Transferring of a real quantum state in a long-distance channel is an important task in the development of quantum information networks. For greatly suppressing the relative phase fluctuations between the signal beam and the corresponding local oscillator beam, the usual method is to transfer them with time-division and polarization-division multiplexing through the same fiber. But the nonclassical states of light are very sensitive to the channel loss and extra noise, this multiplexing method must bring the extra loss to the quantum state, which may result in the vanishing of its quantum property. Here, we propose and realize a suitable time multiplexing method for the transferring and measurement of nonclassical states. Only the local oscillator beam is chopped into a sequence of light pulses and transmitted through fiber with continuous orthogonal-polarized signal beam. Finally, when the local oscillator pulses are properly time delayed compared to the signal beam, the quantum state can be measured in the time sequences without the influence of extra noise in the fiber. Our work provides a feasible scheme to transfer a quantum state in relative long distance and construct a practical quantum information network in metropolitan region.
Transferring of a real quantum state in a long-distance channel is an important task in the development of quantum information networks. For greatly suppressing the relative phase fluctuations between the signal beam and the corresponding local oscillator beam, the usual method is to transfer them with time-division and polarization-division multiplexing through the same fiber. But the nonclassical states of light are very sensitive to the channel loss and extra noise, this multiplexing method must bring the extra loss to the quantum state, which may result in the vanishing of its quantum property. Here, we propose and realize a suitable time multiplexing method for the transferring and measurement of nonclassical states. Only the local oscillator beam is chopped into a sequence of light pulses and transmitted through fiber with continuous orthogonal-polarized signal beam. Finally, when the local oscillator pulses are properly time delayed compared to the signal beam, the quantum state can be measured in the time sequences without the influence of extra noise in the fiber. Our work provides a feasible scheme to transfer a quantum state in relative long distance and construct a practical quantum information network in metropolitan region.
Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a $\ensuremath{-}5.9$-dB squeezed state at 1.3 $\text{\ensuremath{\mu}}\mathrm{m}$ through 3 km of optical fiber. We obtain a $\ensuremath{-}3.2$-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.
Transmission and accurate detection of squeezed states of light will play a central role in the future quantum internet, where quantum cryptography, communication and computation will coexist in an organic system. Their coherent detection has recently proven to be crucial for photonics-based quantum computing [1] , and to provide improvements in continuous variables quantum key distribution [2] [6] and quantum sensing [3] . Often when measuring a quantum state through coherent detection, the local oscillator field is transmitted along with the signal; however, in realistic long-distance optical fiber transmissions, where the needed local oscillator optical power increases exponentially with the distance, the system’s performance deteriorates due to nonlinear Brillouin scattering, and wavelength multiplexing schemes may be needed to avoid the issue. Moreover, transmitting a local oscillator opens security loopholes in quantum key distribution systems, where an eavesdropper can defy the security assessment from the trusted parties by gaining control over the local oscillator [4] . It is therefore very important to design coherent detection schemes that make use of independent local oscillators. We demonstrate the transmission single-mode squeezed light at 1550 nm wavelength through single-mode optical fibers (10 m and 10 km lengths) and its homodyne detection, using an independent local oscillator and a real-time phase-control system. Fig. 1a depicts the implemented physical setup: 1550 nm squeezed light is generated through spontaneous parametric downconversion [5] , couples into a single-mode optical fiber and propagates towards the homodyne detector for quadrature measurement. Real-time phase control consists in detecting the interference between a 40 MHz single-sideband pilot mode -phase-locked to the downconverted pump- and the local oscillator, estimating their instantaneous phase difference and implementing a fast and broad FPGA-based phase control on the local oscillator. As shown in Fig. 1b and 1c, with the presented scheme it was possible to steadily measure up to 3.6 dB of squeezing after a 10 m optical fiber transmission, and up to 1.5 dB of squeezing after a 10 km fiber transmission, with similar performance in terms of phase error and control stability. The presented system is simple, robust and cost-effective, and it will represent a standard class of techniques to detect not only squeezed states of light, but any arbitrary quantum state of light, including Schrödinger cat states, highly useful in quantum computation and error correction.
Abstract We demonstrate the generation, 40 km fiber transmission, and homodyne detection of single-mode squeezed states of light at 1550 nm using real-time phase control of a locally generated local oscillator (LO), often called a ‘real LO’ or ‘local LO’. The system was able to stably measure up to around 3.7 dB of noise suppression with a phase noise uncertainty of around 2.5°, using only standard telecom-compatible components and a field-programmable gate array. The compactness, low degree of complexity and efficacy of the implemented scheme makes it a relevant candidate for long distance quantum communication in future photonic quantum networks.
Continuous variable (CV) quantum entanglement is an essential resource for quantum computation and communication protocols. The use of CV quantum entanglement at a telecommunication wavelength of 1.5m in combination with existing fiber telecommunication networks offers the possibility to implement long-distance quantum communication protocols like quantum key distribution (QKD) and applications such as quantum repeaters, quantum teleportation in the future. In spite of the fact that the optical power attenuation of light in a standard telecommunication fiber is lowest at a wavelength of 1.5m, the entangled states will interact with fiber channels and the disentanglement will occur. It is one of the important factors restricting the development of long distance quantum information. In this paper, CV entangled state at 1.5m telecommunication band is obtained by using a type-II periodically poled KTP (PPKTP) crystal inside a nondegenerate optical parametric amplifier (NOPA). A wedged PPKTP is used for implementing frequency-down-conversion of the pump field to generate the optically entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. By controlling the temperature and the length of the PPKTP crystal, a triply resonant optical parametric oscillator with a threshold of 80 mW is realized. Einstein-PodolskyRosen (EPR)-entangled beams with quantum correlation of 8.3 dB for both the amplitude and phase quadratures are experimentally generated by using a single NOPA at a pump power of 40 mW and an injected signal power of 10 mW when the relative phase between the pump and injected signal is locked to . The generated entangled state is coupled into a single-mode optical fiber, and the transmission characteristics of the generated EPR entangled beams through standard single-mode fibers are investigated experimentally and theoretically. A fiber polarization controller is used to compensate for the polarization state variation induced by random fluctuations of birefringence of the single mode fiber when the light propagates along the fiber, and to keep the polarization of light linear at the fiber output. A 0.21 dB quantum entanglement could still be observed for the EPR-entangled beams transmitted through a 50-km-long single-mode fiber. The theoretical prediction considering the excess noise in fiber channel is in good agreement with the experimental result. The generated CV quantum entanglement is highly suitable for the required experiments, such as CV measurement-device-independence QKD based on standard fibers, owing to the fact that the tolerance of the excess noise in the quantum channel can be enhanced significantly with respect to a coherent state if EPR-entangled beams are used.
Squeezed light plays a vital role in quantum information processing. By nature, it is highly sensitive, which presents significant practical challenges, particularly in remote detection, traditionally requiring complex systems such as active phase locking, clock synchronization, and polarization control. Here, we propose and demonstrate an asynchronous detection method for squeezed light that eliminates the need for these complex systems. By employing radio-frequency heterodyne detection with a locally generated local oscillator and applying a series of digital unitary transformations, we successfully reconstruct squeezed states of light. We validate the feasibility of our approach in two key applications: the distribution of squeezed light over a 10 km fiber channel, and secure quantum key distribution between two labs connected via deployed fiber based on continuous variables using squeezed vacuum states without active modulation. This demonstrates a practical digital reconstruction method for squeezed light, opening new avenues for practical distributed quantum sensing networks and high-performance and long-distance quantum communication using squeezed states and standard telecom technology.
Continuous-variable quantum key distribution (CVQKD) with a real local oscillator (LO) has been extensively studied recently due to its security and simplicity. In this paper, we propose a novel implementation of a high-key-rate CVQKD with a real LO. Particularly, with the help of the simultaneously generated reference pulse, the phase drift of the signal is tracked in real time and then compensated. By utilizing the time and polarization multiplexing techniques to isolate the reference pulse and controlling the intensity of it, not only the contamination from it is suppressed, but also a high accuracy of the phase compensation can be guaranteed. Besides, we employ homodyne detection on the signal to ensure the high quantum efficiency and heterodyne detection on the reference pulse to acquire the complete phase information of it. In order to suppress the excess noise, a theoretical noise model for our scheme is established. According to this model, the impact of the modulation variance and the intensity of the reference pulse are both analysed theoretically and then optimized according to the experimental data. By measuring the excess noise in the 25km optical fiber transmission system, a 3.14Mbps key rate in the asymptotic regime proves to be achievable. This work verifies the feasibility of the high-key-rate CVQKD with a real LO within the metropolitan area.