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Optics Express

Generation of multi-mode squeezed vacuum using pulse pumped fiber optical parametric amplifiers

Nannan Liu, Yuhong Liu, Jiamin Li, Lei Yang, Xiaoying Li

Published Jan 27, 2016• 11 citations
Publisher

Abstract

Multimode squeezed states are essential resources in quantum information processing and quantum metrology with continuous variables. Here we present the experimental generation of squeezed vacuum via the degenerate four wave mixing realized by pumping a piece of dispersion shifted fiber with mode-locked ultrafast pulse trains. The noise fluctuation is lower than the shot noise limit by 1.1 ± 0.08 dB (1.95 ± 0.17 dB after correction for detection losses). The detailed investigation illustrates that the results can be further improved by suppressing Raman scattering and by reshaping the spectrum of the local oscillator to achieve the required mode-matching of the homodyne detection system. Our study is useful for developing a compact fiber source of multi-mode squeezed vacuum.

DOI
10.1364/oe.24.002125

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Multimode squeezed states are essential resources in quantum information processing and quantum metrology with continuous variables. Here we present the experimental generation of squeezed vacuum via the degenerate four wave mixing realized by pumping a piece of dispersion shifted fiber with mode-locked ultrafast pulse trains. The noise fluctuation is lower than the shot noise limit by 1.1 ± 0.08 dB (1.95 ± 0.17 dB after correction for detection losses). The detailed investigation illustrates that the results can be further improved by suppressing Raman scattering and by reshaping the spectrum of the local oscillator to achieve the required mode-matching of the homodyne detection system. Our study is useful for developing a compact fiber source of multi-mode squeezed vacuum.
Using a Sagnac fiber loop functions as a deterministic splitter of photon pairs produced by the frequency degenerate four wave mixing, we show that the background noise of the degenerate photon pairs is contributed by both Raman scattering and frequency non-degenerate four wave mixing. To improve the purity of photon pairs in the high gain regime, in addition to suppressing the noise photons by cooling the nonlinear fiber and by optimizing the detuning between the frequencies of the pump and photon pairs, the walk-off effect of the two pulsed pump fields should be mitigated by managing the dispersion of the fiber. Our investigation is not only the first step towards the generation of multi-mode squeezed vacuum in fiber optical parametric amplifiers pumped with pulsed lights, but also contributes to improving the purity of the fiber sources of degenerate photon pairs.
Nowadays, the realization of quantum computations and communications based on continuous variables has attracted a significant attention due to a substantial expansion of the system dimensionality. The main progress in this area is attributed to the implementation of multimode systems based on squeezed states of light. One of the simplest ways to generate such states relies upon their producing in a single-pass optical parametric amplifier (OPA) using ultrafast pumping. However, for homodyne detection of such multimode states, the profile of the local oscillator (LO) must perfectly match the profile of the measured mode. Usually, this is not the case; therefore a proper treatment of multimode squeezing is required. In this work, we study both theoretically and experimentally the multimode squeezed light generated in type-0 and type-II OPA. We characterize such sources and investigate the degree of squeezing in dependence on the LO spectral profile, employing a pulse shaping technique. The theoretical analysis is performed using the Schmidt-mode theory. This work might have a significant impact on the realization of multimode quantum protocols
Squeezed states of light enable optical measurements with sensitivity beyond the shot noise limit of detection with classical light. Optical fibers represent a major potential platform for quantum squeezed sensing due to the extremely low loss and perfect single-spatial-mode operation. In this work, we demonstrate two-mode intensity squeezing directly within a commercially available optical fiber, based on a four-wave mixing process. The setup operates in a quasi-continuous-wave mode using 15 ns long pulses and relies on standard laser diode sources, telecommunication electro-optic modulators, and simple photo-detectors. The process is seeded by a coherent signal input, generating bright squeezed output states. Absolute noise reduction by 4 dB below the classical shot-noise limit is detected directly with no post-processing, for output signal and idler pulses of 2.4 × 10 6 photons in average. The results are applicable to quantum interferometry over fiber using simple, readily available components.
Continuous-variable quantum networks constitute a valuable resource for a wide range of quantum communication and information protocols. While frequency- and fast time-multiplexing enable the implementation of scalable quantum architectures, a precise characterization of multimode squeezed-light sources is essential. In this work, we increase both the spectral resolution and the measured squeezing values. Both parameters are relevant for the improved generation of so-called cluster states, a key resource for measurement-based quantum computing. Our source is based on parametric down-conversion in a periodically poled KTP waveguide pumped by a pulsed laser. The source produces squeezed states over a specific set of optical spectra. Increased spectral resolution in spectrally resolved homodyne detection allowed us to enhance the squeezing per spectral mode, increasing it from 1 dB to more than 2.4 dB in the first mode when measuring with a homodyne detector with a bandwidth of a few MHz. Secondly, we implemented a pulse-by-pulse characterization of the resource using a fast homodyne detector with an electronic bandwidth on the order of the laser repetition rate (100 MHz). Using this approach, the fast measurements revealed squeezing values of up to 1.5 dB in the first few spectral modes. To fully access the multimode quantum resource and ultimately improve the amount of squeezing, a high quantum efficiency is required. To avoid limitations imposed by poor spatial mode matching between the quantum resource and the local oscillator in homodyne detection, we implemented a wavefront-shaping scheme to characterize and control both the intensity profile and phase of the local oscillator. In preliminary tests this approach increased the mode-matching between the reference beam and the quantum signal by more than 8% while simultaneously providing a detailed characterization of the beam shape and of the influence of waveguide transmission on the beam profile. To achieve a genuine advantage in quantum information processing over classical protocols, the generation of non-Gaussian states is necessary. We therefore present a complete scheme for the creation of non-Gaussian states in a multimode system via mode-selective single-photon addition and subtraction, implemented through nonlinear optical processes in second-order nonlinear materials. We introduce a simulation framework applied to our squeezing source, which can be adapted to other regimes as well. Based on this framework, we are able to predict specific waveguide configurations that enable full control over the generated quantum states while maintaining high purity, with each process exhibiting a Schmidt number close to K=1, corresponding to effective single-mode addressing. generated quantum states while maintaining high purity, with each process exhibiting a Schmidt number close to K=1, corresponding to effective single-mode addressing.
Multimode squeezed light is a key resource for quantum technologies, enabling applications in boson sampling, quantum networks, and optical quantum computation [1],[2]. Parametric down-conversion in synchronously pumped optical parametric oscillators (SPOPOs) offers a versatile platform for generating multimode squeezed states across the time-frequency domain. In these systems, supermodes manifest as independent quantum states in the temporal domain and as frequency combs in the spectral domain [3]. Precise control of these states is crucial for quantum applications and requires an experimental setup capable of fine-tuning multiple parameters to optimize squeezing properties. In this work, we present an experimental setup for generating and characterizing multimode vacuum squeezed states using a SPOPO and a balanced homodyne detection scheme. Two spatial light modulators (SLMs) placed on the pump and local oscillator (LO) beams provide full control over their spectral properties, including amplitude and phase shaping. This flexibility allows us to optimize the experimental conditions and investigate squeezing dependence on key parameters such as the LO spectral detuning and bandwidth. By introducing a programmableπ\piπ-phase shift to the LO via the SLM, we selectively probe the multimode structure, distinguishing between even and odd supermodes. Additionally, the SLM on the pump can introduce an arbitrary chirp, further enhancing control over the system. Our setup, schematized in Figure 1a, is based on a commercial mode-locked Yb-fiber laser operating at 93MHz, emitting pulses centered at 1035nm with a 10nm bandwidth. A portion of the radiation is frequency-doubled to 517.5nm via an LBO crystal to serve as the pump for the SPOPO, while the remaining part is used as the LO for homodyne detection after transmission through the SPOPO cavity, ensuring high beam quality. The SPOPO cavity is frequency-locked to the laser via a Pound-Drever-Hall technique, ensuring stable and optimized operation. Figure 1b presents representative measurements of squeezing levels as a function of the LO spectral bandwidth, showing how its symmetric or antisymmetric phase influences the detection of different supermodes and, consequently, the squeezing level depending on their structure. The measurements were performed using lithium niobate (LNB) nonlinear crystals of 3.0mm and 1.5mm lengths. Our setup allows for future investigations of dispersion effects on squeezing, thus far only theoretically predicted [4], by introducing chirped mirrors for dispersion compensation. Moreover, different nonlinear crystals, such as lithium niobate, beta barium borate (BBO), and lithium triborate (LBO) of various lengths, will be tested to explore the multimode properties of the squeezed states. These investigations will provide insights into the experimental conditions required to achieve single-mode squeezing, crucial for applications such as Gaussian Boson Sampling, with results to be presented at the conference.
Recently, continuous variables (CV) quantum computing, i. e. computing employing squeezed states, has attracted significant attention, as it enables to increase the dimensionality of the system. In CV protocols, homodyne detection plays a key role in the realization of the required gates and operations. Multimode squeezed light generated via parametric down-conversion (PDC) benefits from the broadband bandwidth, which encompasses multiple frequency modes. However, its broadband structure simultaneously requires the precise matching of the local oscillator (LO) with the squeezed light modes for their correct homodyne detection and processing. Achieving such matching is usually a challenging task, since the spectral bandwidth of the local oscillator is typically significantly narrower than the bandwidth of the squeezed-light modes. In this work, we propose and experimentally demonstrate a technique for expanding the bandwidth of the LO by passing it through the microstructured fiber. We validate the proposed concept by measuring squeezing in the modes of multimode squeezed light generated in type-0 and type-II PDC. We show that the broadened LO starts to spectrally match the modes generated via type-II PDC. The presented technique might be beneficial for implementing various CV systems and protocols.