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Photonics Research

Quantum description and measurement for single photon modulation

Jianyong Hu, Yan Liu, Lingling Liu, Bo Hao Yu, Guofeng Zhang, Liantuan Xiao, Suotang Jia

Published Jan 5, 2015• 16 citations
Publisher

Abstract

Single photon modulation has been proposed to overcome the defects of the low signal-to-noise ratio (SNR) and slow process rate of photon counting. In this paper, we present the quantum theory of single photon modulation, and then experimentally investigate the modulation spectroscopy both in the time domain and frequency domain. It is found that the SNR reached 150 in approximately the MHz modulation bandwidth.

DOI
10.1364/prj.3.000024

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Single photon modulation has been proposed to overcome the defects of the low signal-to-noise ratio (SNR) and slow process rate of photon counting. In this paper, we present the quantum theory of single photon modulation, and then experimentally investigate the modulation spectroscopy both in the time domain and frequency domain. It is found that the SNR reached 150 in approximately the MHz modulation bandwidth.
Single-photon detection enables sensitive measurements at ultra-low photon fluxes, supporting long-range sensing and imaging tasks. Photon-counting schemes reconstruct spectra by coherently accumulating interferograms, but phase fluctuations between interfering beams pose challenges for distributed spectroscopy. Here, we introduce a photon-pairing scheme that exploits second-order correlations rather than relying on first-order interference, thereby removing the need for phase alignment of interferograms while enabling long integration times. Our approach enhances the signal-to-noise ratio (SNR) of reconstructed spectra, with SNR increasing linearly with photon count rate, in contrast to the square-root dependence of photon-counting techniques. We accurately reconstruct intensity spectra at an average receiving power of only 5.5 × 10−15 W per comb line over a 400 km fiber link in a distributed configuration, achieving an SNR of over 23.5 dB and demonstrating strong resilience to fiber-induced fluctuations. Our photon-pairing scheme holds strong potential for long-haul dual-comb spectroscopy and opens new avenues for frequency metrology, remote sensing, and quantum networking. Photon-pairing spectroscopy overcomes phase fluctuations to enable sensitive spectral measurements over 400 km of fibre. The study achieves spectral reconstruction at an ultra-low power of 5.5 × 10-15 W per comb line, advancing long-range sensing.
The key security of quantum key distribution (QKD) is guaranteed by the basic principle of quantum mechanics, which makes it possible to combine information theory security communication with one-time encryption. The key is usually encoded on the polarization dimension or phase dimension of a single-photon. It is considered that the birefringence effect of single-mode fiber leads to a random variation of polarization state, which would induce the bit error rate. So it is of great significance to keep the single-photon linear polarization state stable for both polarization encoding QKD system and phase encoding QKD system. By using the single-photon polarization modulation technology, the single-photon linear polarization state periodically varies with the external modulation signal. The flicker noise is suppressed effectively, and the signal-to-noise ratio (SNR) of single-photon counting is increased as indicated by the phase-sensitive detection with a lock-in amplifier (LIA). The error signal is generated by demodulating the modulated single photons and it is used to lock an arbitrary 1550 nm single-photon linear polarization state to the optical axis of in-line polarizer (ILP). The modulation frequency reaches up to 5 kHz, which can eliminate the influence of low frequency flicker noise. The LIA demodulates the single-photon pulses by using 78.1 Hz filter bandwidth, with a time constant of 1 ms and a filter slope of 24 dB. The SNR of error signal is 20. The zero-crossing point of error signal represents the single photon’s linear polarization state aligned to the optical axis of ILP. The linear slope around the zero-crossing point for the polarization state angle versus the error signal amplitude is 1.267 rad/V. When the negative feedback loop does not work, the polarization drift of single-photon pulses is 0.082 rad due to the random environmental noise. However, by using the single-photon polarization modulation technology, the polarization drift of stable single-photon pulses is limited to 0.0011 rad within 2000 s through the precise control with a polarization rotator, and the corresponding Allan deviation reaches the minimal value of 6.7×10–5 at an integration time of 128 ms. The advantages for the single-photon polarization modulation technology are as follows: i) the linear polarization state drift is compensated in real-time at the single-photon level; ii) single frequency polarization modulation can be extended to multiple frequency polarization modulation in order to achieve simultaneous locking of multiple linear polarization states of single-photon pulses; iii) these 1550 nm single-photon pulses with the 0.0011 rad linear polarization state stability can be directly used as the single-photon source in either polarization encoding or phase encoding QKD system.
Employing grating extended-cavity diode lasers as the cooling/trapping and repumping lasers for preparing and manipulating single atoms, we have implemented a large-magnetic-gradient caesium magneto-optical trap (MOT). To detect and evaluate single caesium atoms trapped in MOT, laser-induced-fluorescence (LIF) photons of trapped atoms driven by MOT lasers are collected and counted by an avalanched photodiode worked in photon-counting mode. The dependences of LIF photon-counting signals of single atoms on a cooling laser's intensity, frequency detuning and frequency fluctuation are analysed and investigated. Remarkable improvement of the signal-to-noise ratio of LIF photon-counting signals is achieved by optimizing the cooling laser's intensity and frequency detuning and using the modulation-free polarization spectroscopic technique with feedback to both the slow channel (piezoelectric transducer channel with typical bandwidth of ∼2 kHz in the grating extended cavity) and the fast channel (current modulation channel with typical bandwidth of ∼200 kHz in the current driver).
In this paper, optical communication at the single-photon level is experimentally demonstrated by using a multi-channel frequency coding scheme in which the information is decoded by using the single-photons modulation spectrum. By using the modulation spectrum, the coding scheme could work normally in a channel with high loss and noise. Besides, multiple modulation frequency components could be used in a wide bandwidth regardless of frequency aliasing; therefore, the multi-channel frequency coding scheme makes it possible for high-capacity single-photons communication.