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Development and Characterization of a Second Harmonic Generator and Mach-Zehnder Interferometer towards a Frequency-Dependent Squeezed Light Source

Nathaniel Morrison, P.-F. Cohadon, S. Chua

Published Jul 31, 2019• 0 citations
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Generation of squeezed state at telecommunication wavelength has been recently a very interesting issue due to the lowest optical power attenuation of light at a wavelength of 1550 nm in a standard telecommunication fiber. The low-frequency vacuum squeezed state at 1550 nm in combination with fiber based interferometer offers the possibility to implement quantum precision measurement beyond standard quantum limit (SQL). In this paper, we experimentally realize a quantum-enhanced fiber Mach-Zehnder interferometer (FMZI) for measuring the low-frequency phase modulation signal by using low-frequency vacuum squeezing at 1550 nm. Firstly, the low-frequency vacuum squeezed state at the telecommunication wavelength of 1550 nm is generated by using a degenerate optical parametric oscillator (DOPO). The DOPO is a semi-monolithic construction based on a type I periodically poled KTiOPO4 (PPKTP) crystal and a concave mirror. The pump threshold of DOPO is 270 mW. When the pump power is 120 mW that is below the pump threshold of DOPO and the temperature of PPKTP is controlled at 34.8℃, a vacuum squeezing of 3 dB is generated at an analysis frequency range from 10 kHz to 500 kHz. The quadrature phase vacuum squeezing is obtained by locking the squeezed quadrature angle through using a coherent control scheme, in which two acousto-optic modulators are used to shift the frequency and produce the auxiliary beam acting as a coherent control field. Based on the constructed FMZI, a quantum-enhanced FMZI is realized by injecting the generated low-frequency vacuum squeezed state at 1550 nm into the vacuum channel of FMZI. The relative phase between two injected light fields is locked at π by using the Pound-Drever-Hall (PDH) locking technology, and the relative phase between light fields of its arms in FMZI is also locked at π/2 by using the PDH locking technology. When a phase modulation signal at the frequency of 500 kHz is loaded in the signal arm of FMZI, the noise power spectrum of the output from FMZI is measured by a balance homodyne detect system. A 2 dB quantum improvement beyond shot-noise-level at the frequency of 500 kHz is obtained experimentally by using the quantum-enhanced FMZI. The experimental results demonstrate a potential application in quantum precision measurement beyond the SQL based on fiber sensor technique.

Acta Physica Sinica•Jan 1, 2018•3 citations
We review our recent experimental progress in quantum technology employing amplification effect of four-wave mixing in a rubidium vapor. We have produced an intensity difference squeezed light source at frequencies as low as 1.5 kHz which is so far the lowest frequency at which squeezing has been observed in an atomic system. Moreover, we find that the bandwidth of our squeezed light source can be controlled with light intensity pumping. Using our non-classical light source, we have further developed a nonlinear Mach-Zehnder (MZ) interferometer, for which the maximum fringe intensity depends quadratically on the intensity of the phase-sensing field at the high-gain regime, leading to much better sensitivity than a linear MZ interferometer in which the beam splitters have the same phase-sensing intensity. The quantum technologies developed by our group could have great potential in areas such as precision measurement and quantum information.
We report a bound on the relative frequency fluctuations in nonlinear second harmonic generation. A 1,064 nm Nd:YAG laser is used to read out the phase of a Mach-Zehnder interferometer while PPKTP, a nonlinear crystal, is placed in each arm to generate second harmonic light. By comparing the arm length difference of the Mach Zehnder as read out by the fundamental 1064 nm light, and its second harmonic at 532 nm, we can bound the excess frequency noise introduced in the harmonic generation process. We report an amplitude spectral density of frequency noise with total RMS frequency deviation of 3 mHz and a minimum value of 20 μHz/Hz½ over 250 seconds with a measurement bandwidth of 128 Hz, corresponding to an Allan deviation of 10⁻¹⁹ at 20 seconds.
Generation of squeezed state at telecommunication wavelength has been recently a very interesting issue due to the lowest optical power attenuation of light at a wavelength of 1550 nm in a standard telecommunication fiber. The low-frequency vacuum squeezed state at 1550 nm in combination with fiber based interferometer offers the possibility to implement quantum precision measurement beyond standard quantum limit (SQL). In this paper, we experimentally realize a quantum-enhanced fiber Mach-Zehnder interferometer (FMZI) for measuring the low-frequency phase modulation signal by using low-frequency vacuum squeezing at 1550 nm. Firstly, the low-frequency vacuum squeezed state at the telecommunication wavelength of 1550 nm is generated by using a degenerate optical parametric oscillator (DOPO). The DOPO is a semi-monolithic construction based on a type I periodically poled KTiOPO4 (PPKTP) crystal and a concave mirror. The pump threshold of DOPO is 270 mW. When the pump power is 120 mW that is below the pump threshold of DOPO and the temperature of PPKTP is controlled at 34.8℃, a vacuum squeezing of 3 dB is generated at an analysis frequency range from 10 kHz to 500 kHz. The quadrature phase vacuum squeezing is obtained by locking the squeezed quadrature angle through using a coherent control scheme, in which two acousto-optic modulators are used to shift the frequency and produce the auxiliary beam acting as a coherent control field. Based on the constructed FMZI, a quantum-enhanced FMZI is realized by injecting the generated low-frequency vacuum squeezed state at 1550 nm into the vacuum channel of FMZI. The relative phase between two injected light fields is locked at π by using the Pound-Drever-Hall (PDH) locking technology, and the relative phase between light fields of its arms in FMZI is also locked at π/2 by using the PDH locking technology. When a phase modulation signal at the frequency of 500 kHz is loaded in the signal arm of FMZI, the noise power spectrum of the output from FMZI is measured by a balance homodyne detect system. A 2 dB quantum improvement beyond shot-noise-level at the frequency of 500 kHz is obtained experimentally by using the quantum-enhanced FMZI. The experimental results demonstrate a potential application in quantum precision measurement beyond the SQL based on fiber sensor technique.