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Detecting squeezed light

Svante Fridström, Axel von Malmborg

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

Squeezed light is a non-classical state of light in which the uncertainty in the electricfield strength at certain phases of oscillation is reduced below the vacuum noise level,at the cost of increased uncertainty at other phases. It has applications ranging fromgravitational wave detection to quantum communication, where the bandwidth of thesqueezed states directly limits protocol speed. Squeezed light is commonly measured us-ing balanced homodyne detection, in which the attainable squeezing level is restrictedby the photodetector’s quantum efficiency and dark noise. Currently, no commerciallyavailable photodetector combines high quantum efficiency with low dark noise over alarge bandwidth. This thesis presents the design and construction of a photodetectorintended to address this gap, with the aim of achieving high quantum efficiency and suf-ficient dark noise clearance over at least 1 GHz of bandwidth. The design was guidedby LTspice simulations and implemented on a two-layer printed circuit board. Twocommercially available reference detectors were characterised, illustrating the trade-offbetween high quantum efficiency and large bandwidth. Since the photodiodes for thenew detector had not arrived at the time of completion, its characterisation remains tobe carried out.

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Squeezed light is a non-classical state of light in which the uncertainty in the electricfield strength at certain phases of oscillation is reduced below the vacuum noise level,at the cost of increased uncertainty at other phases. It has applications ranging fromgravitational wave detection to quantum communication, where the bandwidth of thesqueezed states directly limits protocol speed. Squeezed light is commonly measured us-ing balanced homodyne detection, in which the attainable squeezing level is restrictedby the photodetector’s quantum efficiency and dark noise. Currently, no commerciallyavailable photodetector combines high quantum efficiency with low dark noise over alarge bandwidth. This thesis presents the design and construction of a photodetectorintended to address this gap, with the aim of achieving high quantum efficiency and suf-ficient dark noise clearance over at least 1 GHz of bandwidth. The design was guidedby LTspice simulations and implemented on a two-layer printed circuit board. Twocommercially available reference detectors were characterised, illustrating the trade-offbetween high quantum efficiency and large bandwidth. Since the photodiodes for thenew detector had not arrived at the time of completion, its characterisation remains tobe carried out.
Quantum receiver scalability and cost-effectiveness are crucial in the evolving quantum landscape. Application based on squeezed light (e.g. high-precision measurements and quantum communication), can profit especially from integrated and low-noise detectors. The presented die-level receiver is shown in figure 1 a) and combines a novel custom transimpedance amplifier (TIA) with commercial photodiodes (PDs) [1]. This offers a more scalable solution than bulk balanced homodyne detectors (BHDs) while being low-cost compared to fully customized receivers. This design targets a high bandwidth operation while maintaining low-noise properties for the detection of weak quantum signals. To characterize this receiver, we evaluate its performance through measurements of clearance, and common-mode rejection ratio (CMRR). Based on these parameters, a simulation for its performance in the squeezed light detection measurement is presented.
Squeezed light plays a crucial role in state-of-the-art quantum metrology and quantum information experiments. There is significant interest in utilizing squeezed states at high MHz and GHz frequencies. However, past efforts to build suitable photodetectors at these frequencies have yet to yield the required high quantum efficiency. Here, we present the development of a high-frequency balanced photodetector with near-unity quantum efficiency, realized with off-the-shelf components. The detector operates in balanced mode up to approximately 500 MHz, above which the differential frequency response limits its performance. To obtain high sensitivity above 500 MHz, the detector can be efficiently used in an unbalanced homodyne detection scheme. We employ our detector in this unbalanced mode to measure a squeezing comb up to 6.4 GHz, achieving a squeezing level of up to 10.7 dB. By sharing our experience, specifically in identifying the unequal frequency response as a limiting factor, we aim to enable and advance further developments in the field.
Squeezed light plays a crucial role in state-of-the-art quantum metrology and quantum information experiments. There is significant interest in utilizing squeezed states at high MHz and GHz frequencies. However, past efforts to build suitable photodetectors at these frequencies have yet to yield the required high quantum efficiency. Here, we present the development of a high-frequency balanced photodetector with near-unity quantum efficiency, realized with off-the-shelf components. The detector operates in balanced mode up to approximately 500 MHz, above which the differential frequency response limits its performance. To obtain high sensitivity above 500 MHz, the detector can be efficiently used in an unbalanced homodyne detection scheme. We employ our detector in this unbalanced mode to measure a squeezing comb up to 6.4 GHz, achieving a squeezing level of up to 10.7 dB. By sharing our experience, specifically in identifying the unequal frequency response as a limiting factor, we aim to enable and advance further developments in the field.
Squeezing-based quantum metrology has been extensively applied in the field of precision measurement. Performance characteristics of squeezed light are obtained by a balanced homodyne detector. Here, we report a large dynamic range balanced homodyne detector with low-noise and high-signal-to-noise-ratio (SNR) for broadband squeezed light measurement. We perform a comprehensive theoretical analysis of the SNR in a wide bandwidth and experimentally analyze the important printed circuit board (PCB) layout and electronic components. Consequently, in the 100 MHz frequency band, the SNR and common mode rejection ratio (CMRR) rise over 12.6 dB and 55 dB, respectively, and the electronic noise is suppressed below -90 dB. The directly observed squeezing level at 1550 nm with the specified detector is 11.8 dB at 2 MHz and 3.1 dB at 100 MHz.
A photodetector's electronic bandwidth is the frequency range over which beats between different frequencies in the input can be observed in the output. The output of a balanced homodyne detector, the prototypical device for detecting squeezed light, consists of beats between a signal to be measured and a local oscillator. Beats between the signal and quantum local-oscillator fluctuations lead to noise that degrades the observed squeezing. Through wide-band calculations, we show that this noise contains contributions from the entire signal bandwidth, even when this bandwidth is orders of magnitude larger than the detector's electronic bandwidth. This allows us to place an upper limit on the total power that can enter the detector's signal port (both signal and extraneous light) within the detector's optical bandwidth in order for a given level of squeezing to be observable.
Squeezed states of light enable quantum-enhanced measurements but are limited by optical loss, particularly at 2 um where photodiode efficiency is low. We report the first loss-tolerant, audio-band squeezed light detection at 1984 nm by using a phase-sensitive amplifier to amplify the squeezed vacuum prior to detection. This technique increases the effective detection efficiency from 74% to 95% and increases the observed squeezing from 4 dB to 8 dB, the highest level of squeezing observed at this wavelength. Additionally, the shot-to-dark-noise clearance increases, extending the effective measurement bandwidth toward lower frequencies. This approach is largely wavelength-independent, extending high-fidelity quantum measurements to future gravitational-wave detectors and related quantum technologies.