态环境有许多方法可以使脆弱的量子态失相或破坏。自组装量子点（QD）是一种半导体的纳米级岛屿，被另一种具有更大带隙的半导体的海洋包围，提供了克服这一障碍的途径。它们的小尺寸导致强量子化，并且也有效地限制有害的失相机制。强量子化还允许量子点以多种方式模拟人工原子。到目前为止，量子点中最壮观的量子光学实验已经利用了类似原子的两能级系统，由一个空的QD或一个具有由光子吸收产生的电子-空穴对（称为激子）的QD组成。量子点的这种接近理想的两能级行为已被用于高保真单光子源和量子计算的基本元件。然而，量子光学前沿的许多应用需要三能级系统。这可以通过用单个载体给QD充电来实现。由于自旋的两个本征态（向上和向下），一个4能级系统被创建，开辟了许多新的可能性。在零磁场下，自旋态是简并的。磁场的施加由于塞曼效应而提升了自旋态简并度。另一种超越二能级系统的方法是在量子点上加入第二个激子。扩展到一个量子点系统与两个以上的光学耦合的水平在于这个建议的核心。迄今为止，尚未实现对量子点态的完全相干光学操纵。为了实现这一点，可以使用两个激光场，其中每个场与特定的光学跃迁共振。如果QD能级的退相被充分抑制（即它们是相干的），则可以表现出显著的量子干涉效应。一种这样的现象是电磁感应透明（EIT），由此破坏性量子干涉导致吸收在原子跃迁的光学共振处消失。这种效应还可以导致全新的光学性质，例如允许停止光的能力，并且在非线性光学和量子信息处理领域中很重要。在这样的系统中，量子干涉的强度由态的相干性决定。这提供了光学探测系统中的相干性的直接方式。到目前为止，对自组装量子点的光学研究主要集中在光谱学上，直到最近才在单点水平上实现了对弛豫时间（即某个状态的填充时间）的敏感性。然而，潜在应用的一个至关重要的必要条件是直接测量单个量子点中自旋和激子的相干时间。这里提出的实验将实现这一点。这个提议的另一个目的是操纵量子点中激子态的相干时间，使人们能够调整量子干涉的强度和性质，无论是破坏性的还是建设性的。实现这里概述的研究将真正将量子点及其有利的固态环境引入量子光学世界。

The ability to process quantum information has the potential to revolutionize the fields of communication and computing. Photons interact minimally with the environment, making them robust carriers of quantum information. However, processing this information remains very challenging. Perhaps the most feasible approach is to transfer the information of a photon to an atomic state. Hence quantum optics, the study of light-matter interaction at the quantum level, occupies a central role in the emerging field of quantum information processing (QIP). Quantum optics with atomic gasses has produced spectacular results, but scaling up to multiple processing elements and incorporation into devices is difficult in such systems. Hence, a new paradigm stimulated by QIP is the application of quantum optics to solid-state media. However, a solid-state environment has many ways of dephasing or destroying the fragile quantum states. Self-assembled quantum dots (QDs), nano-sized islands of one semiconductor surrounded by a sea of another semiconductor with a larger bandgap, offer a route to overcome this obstacle. Their small size leads to strong quantization and also is effective in limiting the detrimental dephasing mechanisms. The strong quantization also allows QDs to emulate artificial atoms in many ways. To date, the most spectacular quantum optics experiments in quantum dots have exploited the atomic-like two-level system, consisting of an empty QD or a QD with an electron-hole pair (called an exciton) created by the absorption of a photon. This near ideal two-level behaviour of a QD has been exploited for high-fidelity single photon sources and basic elements for quantum computation. However, many applications at the forefront of quantum optics require a three level system. This can be achieved by charging the QD with a single carrier. Due to the two eigenstates for spin (up and down), a 4-level system is created, opening up many new possibilities. At zero magnetic field, the spin states are degenerate. Application of a magnetic field lifts the spin state degeneracy due to the Zeeman effect. Another way to go beyond the two-level system is by adding a second exciton to the QD. The expansion to a QD system with more than two levels coupled optically lies at the heart of this proposal. To date, complete coherent optical manipulation of QD states has not been realized. To achieve this, one can use two laser fields, where each field is resonant with a certain optical transition. If the dephasing of the QD levels is sufficiently suppressed (i.e. they are coherent), dramatic quantum interference effects can be manifest. One such phenomenon is electromagnetic induced transparency (EIT), whereby destructive quantum interference causes the absorption to vanish at the optical resonance of an atomic transition. Such an effect can also lead to radically new optical properties which permit the ability for instance to stop light and is important in the fields of non-linear optics and quantum information processing. The strength of the quantum interference in such a system is determined by the coherence of the states. This provides a direct way to optically probe the coherence in a system. Up to now, optical studies on self-assembled QDs have focused primarily on spectroscopy, and only recently has sensitivity to relaxation times (i.e. how long a certain state is populated) been realized at the single dot level. However, a vital necessity for potential applications is direct measurement of the coherence times of spins and excitons in a single QD. The experiments proposed here will achieve this. An additional aim of this proposal is to manipulate the coherence time of excitonic states in a QD, enabling one to tune the strength and nature, either destructive or constructive, of the quantum interference. Realization of the research outlined here will truly usher QDs and their advantageous solid-state environment into the world of quantum optics.