Integrated two-photon quantum gate

Quantum computing is a radically new technology that promises to revolutionize information processing. It employs so-called quantum bits (qubits) for information storage and processing. In contrast to classical bits which can either be prepared in the state 0 or 1, quantum bits can be prepared in both states at once, i.e. in a so-called superposition state. This allows a massively parallel processing of information and an exponential speed up of quantum calculation compared to classical protocols. One very promising candidate for qubits are optical photons which can be controlled and manipulated with high accuracy. Furthermore, photons are already the backbone of todays optical fibre-based communication technology and can be integrated into miniaturised optical circuits for future on-chip optical quantum computers. Recently, there have been large advances towards photonic quantum computing. However, the essential ingredient is still missing: until now, no logical gate between two optical qubits exist. The physical origin of this problem is that similar to two light beams photons do not interact with each other. An effective photon-photon interaction can, however, be realized by making use of the interaction between photons and atoms, as discovered by Einstein. Here, the quantum state of a photon is first stored in the internal state of a single atom, then, the atom interacts with a second photon and subsequently, the atomic state is read out again, i.e. transferred back into an optical photon. In order to realize this process with high success probability, the atom-photon interaction has to be enhanced thanks to an optical resonator. Here, we plan to use a special optical resonator, a so-called bottle-resonator, which has a size of a few ten microns, with the atom trapped in its vicinity. The light, coupled into the resonator using ultrathin optical fibres, is guided around its circumference for about 100,000 times before it couples back into the fibre. In this type of resonator the direction of propagation fixes the polarisation of the photons and thus the strength of their interaction with the atom. This enables us to deterministically control how the atom absorbs or emits a single photon. We will use this feature to implement a quantum memory, where the quantum state of the photon is written and stored in a single atom and, in a second step, transferred back into a photon. Sending in a second photon while the first photon is stored, then allows us to implement an effective photon-photon interaction which we will employ to realize a universal photon-photon quantum gate.

Nowadays, information processing becomes more and more important. But the miniaturisation of electronics components will soon end. To meet the ever-increasing demand in terms of computational power and speeds, new approaches are required. A promising one is optical computing, using photons as bits of information. Photons are already the backbone of todays optical fibre-based communication technology and can be integrated into miniaturised optical circuits. Optical computing additionally paves the way to future on- chip optical quantum computers using the radically new methods provided by quantum mechanics. There, single photons are used as so-called quantum bits (qubits). In contrast to classical bits, which can either be prepared in the state 0 or 1, they can be prepared in both states at once, i.e. in a so-called superposition state. This allows a massively parallel processing of information and an exponential speed up of quantum calculations compared to classical protocols. Recently, there have been large advances towards photonic computing. However, one essential ingredient of photonic circuits is still missing: integrated components that reliably control the lights propagation direction, such as, e.g., optical circulators. Until now, the miniaturization of such systems comes with high losses which prevents their use for information processing and in particular for quantum applications. In this project, we followed a completely new approach towards the realization of an optical circulator by coupling a single rubidium atom to the light field of a so-called "bottle resonator" a microscopic bulbous glass object where total internal reflection guides the light. If such a resonator is placed in the vicinity of two ultrathin glass fibers, light can couple from the fibers to the resonator and vice versa. Without the atom, the light changes from one glass fiber to the other via the bottle resonator. In this way, however, no sense of circulation is defined: light, can also travel in reverse direction via the same route. In order to break this forward/backward symmetry, we additionally couple an atom to the resonator, which for one propagation direction allows the light to pass through the resonator, while it prevents this process for the other propagation direction. This trick employs a special property of the light: The direction of rotation of the lights electric field depends on the lights propagation direction in the resonator. If the rubidium atom is prepared in a well-defined internal state, its interaction with the light differs for the two directions of circulation. This allows us to realize an integrable optical circulator, which is compatible with low photon levels and which is ready for quantum information processing. Moreover, the asymmetry of the light-atom coupling with respect to the propagation direction of the light in the resonator allows an additional control over the circulator operation: the desired sense of circulation can be adjusted via the internal state of the atom. As the single atom is a quantum system, it can in principle be prepared in a superposition of internal state which then would realize a first quantum-version of an optical circulator.