Quantum Networking with Fibre-Coupled Ions

在项目过程中，我们将开发一个系统，使用光纤中的单光子作为载体，确定性地在遥远的离子之间传输量子信息。这种设备将是所谓量子网络的关键组成部分，在未来，量子网络将像互联网与现代计算机一样连接量子计算机。今天，单量子光（光子）已经被用来长距离安全地传输信息，这一过程称为量子通信。量子力学定律会挫败任何窃听企图。量子效应也可以用来进行计算。研究人员将储存在线性阱中的单个离子作为量子比特，取代了普通计算机中的经典比特。这些量子比特是用激光操纵的。在我们的项目中，我们将通过在离子和光子之间建立一个有效的用户控制接口（量子链路）来结合量子计算和量子通信领域。从离子到光子的量子态转移要求两个系统的强耦合。我们可以通过用两个镜子包围离子来实现这一点，形成一个空腔并增强离子和光子的相互作用。从离子量子比特到光子量子比特的转换过程将通过施加到离子的合适的激光脉冲来操纵。一个光子将产生它的一个属性（偏振）取决于离子的状态。特别有趣的是原子处于两种可能态的叠加态的情况，这种情况在量子力学中是允许的。我们的界面将确保光子的量子态与离子的量子态相同，因此将具有偏振叠加。更有趣的是，当离子并没有将其原始超位态的所有信息传递给光子，而是保留了一部分信息时，离子和发射的光子将处于一种关联或纠缠态，在这种状态下，对单独组分的测量结果是不可预测的，但将两个系统的结果结合起来，我们总能发现完美的相关性。以前，这些状态只在一个位置以受控的方式产生，而我们将能够在长距离上分发纠缠。这将是我们项目的主要成果之一。我们还将逆转这个过程，将入射光子的量子态转换为我们腔中离子的量子态。结合这两个过程，我们可以将量子态从一个离子转移到远处的离子，或者纠缠它们的量子态。这是非常有效的，因为在这个过程中没有留下任何机会。这种纠缠是未来进行高效量子计算的重要资源，要实现我们的目标，我们必须掌握两项技术：首先我们需要将单个离子长时间（小时）存储在非常小的空间区域（小于40 nm）中。这在微观离子阱的帮助下是可能的。围绕离子的腔体的镜子必须具有极高的质量，允许200，000次光子反射而不损失。另外，我们必须把反射镜放在离离子很近的地方，以增强离子和光子之间的相互作用。结合微观陷阱与小镜分离是该项目的主要实验挑战。采用激光加工光纤端面作为反射镜，可以实现腔体的最终小型化。此外，腔发射将直接耦合到光纤中，以实现可靠的长距离传输。

In the course of the project, we will develop a system to transfer quantum information between distant ions deterministically, using single photons in an optical fibre as carrier. This device will be a key building block of a so-called quantum network, which in the future will connect quantum computers like the internet does with present-day computers.Already today single quanta of light (photons) are used to transmit information securely over long distances, a process called quantum communication. The laws of quantum mechanics would foil any attempt of eavesdropping. Quantum effects can also be used to perform computations. Re-searchers use single ions stored in linear traps as quantum bits, replacing the classical bits in ordi-nary computers. These quantum bits are manipulated with laser light. In our project, we will com-bine the areas of quantum computation and quantum communication by building an efficient user-controlled interface (a quantum link) between ions and photons. The transfer of quantum states from ions to photons requires that we strongly couple the two sys-tems. We can achieve this by surrounding the ion with two mirrors, forming a cavity and enhancing the interaction of ions and photons. The conversion process from the ion-qubit to the photon-qubit will be steered with a suitable laser pulse applied to the ion. A photon will be generated with one of its properties (polarization) depending on the state of the ion. Particularly interesting are cases where the atom is in a superposition of two possible states, a situation that is allowed in quantum mechanics. Our interface will make sure that the quantum state of the photon is identical that of the ion and will therefore have a superposition of polarizations. Even more interesting are the cases when the ion doesn't transfer all information on its original su-perposition state to the photon, but retains some of it. The ion and the emitted photon will then be in a linked or entangled state, where the outcome of a measurement on the separate components is unpredictable, but combining the results of the two systems one always finds perfect correlation. Previously these states have been produced in a controlled way only in one location, while we will be able to distribute entanglement over long distances. This will be one of the major achievements of our project. We will also reverse this process and transfer the quantum state of an incoming photon to that of an ion in our cavity. Combining the two processes, we can transfer quantum states from one ion to a distant ion, or entangle their quantum states. This is done very efficiently, as nothing in the process is left to chance. This kind of entanglement is an important resource for performing efficient quantum computation in the future.To achieve our goals, we have to master two technologies: first we need to store a single ion in a very small region of space (less than 40 nm) for a long time (hours). This is possible with the help of a microscopic ion trap. The mirrors of the cavity surrounding the ion must have extremely high quality, allowing 200,000 reflections of the photon without loss. In addition, we must put the mirrors very close to the ion, to enhance the interaction between ion and photon. Combining the micro-scopic trap with a small mirror separation is the main experimental challenge of this project. By employing laser machined end facets of optical fibres as mirrors, we can achieve the ultimate miniaturization of the cavity. Furthermore, the cavity emission will be coupled directly into the fibre for reliable long distance transmission.