Quantum Information is the science that deals with the encoding, transfer, manipulation and measurement of quantum states. In Continous Variables the information is encoded into the X- or P-quadratures of the light field. These variables can take any value and are thus contious.
Med støtte fra Forskningsrådene (FTP) og EU programmet Compas
Nearly all transfer of data is nowadays sent through optical fibers as bursts of light, thus transmitting classical information in term of bits, 0’s and 1’s. Although these light pulses carry classical information, they behave according to the law of quantum mechanics. Therefore, they can also carry quantum information which might define tomorrow’s information. Quantum information communication enables secure key transmission (quantum cryptography) and quantum information processing could solve some computational problems that are intractable on any conceivable classical computer.
Quantum information is traditionally encoded as qubits, that is a quantum superposition of two eigenstates. We, however, study quantum informational systems where the carrier of information is a continuous quantum variable, namely the quadratures of the electromagnetic field.
Quantum information is very vulnerable to noise! One of the main aims in the field of quantum information processing is thus to protect the fragile information from the devastating environment. We are currently investigating three different approaches to overcome decoherence in the environment.
Current research activity
Our current research effort within the field of Quantum Information.
Squeezed and entangled states of light generated with parametric down-conversion
Squeezed and entangled light is the main resource for quantum information and quantum metrology. We use 2nd order nonlinear quasi-phase matched crystals to produce squeezed and entangled light. In order to enhance the nonlinear interaction we surround the crystals with cavities, a so called optical parametrical oscillator (OPO). The OPO we use are bow-tie shaped cavities each with a type I periodically poled Potassium Titanyl Phosphate crystals. The OPO cavities consist each of two curved mirrors of 25 mm radius of curvature and two plane mirrors. Three of the mirrors in each cavity are highly reflective at 1064 nm (R>99.95%) while the output couplers have a transmission of 8-10%. The transmittance of the mirrors at the pump wavelength (532 nm) is more than 95%. Along with the pump we inject a seed beam at 1064 nm, the brightness of which is facilitating the construction of the various phase locks in the setup. To lock the phase of the cavities we use auxiliary counter propagating beams at 1064nm. The two-mode squeezing (or CV entanglement) is produced through interference of two single-mode squeezed states with a relative phase of pi/2 at a 50/50 beam splitter (visibility greater than 99%) to produce the two-mode squeezing. The average measured two-mode squeezing is approximately -3.8 dB below the shot noise level.
Quantum Key Distribution
Quantum Key Distribution is a branch of the quantum information protocols that aims at using the basic laws of nature to guarantee absolutely safe communication. The Heisenberg uncertainty principle makes it impossible to measure both the position and the momentum of a particle, and in similar way it is impossible to measure both of the continuous variables of an optical light field, namely the amplitude and the phase. Since an eavesdropper will leave a mark in the form of excess noise if trying to measure the quantum states, no such excess noise guarantees that no one has been listening, and the data collected can then be used to generate a safe key for encryption of a real message. Since this protocol is all about controlling and measuring quantum noise we have experimentally demonstrated that it is possible to improve the performance of such protocols by using quantum engineered noise in the form of squeezed states.
