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Optica Quantum

Fiber distribution of phase-stabilized GHz-bandwidth squeezed vacuum states of light between two buildings

Sophie Verclas, C. Darsow-Fromm, Benedict Tohermes, R. Schnabel

Published Nov 21, 2025• 1 citation
Publisher

Abstract

One-sided device-independent Quantum Key Distribution (QKD) targets a mathematically proven secure communication between two parties without the need to trust the devices on one side. An appealing kind of high-speed implementation uses GHz-bandwidth two-mode squeezed states, distributed via an optical fiber link. The remote measurements, however, require the precise control of the phase with respect to an optical homodyne local oscillator. Here, we report what is believed to be the first transmission of single-mode squeezed vacuum states with GHz bandwidth over a 1 km optical fiber into a neighboring building combined with phase-stabilized measurements. We directly measure a squeeze factor of up to 4 dB. Our result is an important step towards high-speed QKD with enhanced security level for km-scale areas, e.g., government districts.

DOI
10.1364/opticaq.567418

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One-sided device-independent Quantum Key Distribution (QKD) targets a mathematically proven secure communication between two parties without the need to trust the devices on one side. An appealing kind of high-speed implementation uses GHz-bandwidth two-mode squeezed states, distributed via an optical fiber link. The remote measurements, however, require the precise control of the phase with respect to an optical homodyne local oscillator. Here, we report what is believed to be the first transmission of single-mode squeezed vacuum states with GHz bandwidth over a 1 km optical fiber into a neighboring building combined with phase-stabilized measurements. We directly measure a squeeze factor of up to 4 dB. Our result is an important step towards high-speed QKD with enhanced security level for km-scale areas, e.g., government districts.
Quantum Key Distribution (QKD) is a technology for secure communication between two parties, using the principles of quantum mechanics. Our continuous-variable QKD experiment implements a fiber-based scheme connecting two laboratories in two separated buildings (building A and B). We set up an Gaussian continuous-variable entanglement source in building A, consisting of two squeeze lasers to generate two-mode squeezed states. They are shared between the two buildings via a 1 km optical fiber. In both buildings, the entangled states are measured with self-designed and self-built balanced homodyne detectors. Due to the entanglement, the results are quantum random (truly random) but also correlated and can be used to generate a secret key. Attacks on the channel as well as on devices in building B reduce the entanglement strength and can thus be quantified. A major challenge in this setup is the phase stabilization and measurement synchronization between the two buildings. The fibers pick up extra noise, such as seismic and acoustic and experience temperature drifts. In this work, I will introduce the experiment, discuss the problem of phase noise and our approach to a control scheme. The first results with squeezed (not entangled) states show promising results for a phase lock of entangled states.
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 Squeezed vacuum states of light with bandwidths in the gigahertz range are required for ultrafast quantum sensors, for high-speed continuous-variable (CV) one-sided device independent QKD and for CV optical quantum computers. Here we report on the detailed investigation of the current performance limits of the established method for the generation and direct measurement of squeezed light with GHz bandwidth. We present squeeze factors of diffraction limited laser beams from monolithic periodically poled KTP resonators measured with two laboratory-built balanced homodyne detectors (BHDs) with gigahertz bandwidth. We realize two complete systems without selection of optical or electronic hardware components to test the reproducibility without rejects. As expected, the systems show clear spectral differences. However, both achieve directly measured squeeze factors in the order of 3 dB over a GHz bandwidth, which is achieved here for the first time. Since the speed of CV quantum protocols scales proportionally to the bandwidth used, our layout for BHDs is suitable for increasing the speed of all these protocols.
Squeezed vacuum states of light with bandwidths in the gigahertz range are required for ultrafast quantum sensors, for high-bandwidth QKD and for optical quantum computers. Here we present squeeze factors of monolithic periodically poled KTP (ppKTP) resonators measured with two laboratory-built balanced homodyne detectors with gigahertz bandwidth. We realise two complete systems without selection of optical or electronic hardware components to test the reproducibility without rejects. As expected, the systems show clear spectral differences. However, both achieve directly measured squeeze factors in the order of 3 dB over a GHz bandwidth, which is achieved here for the first time. Our direct measurement of quantum correlation is suitable for increasing the key rate of one-sided, device-independent QKD.
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.
We report the generation of squeezed vacuum states of light at 1550 nm with a broadband quantum noise reduction of up to 4.8 dB ranging from 5 MHz to 1.2 GHz sideband frequency. We used a custom-designed 2.6 mm long biconvex periodically-poled potassium titanyl phosphate (PPKTP) crystal. It featured reflectively coated end surfaces, 2.26 GHz of linewidth and generated the squeezing via optical parametric amplification. Two homodyne detectors with different quantum efficiencies and bandwidths were used to characterize the non-classical noise suppression. We measured squeezing values of up to 4.8 dB from 5 to 100 MHz and up to 3 dB from 100 MHz to 1.2 GHz. The squeezed vacuum measurements were limited by detection loss. We propose an improved detection scheme to measure up to 10 dB squeezing over 1 GHz. Our results of GHz bandwidth squeezed light generation provide new prospects for high-speed quantum key distribution.