Quantum information and -communication

Quantum optics and quantum information encompasses both basic physics and application oriented aspects, notably quantum key distribution, where the first generation systems are already on the market. Indeed, quantum key distribution has been identified as a key technology for the next generation optical networks both in Japan (New Generation Internet Initiative − NGI) and China (863 and 973 programs) and a similar programs exist in Europe. Yet, much work remains to be done before quantum technologies have reached the maturity needed for general systems and applications.

Quantum optics and quantum information encompasses both basic physics and application oriented aspects, notably quantum key distribution, where the first generation systems are already on the market. Indeed, quantum key distribution has been identified as a key technology for the next generation optical networks both in Japan (New Generation Internet Initiative − NGI) and China (863 and 973 programs) and a similar programs exist in Europe. Yet, much work remains to be done before quantum technologies have reached the maturity needed for general systems and applications.

Quantum entanglement

Entanglement implies correlation(s) of quantum nature between subsystems. Entanglement is necessary for tasks like quantum teleportation, quantum computing, and several (but not all) quantum cryptography protocols. Entanglement is starting to be viewed as a physical resource, like energy.

A compact entangled-photon-pair source for quantum key
A compact entangled-photon-pair source for quantum key

With a certain amount of entanglement particular tasks can be performed, while others may require additional entanglement. Therefore, the implementation of quantum technologies is intimately tied with the ability to generate, manipulate, and detect entangled states. Unfortunately entanglement is a complicated resource to generate, quantify, and even qualify. Entanglement between two subsystems can be generated routinely and is relatively well understood. For three or more subsystems, the situation is more complicated as several non-interconvertible (with local transformations) types of entanglement exist. No universally accepted quantitative measure exists, so this constitutes a great theoretical challenge ADOPT will tackle.

Experimentally, we will generate and characterize multi-photon entangled-states (both at 750 and 1550 nm) using Bell inequalities and the so-called witness operators to detect multipartite entanglement. We will also study their entanglement properties, such as persistence under particles loss and measurement. These multipartite entangled states will be used to demonstrate various examples of quantum networking and multi-party protocols and be used as simulators for other quantum systems.

Quantum polarization, imaging, and lithography

A widely used method to entangle photons is to use the polarization degree of freedom. The rational is that this is a robust degree of freedom that relatively easily (and cheaply) can be manipulated. For this reason, for virtually all our experiments this is the degree of freedom we, and other groups, are using. At present, the description of polarization itself is under considerable debate and reconsideration, in particular for near-fields, both in classical and quantum optics. It is likely that concepts and ideas from these respective fields will influence and fertilize each other. Near-field quantum-polarization is related to multimode quantum interference. With the combination of mode and state engineering, we expect to demonstrate both phase (or rotation) super-resolution and phase super-sensitivity.

Entanglement is also useful for imaging and lithography. So far, relatively simple (two-mode, few-photon) states have been used to demonstrate, e.g., ghost imaging. We believe that more general entangled states may be even more useful in imaging applications. Imaging naturally leads to multimode quantum interferometry, and this is another topic that is ripe both for theoretical and experimental research within ADOPT.

Photonic quantum information and quantum memories

Quantum information opens new routes for implementing information technologies exploring quantum states generated, e.g., in periodic structures, notably waveguides and fibers. We are studying quantum information both from fundamental aspects as well as from an application context, both at 750 and 1550 nm wavelengths. We are performing studies and demonstrations of secure multiparty protocols, quantum teleportation/cloning, higher dimensional/composite Hilbert space encoding, entanglement swapping, quantum error filtering/error rejection and decoherence-free communication.

A crucial challenge in quantum communication is the realization of a quantum memory, i.e., the temporary conversion and storage of photonic quantum information into matter and then re-conversion into light again. Some possibilities are to work with either atomic clouds, rare earth doped crystals, or potentially with tailored semiconductor or ferroelectric structures. This is a novel area for us. The quantum memory project naturally connects to the metamaterial project (converting quantum states from light to plasmons and back while preserving the quantum character or “freezing” optical quantum states in photonic crystals).

 See even

Quantum electronic and quantum optics group, KTH

Quantum and field theory, Stockholm University

Teleportation ― dos and don’ts

Listen to Prof. Gunnar Björk discuss teleportation as a future transportation solution. Learn the dos and the don’ts, and hear where the science stands today. A talk held in the context of the Transportation platform event arranged by TEDxKTH at Bromma Airport on May 2, 2012. Find the talk here …
 

Page responsible:Max Yan
Belongs to: School of Engineering Sciences (SCI)
Last changed: May 14, 2012