Johannes Tiedau

Limits on the heralding efficiencies and spectral purities of spectrally filtered single photons from photon-pair sources

Photon pairs produced by parametric down-conversion or four-wave mixing can interfere with each other in multiport interferometers or carry entanglement between distant nodes for use in entanglement swapping. This requires the photons to be spectrally pure to ensure good interference and have high heralding efficiency to know accurately the number of photons involved and to maintain high rates as the number of photons grows. Spectral filtering is often used to remove noise and define spectral properties. For heralded single photons high purity and heralding efficiency are possible by filtering the heralding arm, but when both photons in typical pair sources are filtered, we show that the heralding efficiency of one or both of the photons is strongly reduced even by ideal spectral filters with 100% transmission in the passband: any improvement in reduced-state spectral purity from filtering comes at the cost of lowered heralding efficiency. We consider the fidelity to a pure, lossless single photon, symmetrize it to include both photons of the pair, and show this quantity is intrinsically limited for sources with spectral correlation. We then provide a framework for this effect for benchmarking common photon-pair sources and present an experiment where we vary the photon filter bandwidths and measure the increase in purity and corresponding reduction in heralding efficiency.

Quantum state and mode profile tomography by the overlap

VS acknowledge support from the National Council for Scientific and Technological Development (CNPq) of Brazil, grant 304129/2015-1, and by the Sao Paulo Research Foundation (FAPESP), grant 2015/23296-8. DM acknowledge support from the EUproject Horizon-2020 SUPERTWIN id.686731, the National Academy of Sciences of Belarus program ‘Convergence’ and FAPESP grant 2014/21188-0. NK acknowledges the support from the Scottish Universities Physics Alliance (SUPA) and from the International Max Planck Partnership (IMPP) with Scottish Universities. JT and CS acknowledge support from European Union Grant No. 665148 (QCUMbER). TB acknowledges support from theDFG under TRR 142.

Discorrelated quantum states

Using the building blocks of quantum optics - single photons, coherent states, beam splitters and projective measurement - we construct a two-mode quantum state for which coincident photon number terms in each mode are removed.

Limits of the time-multiplexed photon-counting method

The progress in building large quantum states and networks requires sophisticated detection techniques to verify the desired operation. To achieve this aim, a cost- and resource-efficient detection method is the time multiplexing of photonic states. This design is assumed to be efficiently scalable; however, it is restricted by inevitable losses and limited detection efficiencies. Here, we investigate the scalability of time-multiplexed detectors under the effects of fiber dispersion and losses. We use the distinguishability of Fock states up to

Extracting the physical sector of quantum states

n=20

Measuring single photons to bright light with a logarithmic optical detector

after passing the time-multiplexed detector as our figure of merit and find that, for realistic setup efficiencies of

A high dynamic range optical detector for measuring single photons and bright light.

\eta=0.85

Incomplete Detection of Nonclassical Phase-Space Distributions

, the optimal size for time-multiplexed detectors is 256 bins.

Engineering integrated sources of entangled photon pairs

The physical nature of any quantum source guarantees the existence of an effective Hilbert space of finite dimension, the physical sector, in which its state is completely characterized with arbitrarily high accuracy. The extraction of this sector is essential for state tomography. We show that the physical sector of a state, defined in some pre-chosen basis, can be systematically retrieved with a procedure using only data collected from a set of commuting quantum measurement outcomes, with no other assumptions about the source. We demonstrate the versatility and efficiency of the physical-sector extraction by applying it to simulated and experimental data for quantum light sources, as well as quantum systems of finite dimensions.