In the Cavity QED Experiment, we explore the interactions between atoms and photons in the context of quantum metrology. More specifically, our goal is to trap and entangle atoms in a high finesse cavity in order to create an atomic spin-squeezed state. The creation of such a state has great potential to improve the precision of all sensors that use atomic clock states by suppressing their atomic projection noise. Sensors that use atomic clock states include atomic clocks, gravimeters, gravity gradiometers, accelerometers and gyroscopes. The improvement of these sensors is important as they have a wide variety of applications such as in inertial navigation systems (INS), oil and mineral exploration, gravitational wave detection and global positioning systems (GPS).
Introduction to spin-squeezed states
Squeezed states in a cavity QED system can be generated through QND measurement of the collective atomic state.
The clock states of Rubidium can be described in terms of a spin formalism where the F=1 state is ‘spin down’ and the F=2 state is ‘spin up’. One can then measure the overall ‘spin’ of the system by probing the cavity with 780 nm light. The QND measurement allows generation of entanglement since there is no ‘which-way’ information, meaning one does not know which atoms cause the frequency shift of the cavity resonance.
Firstly, 87Rb atoms are placed inside the dual-wavelength high-finesse cavity using a magneto-optical trap (MOT). The atoms are then loaded into a 1560nm optical lattice. After the loading is complete, the atoms are put into a superposition of the 5 2S1/2 F=1 and 5 2S1/2 F=2 hyperfine ground states, or simply put the “ground state” and the “excited state” of a two level atom system also known as an atomic clock state. This is done by radiating the atoms with a π/2 microwave beam. The cavity is then probed with a 780nm laser (using a homodyne setup). This allows us to determine the shift in resonance frequency of the cavity, which is directly proportional to the total angular momentum of the Rb atoms inside the cavity. However, since no information about individual atoms are obtained, the atoms are in an entangled state known as a spin-squeezed state.
The dual wavelength cavity allows connement of the atoms on the intensity peaks of the 780 nm probe light. The atoms are thus uniformly and maximally coupled to the probe light. Uniform coupling to the cavity lets you reap the full benefitt of squeezed states in existing detection schemes like fluorescence detection.
The optical detection outline
The master laser is a RIO 1560 nm ECDL laser, which is locked to a ‘scrubbing cavity’ to reduce noise. The transmission of the cavity is then amplified and locked to the science cavity. The 780 nm probe is generated by frequency doubling the stabilized 1560 nm light, simplifying the setup by eliminating the need for an extra laser. The cavity frequency shift per atom is on the order of 10 Hz, requiring excellent frequency stability to achieve single atom resolution
The fundamental limit to squeezing of atom in a cavity is spontaneous emission. We therefore use a homodyne setup to measure the cavity resonance, as other schemes inherently use more photons. Path-length stabilization is necessary to perform homodyne measurements. We use two 10 nW sidebands oset by 2 MHz from the cavity resonance to stabilize the path-length to the theoretical limit at 5 μrad/rtHz.
The immediate next step is to observe quantized jumps in the atom number using the uniform registration of our dual-wavelength cavity. Once that is done, we can generate squeezed states with more than 10 dB of squeezing in the variance.
In the long-term, we hope to find ways to implement this method of squeezing in atom interferometer experiments.