The spatial degree of freedom (DOF) has significant applications in the emerging high-dimensional quantum information processing. Using a mode-maintaining few-mode fiber (FMF) and a dual-Sagnac-loop configuration, we generate polarization-entangled photon pairs and transform the entanglement into spatial DOF. We obtain Bell states encoded in the linearly polarized (LP) and orbital angular momentum (OAM) mode bases, respectively. The entangled photons from the FMF are in well-defined spatial modes, providing great advantages for the entanglement generation, manipulation, and transmission. The measured fidelities of the obtained entangled states are more than 97%. Our generation-and-transformation protocol can also be adopted on other integrated waveguide platforms to develop robust and miniaturized sources of spatial entanglement.

Observation of the universe demands telescopes with high resolution. In the optical band, traditional interference requires bringing interfering fields together, which limits the resolution due to the restricted length of baseline. Here we demonstrate the very long-baseline interferometer (VLBI) in optical band, where two interfering fields never met each other. In particular, we report the first quantum interference observation when the input of VLBI is single-photon state. Interference is recovered after measuring the amplitudes of photon fields and digitally processing the signals of quantum receivers. Moreover, we analyze interference in time and spectral domains for broadband thermal light input and show that the ultrahigh spectral resolution can improve the precision of radial velocity to 0.08 centimeters per second, which is 2 orders of magnitude better than that achievable at the current stage. Further, we apply the spectrally resolved interference in distinguishing two independent sources with angular resolutions beyond diffraction limit. Our investigations have a profound effect on the VLBI, quantum optics, and precision measurement.

Time-domain multiplexed continuous-variable quantum states provide a promising route toward large-scale quantum networks. Existing platforms are based on continuous-wave pumped optical parametric systems, where the durations of temporal modes are on the order of nanoseconds. Here we demonstrate the time-domain multiplexed squeezing localized in a train of ultrashort pulses by exploiting unbalanced SU(1,1) interferometer (USUI) with a mode-locked laser serving as pump. Using the pulse-resolved measurement, we reveal the correlation structure of the state is unique and fundamentally different from previous approaches. To reach the ideal intensity squeezing, in principle, both the gain of USUI and mode number M involved in joint measurement should tend to infinity, illustrating the feature of global multimode squeezing. We conduct proof-of-principle experiments, in which the temporal mode duration is down to 10 ps. We verify the intensity squeezing degree R_d depends on both the gain of USUI and M. The results show R_d improves with the increase of M for M<10 and R_d is lower than shot noise level by ∼0.9 dB for M>10 when the gain of USUI is fixed. Our investigations demonstrate the emission from high gain USUI is novel, which not only possesses the unique coherent feature but also enables the realization of ultra-large-scale quantum states.

Abstract The spatial degree of freedom has enabled a wide range of applications in quantum information processing (QIP), such as increasing the capacity of quantum channels and realizing novel QIP protocols. In such applications, generating and manipulating quantum states in different spatial modes is one of the primary tasks. In this paper, by utilizing a telecom-band panda-type mode-maintaining few-mode fiber (MM-FMF) with a Sagnac loop configuration, we generate polarization-entangled photon pairs in higher-order spatial modes, namely the LP 11 modes, via intermodal spontaneous four-wave mixing process. The polarization- and spatial-mode-maintaining capabilities of the MM-FMF, combined with the automatic phase-stabilization of the Sagnac loop, provide a robust architecture for entanglement generation. We verify the polarization entanglement by two-photon interference measurements, yielding raw visibilities exceeding 76%. The visibilities are mainly limited by spontaneous Raman scattering noise, which could be reduced by cooling the fiber. The photon pairs directly emerging from MM-FMF are in a well-defined spatial mode, which greatly facilitates further manipulation and transmission of the entanglement. Our work demonstrates that the MM-FMF is a robust platform for developing devices and protocols exploiting the spatial degree of freedom in QIP applications.

Quantum information technology is an emerging discipline that combines quantum mechanics with information science,promising revolutionary advancements in computing,communication,and precision measurement.Optical quantum systems,known for their low transmission loss and low-noise coupling at room temperature,are crucial components of quantum information technology.To effectively utilize the low noise and strong quantum correlation properties of optical quantum systems,phase-locked control technology is widely applied in quantum state generation,mode regulation,and quantum state detection.This article reviews the research progress in phase-locked control technology within the field of optical quantum information field,including the principles of the technology,its typical applications in optical quantum information systems,and implementation schemes under weak light intensity conditions,and prospects for future development directions.

We study intensity correlations between multiple-temporal modes in unbalanced nonlinear interferometers using photon counting techniques. Experimentally, each mode correlates with five neighboring modes, two of which are within quantum regime.

Utilizing the phase-matching conditions of inter-modal four-wave mixing in an elliptical-core few-mode fiber supporting three non-degenerate modes, we experimentally demonstrate schemes for generating orbital-angular-momentum (OAM)-entangled photon pairs with high mode purity and for achieving highly mode-selective frequency conversion of beams in OAM-compatible (LP11a, LP11b) mode basis. These techniques expand the toolbox for using OAM modes in both classical and quantum communications and information processing.

Twin beams are useful tools in quantum technologies. However, due to their asymmetry in photon number distribution, a scheme needs to be optimized in order to fully exploit their quantum advantage. Here, we demonstrate a detection scheme for bright pico-second twin beams generated from a fiber optical parametric amplifier, in which the electronic gain of the detecting process is optimized in real time by digital signal processing without accurately calibrating the detection channels. Twin beams of −7.5 dB intensity difference squeezing (IDS) (the highest in an optical fiber system) is generated at a brightness of about 0.1 W in peak power. Our result paves the way for applications of twin beams in time-varying channels with asymmetric loss or noise.

Hanbury-Brown and Twiss (HBT) effect is the foundation for stellar intensity interferometry. However, it is a phase insensitive two-photon interference effect. Here we extend the HBT interferometer by mixing intensity-matched reference fields with the input fields before intensity correlation measurement. With the freely available coherent state serving as the reference field, we experimentally demonstrate the phase sensitive two-photon interference effect when the input fields are thermal fields in either continuous wave or non-stationary pulsed wave and measure the complete complex second-order coherence function of the input fields without bringing them together from separate locations. Moreover, we discuss how to improve the signal level by using the more realistic continuous wave broadband anti-bunched light fields as the reference field. Our investigations pave the way for developing new technology of remote sensing and interferometric imaging with applications in long baseline high-resolution astronomy.

Multi-channel squeezed light sources are extremely desired in establishing quantum metrology systems. Using a single optical fiber parametric amplifier, we prepare a two-channel intensity difference squeezed light source. The directly measured squeezing degrees of the two channels are 3.18 +/- 0.18 dB and 2.85 +/- 0.18 dB, respectively. The squeezing degree can be further improved by cooling the fiber and reducing injected seeds power and the number of channels can be further increased by injecting more seeds with different wavelengths. The generated two channels of intensity difference squeezing lights can be used in different kinds of quantum measurement and different quantum information processes. Our study is helpful for energy saving and promotion of quantum technology.

Objective Infrared sensing and measurement are crucial for both basic scientific research and engineering applications. However, compared to visible light devices, infrared light sources and detectors still require further improvement in terms of efficiency, dark noise, and cost. Recently, the infrared quantum sensing technology based on nonlinear interferometers has provided a new approach to address these issues. In nonlinear interferometers, the beam splitters are replaced by parametric processes, in which pump photons scatter into correlated signal and idler photons. Since the signal and idler photons can be in different wavelengths (e.g., infrared and visible), placing a sample in the infrared arm and measuring the interference pattern in the visible arm can still reveal the infrared optical properties of the sample. In this case, there is no need for infrared light sources and detectors. Most reported nonlinear interferometers for infrared sensing and measurement employ nonlinear crystals as the nonlinear medium. In this paper, we employ a photonic crystal fiber as the nonlinear medium to build a Michelson-type nonlinear interferometer. The wavelength of the signal photons from the interferometer is in the near-infrared band, and the idler photons are located near the visible light band. By changing the wavelength of the pump light, the wavelengths of the signal and idler photons can be continuously tuned without other measures such as temperature control. The experimental results demonstrate the application potential of nonlinear interferometers based on photonic crystal fibers in infrared quantum sensing. Methods We use a 25 cm long photonic crystal fiber as the nonlinear medium. The zero-dispersion wavelength of the fiber is about 1062 nm. The calculated phase matching curves of the four-wave mixing process in our fiber are shown in Fig. 2(b). When the central wavelength of the pump light is in the range of 890-1060 nm, the wavelength of the signal light is in the infrared band of 1170-3466 nm, while the corresponding idler wavelength is in the visible light band of 510-969 nm. Moreover, the signal and idler wavelengths can be continuously tuned by changing the wavelength of the pump light. We construct a Michelson-type nonlinear interferometer as shown in Fig. 3. The pump source is a mode-locked fiber laser with a repetition rate of 49.2 MHz. First, the pump light output is fed into a 10 cm long single-mode fiber, where it is broadened to approximately 40 nm by self-phase modulation. Then, by using a grating-based tunable bandpass filter (F), the central wavelength of the pump light is continuously tunable between 1015 nm and 1055 nm with a bandwidth of 0.9 nm. The pump light first enters the photonic crystal fiber in the forward direction, generating the signal and idler photon pairs through four-wave mixing. A long pass filter (LPF) then splits the forward output light field into two arms. The pump and idler light fields are reflected by the LPF, while the signal light field is transmitted. The signal light returns along its original path after passing through a mirror mounted on a piezoelectric transducer (PZT) stage. The pump and idler light fields, after traveling different paths, are also reflected and return along their original paths. The reflected light from the two arms rejoins at the LPF and enters the photonic crystal fiber in the opposite direction. At this time, the three light fields undergo a phase-sensitive four-wave mixing process, and nonlinear interference occurs in the signal and idler light fields. We place a short pass filter (SPF) at the reverse-output (i.e., input) of the photonic crystal fiber. The reverse-output pump and signal light fields are reflected by the SPF, while the idler light field is transmitted. The separated idler photons are detected by a fiber-coupled single-photon detector (SPD). Results and Discussions The experimental results are shown in Fig. 4. When the pump wavelengths are 1049.8 nm, 1035.9 nm, and 1029.4 nm, the corresponding wavelengths of the idler (signal) photons are 858.8 nm (1350 nm), 783.0 nm (1530 nm), and 756.6 nm (1610 nm), respectively. For each case, we apply a triangular high-voltage signal with a frequency of 0.15 Hz and a voltage amplitude of approximately 40 V to the PZT to scan the phase of the signal light field. The data points in Figs. 4(a1) - (a3) represent the measured idler photon count rates under different pump wavelength conditions. The sampling time for each data point is 0.012 s, and only the detector dark counts are subtracted during data processing. It is seen that although the phase shift is introduced in the signal light field, interference fringes are also observed in the idler light field. We fit the data using a sine function, and the visibility of the fringe is 19.2 degrees o, 15.4 degrees o, and 14.2 degrees o, respectively. To verify that the interference fringes in the idler light field originate from a nonlinear interference process, we block one arm of the signal light, retaining only the pump and idler light fields and returning them to the photonic crystal fiber. The idler photon count rate is measured under the same conditions, and the results are shown in the data points in the Figs. 4 (b1) -(b3). The experimentally measured interference visibility is 14 degrees o -19 degrees o, decreasing with increasing signal light wavelength. The factors limiting the interference contrast in this device primarily include two factors: transmission loss between the two-stage nonlinear media and the walk-off effect between different light fields introduced by fiber dispersion. Therefore, we can point out two approaches to improving interference: reducing losses in fiber coupling and optical devices, and minimizing the fiber length while maintaining a certain photon generation rate. Conclusions We demonstrate a wavelength-tunable nonlinear interferometer in the infrared band based on a photonic crystal fiber. Theoretical calculations show that when the pump wavelength is between 890 nm and 1060 nm, the signal wavelength satisfying the four-wave mixing phase matching condition is in the infrared band of 1170 nm to 3466 nm, while the idler wavelength is in the near visible light band of 510 nm to 969 nm. We experimentally characterize our nonlinear interferometer when the wavelengths of the signal photons are 1350 nm, 1530 nm, and 1610 nm, respectively, and obtain interference visibility of up to 19.2%. The wavelengths of the obtained signal photons are consistent with the theoretical calculations, confirming the accuracy of the theoretical results. The results of this paper reveal the potential of photonic crystal fibers for developing integrated infrared wavelength-tunable nonlinear interferometers, and pave the way for further applications in infrared quantum sensing technology.

We report a single-photon interference experiment where the interfering paths are in orthogonal polarization and temporal modes. This is achieved through amplitude measurement by homodyne detection so that interference occurs in photo-current after measurement.

Polarization entangled signal and idler photons in the higher-order LP11a and LP11b modes, respectively, are generated via inter-spatial-mode spontaneous four-wave mixing in a 17-m-long mode maintaining few-mode fiber with a panda type elliptical core. © 2024 The Author(s)

We demonstrate an all-fiber photon pair source based on a dual-layer high index doped silica glass (HDSG) spiral waveguide, which consists of two layers of HDSG and one silicon oxide layer in between. Due to this dual-layer structure, the waveguide has a flattened and near-zero dispersion at the 1550 nm telecom-band, and thus the bandwidth of the photon pairs generated via spontaneous four-wave mixing (SFWM) can be more than 30 nm. We pump the waveguide by using a pulse train with central wavelength of 1550.92 nm, and collect the signal and idler correlated photons at 1554.13 nm and 1547.72 nm, respectively. We obtain a coincidence to accidental coincidence ratio (CAR) of similar to 65 when the photon pair production rate is similar to 3x10(-5) pairs/pulse.

Femtosecond fiber lasers are widely utilized across various fields and also serve as an ideal platform for studying soliton dynamics. Bound-state solitons, as a significant soliton dynamic phenomenon, attract widespread attention and research interest because of their potential applications in high-speed optical communication, all-optical information storage, quantum computing, optical switching, and high-resolution spectroscopy. We investigate the effects of pump power variations on the formation of mode-locked solitons and bound-state solitons in a femtosecond fiber laser with a Cr2S3 saturable absorber (SA) through numerical simulations while observing the transition, formation, and break-up process of bound soliton pulses. By optimizing the cavity structure and adjusting the net dispersion, the mode-locked soliton is obtained based on this SA. This is the narrowest solitons produced by this SA to date, exhibiting the smallest time-bandwidth product. Moreover, stable double-bound solitons and unique (2 + 1) triple-bound solitons are successfully obtained. The diverse bound-state solitons not only demonstrate the excellent nonlinear absorption properties of Cr2S3 as a saturable absorber but also expand the scope of applications for Cr2S3 saturable absorbers in fiber lasers.

Interference effects are usually observed by intensity measurement. Path indistinguishability by the quantum complementarity principle requires projection of the interfering fields into a common indistinguishable mode before detection. On the other hand, the essence of wave interference is the addition of amplitudes of the interfering fields. Therefore, if amplitudes can be directly measured and added, interference can occur even though the interfering fields are in well-distinguishable modes. Here, we make a comprehensive study in both theory and experiment of a technique by homodyne measurement of field amplitudes to reveal interference. This works for both classical and quantum fields even though there exists distinguishability in the interfering paths of light. This directly challenges the complementarity principle. We present a resolution of this issue from the viewpoint of measurement that emphasizes either particle or wave. This technique is particularly useful for recovering interference in unbalanced interferometers with path imbalance beyond a coherence length of the input field and can be applied to remote sensing to extend the applicable range. Since the amplitude-based interference phenomena studied here are fundamentally different from the traditional intensity-based interference phenomena, our approach leads to a new paradigm to study coherence between optical fields.

We demonstrate an adaptive electrical gain optimized detection scheme for pulsed twin beams in optical fiber. Intensity difference squeezing level of -7.5 dB has been measured without the need of accurately calibrating channel losses.

Quantum frequency conversion is one of the important tools for quantum information processing. So far, the frequency conversion of continuous variable quantum state in radio frequency range has not been demonstrated yet. Here, we experimentally demonstrate the optical frequency fine tuning of a squeezed vacuum state by using an acousto-optic modulator based bi-frequency interferometer. The systematic efficiency of the frequency tuning device is 91%, which is only confined by the optical transmission efficiency of the acousto-optic modulators. The amount of frequency tuning is 80 MHz, which is orders of magnitude larger than the line-width of the laser used to generate the squeezed state and can, in principle, be further extended. A squeezed vacuum state with -3.47 ± 0.02 dB squeezing level is sent to the frequency tuning device, and a squeezing level of -1.98 ± 0.02 dB is directly measured by a fiber coupled homodyne detector after the frequency tuning. Our scheme can also be applied to a variety of other quantum optical states as well and will serve as a handy tool for quantum networks.

We demonstrate the generation of correlated photon pairs by using a hybrid integrated quantum photonic platform, where the dual-layer platform consists of a high-index doped silica glass (HDSG) layer to accommodate low-loss linear components and an SiN-based layer to accommodate the photon source. Leveraging the low-loss fiber coupling to the HDSG waveguide and the high nonlinearity of the SiN waveguide, we experimentally realize integrated source of photon pairs with high heralding efficiency. The directly measured photon pair rate is up to 87 KHz (corresponding to 1.74 × 10−3 pairs per pulse) when the coincidence-to-accidental ratio is greater than 10. The raw heralding efficiency can reach 18%. If the filtering loss is excluded, the heralding efficiency can further reach 29%.

Characterizing the temporal-spectral profile of single photons is essential for quantum information protocol utilizing temporal mode for encoding. Based on the phase retrieval algorithm, we present a method to reconstruct the phase spectrum difference between two wave packets from their Hong-Ou-Mandel dip, and intensity spectra. Our confirmatory experiment with weak coherent wave packets demonstrated the accuracy of the reconstructed phase spectrum difference to within plus or minus 0.1 rad. This method is generalizable to the measurement of unknown single-photon wave packets with the aid of a reference wave packet, requiring only the collection of one-dimensional data, which simplifies and expedites the process.