Quantum metrology using tailored non-classical states

authored by
Jonas Junker
supervised by
Michele Heurs

Squeezed states of light play a significant role in various technologies ranging from high-precision metrology such as gravitational wave detection to quantum information.These quantum states are prepared to carry particular characteristics depending on their application. For instance, some applications require squeezing in one, others only in the combination of two distinct optical modes. Furthermore, squeezing can appear constant for all frequencies or frequency-dependently. In this thesis, novel quantum optical methods employing different, tailored non-classical light sources, are developed and described. The individual squeezed states are controlled and characterised, each tailored for a particular application. In high-precision spectroscopy, the measurement sensitivity is often limited by technical noise at low frequencies. The first publication shows that small phase signals at low-frequency are resolvable without increasing the laser power. We use a phase-modulated field, shifting the signal to high frequencies where technical noise is circumvented. In addition, the field is squeezed by 6 dB at high frequencies to reduce shot noise arising from quantum fluctuations. Our approach resolves sub-shot-noise signals at 100 Hz and 20 kHz on a reduced noise floor. In opto-mechanical sensors such as gravitational wave detectors, the fundamental measurement limitation arises from the combination of shot noise and quantum back-action noise induced by quantum radiation pressure noise. A conventional fixed-quadrature squeezed state generated by a resonant optical parametric oscillator (OPO) can only fight one of these two contributions simultaneously. To cancel both quantum noise contributions, a particularly frequency-dependent squeezed state is required. Our second publication shows that a detuned OPO generates frequency-dependent squeezing. It can be used as an approximate effective-negative mass oscillator in an all-optical coherent quantum noise cancellation scheme and is suitable to coherently cancel quantum noise. Our generated state, which is reconstructed by quantum tomography, rotating over megahertz frequencies, exhibits a rotation angle of 39° and a maximal squeezing degree of 5.5 dB. Two-mode squeezed quantum states are resources required in modern applications such as quantum information processing. In the third publication, we address the challenge of determining the ten independent entries of a two-mode squeezed state’s covariance matrix to fully characterise the quantum state. We demonstrate a full reconstruction of a 7 dB two-mode squeezed state using only a single polarisation-sensitive homodyne detector, which avoids additional optics and potential loss channels. The findings of this thesis are relevant for experiments in high-precision quantum metrology, e.g. in spectroscopy or gravitational wave detectors operating at the standard quantum limit. The insights gained on the generating and handling non-classical states enable advances in quantum information technology.

Institute of Gravitation Physics
QUEST-Leibniz Research School
Doctoral thesis
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Electronic version(s)
https://doi.org/10.15488/13411 (Access: Open)

Details in the research portal "Research@Leibniz University"