Liantuan Xiao

Experimental quantum secure direct communication with single photons

Quantum secure direct communication is an important mode of quantum communication in which secret messages are securely communicated directly over a quantum channel. Quantum secure direct communication is also a basic cryptographic primitive for constructing other quantum communication tasks, such as quantum authentication and quantum dialog. Here, we report the first experimental demonstration of quantum secure direct communication based on the DL04 protocol and equipped with single-photon frequency coding that explicitly demonstrated block transmission. In our experiment, we provided 16 different frequency channels, equivalent to a nibble of four-bit binary numbers for direct information transmission. The experiment firmly demonstrated the feasibility of quantum secure direct communication in the presence of noise and loss.

Direct measurement of the photon statistics of a triggered single photon source.

We studied intensity fluctuations of a single photon source relying on the pulsed excitation of the fluorescence of a single molecule at room temperature. We directly measured the Mandel parameter Q(T) over 4 orders of magnitude of observation time scale T by recording every photocount. On time scale of a few excitation periods, sub-Poissonian statistics is clearly observed and the probablility of two-photons events is 10 times smaller than Poissonian pulses. On longer times, blinking in the fluorescence, due to the molecular triplet state, produces an excess of noise.

Frequency stabilization of an external-cavity diode laser with a thin Cs vapour cell

We demonstrate a novel method of stabilizing an external-cavity diode laser frequency to an atomic transition. This technique employs a sub-Doppler spectrum of Cs atoms in a thin 150 µm vapour cell. We obtain the result of the frequency fluctuation to be less than 0.8 MHz using the third derivative signal, with the root of Allan variance of the error signals reaching a minimum of 5.9 × 10−11 for an averaging time of 200 s.

Absolute frequency stabilization of a diode laser to cesium atom-molecular hyperfine transitions via modulating molecules

We have demonstrated a robust method of directly stabilizing diode laser frequency to the cesium atom-molecular hyperfine transitions. The trap loss fluorescence spectroscopy was applied to yield the error signal based on modulating molecules with ultralow modulation frequency of 1.2Hz. The excursions over 300s of the frequency of the laser were bounded by 1.5MHz. The root of Allan variance of the error signals reached a minimum of 4.8×10−11 for an averaging time of 100s.

Beat frequency quartz-enhanced photoacoustic spectroscopy for fast and calibration-free continuous trace-gas monitoring

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a sensitive gas detection technique which requires frequent calibration and has a long response time. Here we report beat frequency (BF) QEPAS that can be used for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications. The resonance frequency and Q-factor of the quartz tuning fork (QTF) as well as the trace-gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the QTF is demodulated at its non-resonance frequency. Hence, BF-QEPAS avoids a calibration process and permits continuous monitoring of a targeted trace gas. Three semiconductor lasers were selected as the excitation source to verify the performance of the BF-QEPAS technique. The BF-QEPAS method is capable of measuring lower trace-gas concentration levels with shorter averaging times as compared to conventional PAS and QEPAS techniques and determines the electrical QTF parameters precisely.

Overtone resonance enhanced single-tube on-beam quartz enhanced photoacoustic spectrophone

A single-tube on-beam quartz enhanced photoacoustic spectroscopy (SO-QEPAS) spectrophone, which employs a custom-made quartz tuning fork (QTF) having a prong spacing of 700 μm and operating at the 1st overtone flexural mode, is reported. The design of QTF prong geometry allows the bare QTF to possess twice higher Q-factor values for the 1st overtone resonance mode falling at ∼17.7 kHz than in the fundamental resonance mode at ∼2.8 kHz, resulting in an 8 times higher QEPAS signal amplitude when operating in the 1st overtone resonance mode. Both the vertical position and length of the single-tube acoustic micro-resonator (AmR) were optimized to attain optimal spectrophone performance. Benefiting from the high overtone resonance frequency and the quasi 1st harmonic acoustic standing waves generated in the SO-QEPAS configuration, the AmR length is reduced to 14.5 mm. This allows the realization of compact spectrophone and facilitates the laser beam alignment through the QTF + AmR system. The signal enhancemen...

Signal-to-noise ratio improvement of photon counting using wavelength modulation spectroscopy

Photon counting has been widely used in weak light detection. However, the low signal-to-noise ratio (N, N is the mean counts) significantly limits the detection sensitivity. In this letter, we present a method that uses wavelength modulation spectroscopy to improve the signal-to-noise ratio of photon counting. Five times improvement of signal-to-noise ratio in 1Hz bandwidth at best has been obtained in 2f harmonic detection.

Impact of Humidity on Quartz-Enhanced Photoacoustic Spectroscopy Based CO Detection Using a Near-IR Telecommunication Diode Laser

A near-IR CO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is evaluated using humidified nitrogen samples. Relaxation processes in the CO-N2-H2O system are investigated. A simple kinetic model is used to predict the sensor performance at different gas pressures. The results show that CO has a ~3 and ~5 times slower relaxation time constant than CH4 and HCN, respectively, under dry conditions. However, with the presence of water, its relaxation time constant can be improved by three orders of magnitude. The experimentally determined normalized detection sensitivity for CO in humid gas is 1.556×10−8 W⋅cm−1/Hz1/2.

Electric field induced fluorescence hysteresis of single molecules in poly(methyl methacrylate)

Single molecule (SM) chips could serve as the fundamental devices in quantum information processing. In this context, a chip with the non-polar SMs of squaraine-derived rotaxanes embedded in a polar poly(methyl methacrylate) matrix was realized and the SM fluorescence hysteresis induced by the electric field was observed at room temperature. Here, we presented a model considering both of the electron transfer and space charge relaxation processes to explain the fluorescence hysteresis effect, and the model-based simulations agreed reasonably well with the experimental results.

Single molecules probe the polarization dynamics of poly (methyl methacrylate) in external electric field

We demonstrate the electric field (EF) induced polarization dynamics of poly (methyl methacrylate) (PMMA) by observing the fluorescence modulation of embedded non-polar single squaraine-derived rotaxane molecules. It is established that interaction between the molecular energy level and the potential valley formed by surrounding PMMA matrix can be detuned by the EF, which induces the changing of electron transfer rates between them effectively. The EF-induced response time of the fluorescence quenching or enhancement and the fluorescence recovery time reflect the diverse polarization and relaxation dynamics of PMMA.