Molecular Microcavity Photon Source

光子——光的量子粒子——在量子科学和技术中扮演着重要的角色。信息很容易存储在它们里面。它们易于操纵，运输方便，不受周围环境的干扰。来自强衰减激光束的光子每次到达一个，但这种类型的光源在大多数量子应用中并不实用，因为我们不知道光子何时到达。这个问题通常是通过让激光穿过一个晶体来解决的，这个晶体偶尔会随机地把一个光子分成两个。当需要一个光子时，我们等待两个光子中的一个被检测到（并被破坏），然后我们知道另一个光子已经准备好在我们的量子应用中使用。秘密消息的通信是该方法的一个著名应用。如果一次只需要一个或两个光子，这种方法就可以很好地工作，但许多应用需要几个相同的光子（甚至几十个光子），要么同时进行，要么有精确的时间延迟。不幸的是，分裂的随机性不允许这样做。我们需要一种能够快速可靠地传输光子的光源，无论何时，它们都需要用于量子信息处理或复杂量子系统的模拟等应用。光子应该是相同的，这样它们才能表现出量子干涉——即一个粒子与另一个粒子的干涉——这是量子粒子的基本特征，从而产生了著名的量子力学的力量。这样的来源尚不存在。在这里，我们建议建立一个能快速和按需产生相同光子的光源。我们的方法是将有机染料（DBT）的单个分子嵌入到一个小型光学谐振器中，并与光纤耦合。当需要一个光子时，分子被一个明亮的光脉冲激发，然后释放一个光子。谐振器迫使光子以特定的频率发射，并进入与光纤耦合的特定方向，从而使光子彼此无法区分。这有时被称为珀塞尔效应，以埃德·珀塞尔（Ed Purcell）的名字命名，他首先在磁共振的背景下注意到了这一点。其他研究人员曾试图利用珀塞尔效应制造一个好的光子源，使用各种发射体，如量子点或晶体中的色心，但由于发射体的光学特性不足，这些都无法产生高产量的相同光子。我们的建议的新颖之处在于DBT分子，正如我们最近所展示的，DBT分子对于这个应用程序具有接近理想的特性。通过将这个分子加入到我们的一个空腔中，我们期望每次我们需要一个光子时都能产生一个光子，我们可以期望这些光子是相同的。我们还计划通过在腔内施加电场来调节分子。我们的设计将允许我们堆叠几个微型光子源，并选择光子是相同的还是调谐到不同频率的阵列。这些可以用来制造复杂的量子态光，适用于目前缺乏合适光源的量子技术的一系列应用。除了直接应用于量子信息处理之外，我们的DBT分子光源为纳米光学和纳米电子学提供了一个有前途的新元素。只要有一点想象力，我们就可以看到，有一天，这些分子可能会通过添加化学基团而得到增强，将它们变成针对特定分子的定制传感器，通过光或直接通过有机半导体进行敏感读出。简而言之，我们将在空腔中使用DBT分子按需制造相同的光子，满足量子技术的迫切需求。通过利用这种新型量子发射器，我们期望在纳米光学和纳米电子学的科学和技术方面取得根本性的进步。

Photons - quantum particles of light - have an important role to play in quantum science and technology. Information is easily stored in them. They are readily manipulated, conveniently transported and not much disturbed by their surroundings. The photons from a strongly attenuated laser beam arrive one at a time, but this type of source is not practical to use in most quantum applications because we do not know when the photons will come. That problem is normally addressed by passing the laser light through a crystal which occasionally, and randomly, splits a photon into two. When a photon is needed, we wait for one of the two to be detected (and destroyed), then we know that the other photon is ready for use in our quantum application. The communication of secret messages is one famous application of this approach. This works well if only one or two photons are needed at a time, but many applications require several identical photons (or even dozens of them), either simultaneously or with accurate time delays. Unfortunately, the random nature of the splitting does not allow that. We need a source that can deliver photons rapidly and reliably whenever they are needed for applications such as quantum information processing, or the simulation of complex quantum systems. The photons should be identical so that they can exhibit quantum interference - i.e. interference of one particle with another - which is the essential feature of quantum particles giving rise to the famous power of quantum mechanics. Such a source does not yet exist.Here we propose to build a source that produces identical photons, rapidly and on demand. Our method is to embed individual molecules of an organic dye (known as DBT) inside a small optical resonator, coupled to an optical fibre. When a photon is required, the molecule is excited by a bright pulse of light, after which it emits a photon. The resonator forces the photon to be emitted at a specific frequency and into the specific direction that couples to the fibre, thereby making the photons indistinguishable from each other. This is sometimes called the Purcell effect after Ed Purcell, who first noticed it in the context of magnetic resonance. Other researchers have tried to use the Purcell effect to make a good photon source, with various emitters such as quantum dots or colour centres in a crystal, but these have not been able to produce a high yield of identical photons because of the inadequate optical properties of the emitter. The novel aspect of our proposal is the DBT molecule, which has near ideal properties for this application, as we have recently shown. By incorporating this molecule into one of our cavities, we expect to produce a photon essentially every time we ask for one, and we can expect these photons to be identical. We also plan to tune the molecule by applying an electric field inside the cavity. Our design will allow us to stack up several miniature photon sources and choose whether the photons are identical or are tuned to an array of different frequencies. These can then be used to make complex quantum states of light suitable for a range of applications in quantum technology where suitable sources are currently lacking.Beyond the immediate application to quantum information processing, our DBT molecular light source provides a promising new element for nano-optics and nano-electronics more generally. With a little imagination, we can see that these molecules may one day be enhanced by the addition of chemical groups to turn them into custom sensors for specific molecules, with sensitive readout by the light, or perhaps directly through an organic semiconductor.In short, we will use DBT molecules in cavities to make identical photons on demand, satisfying an immediate need of quantum technology. By working to utilise this new type of quantum emitter, we expect to make a fundamental advance in the science and technology of nano-optics and nano-electronics.