TY - BOOK

T1 - A high-frequency squeezing comb

T2 - generation, detection & characterisation

AU - Wilken, Dennis

PY - 2024

Y1 - 2024

N2 - Squeezed light is a nonclassical state that exhibits reduced noise in one quadrature compared to the vacuum state. Currently, its main application is reducing quantum noise in gravitational wave detectors and, hence, increasing their sensitivity. Besides application in metrology, squeezed states are a resource of bipartite entanglement that can be harvested for applications in quantum information with a focus on quantum communication but extended to cluster states, a prospect for quantum computing. Squeezed states are predominantly generated with nonlinear crystals in optical resonators. These resonators squeeze the vacuum fluctuations for all frequencies resonant with the cavity, yielding a comb structure, the squeezing comb. The tooth separation corresponds to the free spectral range (FSR), which scales antiproportionally with the resonator's roundtrip length. Typically, these resonators are a few centimetres to a few decimeters long and, hence, have tooth separation up to several gigahertz. These frequencies make detecting and accessing higher FSRs a challenge. High-frequency photodetectors are being developed for this purpose, but their quantum efficiency still needs to improve to measure high squeezing values. In this work, I offer a solution for measuring a squeezing comb broadband. On the one hand, I have built a 1.5 m-long highly efficient squeezed light source with an FSR of only 200 MHz providing up to 20 times higher spectral tooth density than linear models. With the squeezer, it was possible to measure squeezing values of up to 11.8 dB. Furthermore, I developed high-speed photodetectors with high quantum efficiency to measure squeezing not only at the first FSR but at the first 31 FSRs, corresponding to a bandwidth of 6.2 GHz. However, due to asymmetries between the two photodiodes, this detector only allowed balanced measurements below 500 MHz. Above the unconventional unbalanced homodyne detection had to be used. Here, I demonstrate, for the first time, homodyne detection of a squeezing comb in the gigahertz range with high squeezing values. In a further experiment, I investigated the entanglement between the squeezing comb's frequency sidebands. Therefore, I spatially divided the squeezing comb using an unbalanced Mach-Zehnder interferometer (UMZI) and performed measurements between uncorrelated sidebands. Combining the measurements, I could confirm Einstein-Podolsky-Rosen (EPR) entanglement. The squeezer and the photodetector have already enabled several experiments in quantum metrology and, together with the UMZI, provide a basis for further decisive experiments in quantum information, such as entanglement swapping or quantum teleportation. Such experiments are essential in contributing to the success of squeezing combs and the technology based on them, which is currently being fuelled by their integration into chips.

AB - Squeezed light is a nonclassical state that exhibits reduced noise in one quadrature compared to the vacuum state. Currently, its main application is reducing quantum noise in gravitational wave detectors and, hence, increasing their sensitivity. Besides application in metrology, squeezed states are a resource of bipartite entanglement that can be harvested for applications in quantum information with a focus on quantum communication but extended to cluster states, a prospect for quantum computing. Squeezed states are predominantly generated with nonlinear crystals in optical resonators. These resonators squeeze the vacuum fluctuations for all frequencies resonant with the cavity, yielding a comb structure, the squeezing comb. The tooth separation corresponds to the free spectral range (FSR), which scales antiproportionally with the resonator's roundtrip length. Typically, these resonators are a few centimetres to a few decimeters long and, hence, have tooth separation up to several gigahertz. These frequencies make detecting and accessing higher FSRs a challenge. High-frequency photodetectors are being developed for this purpose, but their quantum efficiency still needs to improve to measure high squeezing values. In this work, I offer a solution for measuring a squeezing comb broadband. On the one hand, I have built a 1.5 m-long highly efficient squeezed light source with an FSR of only 200 MHz providing up to 20 times higher spectral tooth density than linear models. With the squeezer, it was possible to measure squeezing values of up to 11.8 dB. Furthermore, I developed high-speed photodetectors with high quantum efficiency to measure squeezing not only at the first FSR but at the first 31 FSRs, corresponding to a bandwidth of 6.2 GHz. However, due to asymmetries between the two photodiodes, this detector only allowed balanced measurements below 500 MHz. Above the unconventional unbalanced homodyne detection had to be used. Here, I demonstrate, for the first time, homodyne detection of a squeezing comb in the gigahertz range with high squeezing values. In a further experiment, I investigated the entanglement between the squeezing comb's frequency sidebands. Therefore, I spatially divided the squeezing comb using an unbalanced Mach-Zehnder interferometer (UMZI) and performed measurements between uncorrelated sidebands. Combining the measurements, I could confirm Einstein-Podolsky-Rosen (EPR) entanglement. The squeezer and the photodetector have already enabled several experiments in quantum metrology and, together with the UMZI, provide a basis for further decisive experiments in quantum information, such as entanglement swapping or quantum teleportation. Such experiments are essential in contributing to the success of squeezing combs and the technology based on them, which is currently being fuelled by their integration into chips.

U2 - 10.15488/18014

DO - 10.15488/18014

M3 - Doctoral thesis

CY - Hannover

ER -