Quantum entanglement increases information carried by photons
The use of the photons' quantum properties such as energy and spin to realise optical quantum networks is particularly appealing because photons are immune to disturbances from the surrounding environment. In addition, they are readily manipulated and also subject to high-efficiency detection. Moreover, while strong interactions at the single-photon level are difficult to achieve, it is possible to initiate them among multiple photon sources. Scientists working on the EU-funded project QOCO (Quantum optical control) created optical quantum networks with an optical parametric oscillator driven by femtosecond pulse trains. The femtosecond pulse trains were produced by a mode-locked titanium-sapphire oscillator delivering 76 MHz trains of 140 femtosecond pulses. The second harmonic (~ 280 femtoseconds) served to pump the optical parametric oscillator synchronously. Specifically, femtosecond pulse trains contained up to 10 000 individual frequency modes. The simultaneous injection of all these modes produced an intricate network of frequency correlations. The scientists exploited an optical parametric oscillator to access these states. The optical parametric oscillator was selected to have a free spectral range (or differently, finesse) matching the repetition rate of the pulse trains. To fully characterise the spectral entanglement in this frequency comb, the scientists utilised ultrafast pulse shaping. Each possible combination of spectral bands was found to be entangled. This multicolour entanglement imports the classical concept of wavelength-division multiplexing to the quantum domain, increasing the quantum channel capacity. The absence of any partially separable form implied that the optical parametric oscillator is a practical, compact source of massively entangled quantum states. The network of quantum channels present in the broad frequency comb structure should find numerous applications in quantum metrology as well as quantum computing.
Keywords
Quantum entanglement, photons, optical quantum networks, femtosecond pulse trains, quantum metrology
