Temporally multimode squeezed states have been a topic of recent interest due to their applications in quantum communication, information processing, and sensing. Characterizing the mode shapes is crucial for effectively manipulating these states, but current mode shape and state characterization techniques necessitate constraining assumptions and complicated experimental setups. Here, we propose a characterization technique that simultaneously recovers the complex temporal mode shapes and quadrature variances of ultrafast multimode squeezed states based on frequency resolved optical gating (FROG) using an optical parametric amplifier (OPA). FROG is a promising tool for quantum state characterization due to its flexibility of implementation and high temporal resolution. Using an OPA as the nonlinear process in FROG has the benefit of amplifying weak quantum states to a detectable level while preserving quantum information. Numerical simulations demonstrate the recovery of the mode shapes and levels of squeezing and anti-squeezing of ultrafast multimode squeezed states. This scheme offers a practical experimental approach to measuring arbitrary temporal mode shapes and characterizing large-scale multimode ultrafast Gaussian quantum states.
Temporally multimode squeezed states have been a topic of recent interest due to their applications in quantum communication, information processing, and sensing. Characterizing the mode shapes is crucial for effectively manipulating these states, but current mode shape and state characterization techniques necessitate constraining assumptions and complicated experimental setups. Here, we propose a characterization technique that simultaneously recovers the complex temporal mode shapes and quadrature variances of ultrafast multimode squeezed states based on frequency resolved optical gating (FROG) using an optical parametric amplifier (OPA). FROG is a promising tool for quantum state characterization due to its flexibility of implementation and high temporal resolution. Using an OPA as the nonlinear process in FROG has the benefit of amplifying weak quantum states to a detectable level while preserving quantum information. Numerical simulations demonstrate the recovery of the mode shapes and levels of squeezing and anti-squeezing of ultrafast multimode squeezed states. This scheme offers a practical experimental approach to measuring arbitrary temporal mode shapes and characterizing large-scale multimode ultrafast Gaussian quantum states.
Offering terahertz of bandwidths and femtosecond timescales, ultrafast optics is enabling both the study of fundamental quantum optical phenomena and the advancement of quantum-enhanced applications. However, unlocking the full potential of ultrafast quantum optics requires accessing the temporal characteristics of ultrashort quantum pulses across ultrabroad bandwidths. This is particularly important in the near-infrared and visible range of the optical spectrum, which, unlike the terahertz and long-wave infrared, has remained beyond the reach of current techniques. Here, we break this barrier by translating frequency-resolved optical gating (FROG), a widely used technique for ultrafast classical pulse characterization, to the quantum regime. We show how such a quantum FROG can measure complex temporal modes and sub-optical-cycle quadrature covariances in the near-infrared, enabling complete characterization of microscopic Gaussian states. We experimentally use the quantum-FROG to report the measurement of quadrature correlations, complex temporal modes, and squeezing levels of multimode ultrafast squeezed vacuum states generated on a nanophotonic chip. We access multimode squeezing levels of a femtosecond quantum pulse approaching 7 dB and demonstrate FROG-based measurement bandwidths exceeding 100 THz. Quantum FROG enables measurement of previously inaccessible quantum features of ultrashort pulses at the sub-optical-cycle regime and highlights a practical path to accessing terahertz of bandwidths in quantum optics for applications in computing, sensing, and imaging.
Photonics offers the potential for large-scale, room-temperature, and ultrafast quantum operations. Multiplexing pulses of light allows for high throughput speeds and dense encoding, with tens of thousands of optical modes being present in a time as short as a microsecond. However, the measurement speed of electronic devices such as photodetectors is currently one bottleneck in such time-multiplexed scalability, because many quantum optical protocols rely on measurement, including state preparation, feed-forward, and feedback. Furthermore, characterization of quantum states and modes is essential for their effective use. Using nanophotonic optical parametric amplifiers (OPAs) presents one way to circumvent the measurement limitations of current quantum photonic schemes. Their large amplification bandwidth, enabled by dispersion engineering on nanophotonic integrated platforms, allows information encoded in femtosecond-scale, THz-bandwidth, multimode quantum optical states to be accessed with nonlinear optical interactions. In this work, we show how ultra-broadband integrated nanophotonic OPAs can be used to measure ultrafast and multimode quantum states of light. First, we explore the single-photon detection capabilities of OPAs, showing that current OPAs operating in the Gaussian regime are capable of 250 MHz photon count rates with 26% efficiency and a 2% dark count probability. We also show how non-Gaussian operation through pump depletion, with performance that approaches state-of-the-art photon detectors in terms of efficiency and dark count rate while retaining ultrafast operation, can become experimentally possible with a higher nonlinear coupling rate and lower loss. Next, we use nanophotonic OPAs to both generate and characterize multimode ultrafast squeezed vacuum. We use the photocurrent distribution of amplified squeezed vacuum to recover 2.41 dB of squeezing in one mode of a 154-fs multimode squeezed pulse and reconstruct its Wigner distribution. Finally, we investigate the capabilities of broadband nanophotonic OPAs to determine the temporal mode structure and quadrature variances of ultra-broadband temporally multimode quantum states by adapting frequency-resolved optical gating to the quantum regime. We numerically show the successful full characterization of a multimode squeezed state, even in the presence of noise. Together, these results establish OPAs as a valuable measurement device for measuring ultrafast quantum pulses and learning the structure of multimode quantum states, and they provide one building block towards a framework for scalable continuous-variable quantum photonics in which state generation, manipulation, and characterization occur within the same nanophotonic platform.
Multimode squeezed light is an increasingly popular tool in photonic quantum technologies, including sensing, imaging, and computation. Meanwhile, the existing methods of its characterization are technically complicated, which reduces the level of squeezing, and mostly deal with a single mode at a time. Here, for the first time, to the best of our knowledge, we employ optical parametric amplification to characterize multiple squeezing eigenmodes simultaneously. We retrieve the shapes and squeezing degrees of all modes at once through direct detection followed by modal decomposition. This method is tolerant to inefficient detection and does not require a local oscillator. For a spectrally and spatially multimode squeezed vacuum, we characterize eight strongest spatial modes, obtaining squeezing and anti-squeezing values of up to −5.2 ± 0.2 dB and 8.6 ± 0.3 dB, respectively, despite the 50% detection loss. This work, being the first exploration of an optical parametric amplifier’s multimode capability for squeezing detection, paves the way for the real-time detection of multimode squeezing.
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
Optical quantum information science plays a central role in the development of advanced photonic technologies, with applications ranging from quantum imaging and communication to continuous-variable (CV) quantum computing. The efficient realization of these technologies depends not only on the generation of nonclassical states of light but also on their proper detection. Optical parametric amplification (OPA) has long been known as both a fundamental nonlinear process for generating nonclassical light and as a method to boost detection efficiency through pre-amplification. Recently, OPA has been revisited as a tool for all-optical quadrature detection, offering a powerful alternative to conventional homodyne detection by enabling broadband, loss-tolerant squeezing measurements across various optical platforms. In this thesis, I establish optical parametric amplification as a versatile tool for the detection of CV quantum states through all-optical quadrature detection. I extend squeezing detection to high-dimensional scenarios by exploiting the intrinsic multimode nature of OPA, thereby enabling, for the first time to the best of my knowledge, the real-time monitoring of multimode squeezed light, a long-standing challenge in quantum optics. This approach further permits the simultaneous monitoring of multiple CV Einstein–Podolsky–Rosen (EPR) mode pairs, providing a framework for scalable detection of multimode entanglement, namely optical cluster states, which constitute essential resources for CV quantum computing. Furthermore, I demonstrate OPA as a platform for loss-tolerant and broadband quantum state tomography, applying it to both Gaussian and non-Gaussian states. This paves the way for the scalable tomography of multimode quantum states. Finally, I show that OPA enables the certification of the non-Gaussian and nonclassical character of quantum states without the need for photon-number-resolving detectors, relying instead on post-amplification intensity statistics. In particular, I amplify a single photon sufficiently strongly that its non-Gaussian character becomes accessible through conventional intensity detectors. Altogether, this work establishes optical parametric amplification as a powerful and scalable platform for high-dimensional continuous-variable optical quantum technologies.
We demonstrate broadband phase-sensitive amplification (PSA) measurement of squeezed light generated by a waveguide optical parametric amplifier (OPA) with external dispersion compensation. In broadband systems, group velocity dispersion (GVD) induces a frequency-dependent rotation of the squeezing axis, which limits the observable bandwidth in PSA measurements. To overcome this limitation, we introduce external dispersion compensation between two OPAs and suppress the quadrature rotation over a wide frequency range. As a result, we observe a maximum squeezing of 5.9 dB near the carrier frequency and more than 5 dB of squeezing up to a frequency offset of 4.5 THz from the carrier. Furthermore, squeezing below the shot-noise level is confirmed up to a frequency offset of 6 THz from the carrier, corresponding to the accessible phase-matching bandwidth of the waveguide OPA. Our results establish a practical method for broadband characterization of squeezed light and provide a key step toward ultrafast continuous-variable quantum information processing.