ObjectiveHigh performance balanced homodyne detectors with high bandwidth, high quantum efficiency, reasonable gain, and high common-mode rejection ratio are critical in the fields of high-speed quantum communication, broadband squeezed light and entangled light detection, and broadband quantum precision measurement. However, it is challenging to design and develop GHz bandwidth balanced homodyne detectors with overall high performance. For example, the actually achieved bandwidth and gain are usually limited by the distributed capacitance of the circuit board. Furthermore, high common-mode rejection ratio, linear response and low noise performance are also technically challenging due to fabrication and soldering processes of the printed circuit board (PCB).MethodsIn comparison with other high bandwidth balanced homodyne detectors developed in previous works, we have adopted a design that separates alternating current (AC) and direct current (DC). The DC output helps us monitor the optical power and ensures the balance of the two photodiodes, which is crucial for balanced homodyne detection in quantum optical experiments. Our circuit board design involves placing two photodiodes on opposite sides of the PCB. This makes it easier to ensure the circuit lengths of the two photodiodes are exactly the same and reduce the distributed capacitance of the circuit board. This is important for enhancing the detector bandwidth and improving the common-mode rejection ratio. Additionally, this design is more convenient for quantum optical experiment operations.Results and DiscussionsWe used two photodiodes with 95% quantum efficiency, low capacitance and dark current for our balanced homodyne detector. The DC gain of the balanced homodyne detector is 1003.6 V/W, which is at a reasonable level for optical power monitor. We also took the noise power spectra of the balanced homodyne detector at different optical powers. We demonstrate the detector response is linear at five different frequencies. An effective bandwidth of 1 GHz and a maximum signal-to-noise ratio of 12 dB were realized at 1550 nm. The common-mode rejection ratio of the homodyne detector was tested by modulating the input beam with an electro-optical modulator. By placing two photodiodes symmetrically on the front and back sides of the double-sided PCB board, a common-mode rejection ratio of 60.65 dB at 20 MHz was achieved. Subsequently, the balanced homodyne detector was used to measure 1550 nm quadrature amplitude squeezed light. Up to -2.5 dB squeezing was observed over a bandwidth range from 3.5 MHz to 71.5 MHz.ConclusionsIn this paper, we designed and developed a balanced homodyne detector with GHz bandwidth, high quantum efficiency, reasonable gain, and high common-mode rejection ratio. The results show that the balanced homodyne detector with high bandwidth and high common-mode rejection ratio are useful in broadband squeezed state measurement.