QAP – Qubit Applications Project

The EU Integrated Project Qubit Applications (or QAP) is a partnership of 35 academic and industrial groups at the cutting edge of quantum information research. Initiated in 2005, QAP’s mission is to develop and implement novel applications for quantum information processing, and to explore theoretical concepts of quantum information. QAP partners have published over 1205 papers in a variety of journals to date, including prestigious titles such as Nature, Science, Physical Review Letters, Nature Photonics among others. These papers mark a significant contribution to the worldwide effort to understand, control and utilize quantum systems, and reflect the diverse range of interests within the collaboration.

The following paragraphs give a very brief introduction to some of the core physical ideas underlying the whole quantum information research area. A more detailed description of the research in each of the QAP subprojects can be found below, including descriptions of some successes to date.

From Quantum mechanics to Quantum Information and Quantum Communication

Among the greatest achievements of 20th century physics was the creation of quantum mechanics. This physical theory describes the behaviour of the microscopic particles that make up the world around us. Everything - from the oxygen molecules in your lungs to the photons (light particles) entering your eyes, from the atoms making up your body to the electrons running your computer - obeys the laws of quantum mechanics. Perhaps the most exciting outcome of the theory is the surprising nature of its predictions for the physics of fundamental particles. For example, while quantum mechanics delivers astonishingly accurate results, it says that fundamental particles are inherently governed by probabilities which cannot be avoided. This probabilistic behaviour has dramatic implications when we consider using quantum systems (systems of photons, atoms etc.) to store, manipulate, and transmit information. In conventional or “classical” computers, information is stored as bits (0’s and 1’s) which are represented, for example, as voltage pulses involving millions of electrons. In a quantum system, information and the laws which govern it must be in accordance with quantum mechanics. While this precludes us from using quantum systems for classical computation, scientists have found that the necessary reformulation of information processing in accordance with quantum mechanics – or quantum information processing – is a tremendously powerful concept for information processing and communication. The main goal of the QAP research effort is to design and build some of the small scale systems possible using present technologies while developing new ideas for future quantum information processing and communication.

The Qubit

The fundamental unit of information in quantum information processing and communication (QIPC) is the quantum bit or qubit. In classical computation, a bit can be in one of two states – either 0 or 1. As an illustrative example, consider the direction of an arrow to represent the state of the bit with up corresponding to 0 and down corresponding to 1. In classical computation the arrow direction is restricted to be in exactly one of these directions. However, quantum mechanics places no such restriction on the direction of the arrow – it can point in any direction, meaning that quantum systems are potentially very powerful for computation. This extra freedom of quantum systems is called superposition, where we describe the arrow as being in a superposition of both 0 and 1 at the same time, unless it points exactly up or down. The phenomenon is found everywhere in the microscopic world of quantum physics, making the qubit a very useful concept. Examples include the direction of an atom’s internal magnetic field, the polarization of a photon, and the energy of a vibrating atom – all of which can be used as qubits. QAP researchers are using a variety of different systems as qubits including photons, individual atoms and solid-state systems.

Entanglement

Entanglement describes a phenomenon whereby two or more quantum particles can be made to interact so that the physical properties (eg. energy, momentum etc) of one particle are strongly correlated with the other particle(s). These correlations cannot be explained unless we use quantum mechanics to describe the particles as belonging, in some sense, to the same entity – even though they may be separated by large distances. It was this behaviour that prompted Albert Einstein to famously criticize quantum mechanics as an incomplete physical picture of reality. Many experiments have since shown that entanglement does occur and that it is correctly described by quantum mechanics. A major objective of the QAP project is to develop quantum repeaters and scalable quantum networks capable of transmitting quantum entanglement over large distances. This will be crucial for future quantum technologies which will use entanglement as a necessary resource.

Challenges of QIPC

While qubits are potentially found in many physical systems, their storage, manipulation and transmission represent significant technical challenges. For example, the ideal way to transmit quantum information is using photons (often called “flying qubits”) because of their high speed and weak interactions with their surroundings. However, these properties make them difficult to store and modify. Atoms and solid-state systems are more appropriate for information storage and manipulation due to their strong interactions with their environment. Key to any future implementations of QIPC is therefore the ability to make flying qubits (photons) interact with stationary qubits (eg. atoms) in a reliable way that preserves the quantum information accurately when we transfer it from one qubit to another. This specific challenge is being addressed by the QAP subproject: Quantum memories and interfaces.

Many QIPC ideas are predicated on the ability to send specific quantum states of light (ie. photons with specific quantum mechanical properties) to distinct locations. Such a set of linked locations is called a quantum network. For example, it might be necessary to generate a pair of entangled photons and send each photon to a separate destination where it must be detected. When you consider that a standard 100W light bulb emits approximately 6x10¹⁶ (sixty million billion) visible photons every second, it is clear that deterministic generation, distribution and detection of specially conditioned photons is a non-trivial pursuit. The QAP Quantum Networks subproject is building on existing methods of generating and distributing entangled photons, and extending these to implement a scalable quantum network.

All transmission of information is susceptible to degradation as the information propagates. For example, modern fibre-optic telecommunications networks suffer from distortion of the input signal due to attenuation and fibre distortion. This problem is resolved by amplifying the optical signal at intervals between the origin and destination of the signal. Unfortunately, such amplification schemes are not possible in quantum communication channels because it is not possible to amplify or copy quantum information. This result is the No-Cloning Theorem of quantum mechanics. The Quantum Repeater's subproject aim to bypass this problem using a quantum repeater scheme. This involves breaking a communication channel into smaller links, and distributing entanglement between the links to allow effective transmission of quantum information.