Strongly squeezed light at telecommunication wavelengths is the necessary resource for one-sided device-independent quantum key distribution via fibre networks. Reducing the optical pump power that is required for its generation will advance this quantum technology towards efficient out-of-laboratory operation. Here, we investigate the second-harmonic pump power requirement for parametric generation of continuous-wave squeezed vacuum states at 1550nm in a state-of-the-art doubly-resonant standing-wave PPKTP cavity setup. We use coarse adjustment of the Gouy phase via the cavity length together with temperature fine-tuning for simultaneously achieving double resonance and (quasi) phase matching, and observe a squeeze factor of 13dB at 1550nm from just 12mW external pump power at 775nm. We anticipate that optimizing the cavity coupler reflectivity will reduce the external pump power to 3mW, without reducing the squeeze factor.
We propose and demonstrate a pump-phase locking technique that makes use of weak pump depletion (WPD) - an unavoidable effect that is usually neglected - in a sub-threshold optical parametric oscillator (OPO). We show that the phase difference between seed and pump beam is imprinted on both light fields by the non-linear interaction in the crystal and can be read out without disturbing the squeezed output. Our new locking technique allows for the first experimental realization of a pump-phase lock by reading out the pre-existing phase information in the pump field. There is no degradation of t…
Conditions to generate high-purity entangled vacuum-evacuated coherent states ($|0>|α>^0 - |-α>^0 |0>$ were studied for two cascade-placed beam splitters, with one squeezed state input and two coherent state inputs whenever a single photon is detected. Controlling the amplitudes and the phases of the beams allows for various amplitudes of the vacuum-evacuated coherent states ($|α>^ 0 = |α> -e^{-|α|^2} |0> $) up to $α= 2.160$ to be manipulated with high-purity.
Quantum frequency conversion, the process of shifting the frequency of an optical quantum state while preserving quantum coherence, can be used to produce non-classical light at otherwise unapproachable wavelengths. We present experimental results based on highly efficient sum-frequency generation (SFG) between a vacuum squeezed state at 1064 nm and a tunable pump source at 850 nm $\pm$ 50 nm for the generation of bright squeezed light at 472~nm $\pm$ 4~nm, currently limited by the phase-matching of the used nonlinear crystal. We demonstrate that the SFG process conserves part of the quantum c…
Multipartite entanglement is a key resource for various quantum information tasks. Here, we present a scheme for generating genuine tripartite entanglement via nonlinear optical processes. We derive, in the Fock basis, the corresponding output state which we termed the coupled three-mode squeezed vacuum. We find unintuitive behaviors arise in intensity squeezing between two of the three output modes due to the coupling present. We also show that this state can be genuinely tripartite entangled.
We propose a method for tailoring the frequency spectrum of bright squeezed vacuum by generating it in a nonlinear interferometer, consisting of two down-converting nonlinear crystals separated by a dispersive medium. Due to a faster dispersive spreading of higher-order Schmidt modes, the spectral width of the radiation at the output is reduced as the length of the dispersive medium is increased. Preliminary results show 30\% spectral narrowing.
We report a novel source of twin beams based on modulational instability in high-pressure argon-filled hollow-core kagomé-style photonic-crystal fibre. The source is Raman-free and manifests strong photon-number correlations for femtosecond pulses of squeezed vacuum with a record brightness of ~2500 photons per mode. The ultra-broadband (~50 THz) twin beams are frequency tunable and contain one spatial and less than 5 frequency modes.
Entangled coherent states are shown to emerge, with high fidelity, when mixing coherent and squeezed vacuum states of light on a beam-splitter. These maximally entangled states, where photons bunch at the exit of a beamsplitter, are measured experimentally by Fock-state projections. Entanglement is examined theoretically using a Bell-type nonlocality test and compared with ideal entangled coherent states. We experimentally show nearly perfect similarity with entangled coherent states for an optimal ratio of coherent and squeezed vacuum light. In our scheme, entangled coherent states are genera…
A novel two-mode non-degenerate squeezed light is generated based on a four-wave mixing (4WM) process driven by two pump fields crossing at a small angle. By exchanging the roles of the pump beams and the probe and conjugate beams, we have demonstrated the frequency-degenerate two-mode squeezed light with separated spatial patterns. Different from a 4WM process driven by one pump field, the refractive index of the corresponding probe field $n_{p}$ can be converted to a value that is greater than $1$ or less than $1$ by an angle adjustment. In the new region with $n_{p}<1$, the bandwidth of the…
We study the quantum correlations induced by spontaneous parametric down-conversion (SPDC) of a frequency comb. We derive a theoretical method to find the output state corresponding to a pump with an arbitrary spectral profile. After applying it to the relevant example of a spectrally chirped pump, we run an optimization algorithm to numerically find the pump profiles maximizing some target functions. These include the number of independently squeezed modes and the variances of nullifiers defining cluster states used in many continuous-variable quantum information protocols. To assess the adva…
The numerical conditions to generate a high-fidelity Yurke-Stoler states ($|α>+ e^{i ψ} |-α>$)were found for two cascade-placed beam splitters with one squeezed state input and two coherent state inputs. Controlling the amplitude and the phases of beams, allows for various Yurke-Stoler states to be manipulated with ultra high-fidelity, and the expected theoretical fidelity is of more than 0.9999.
In this article, we use quantum Langevin equations to provide a theoretical understanding of the non-classical behavior of Kerr optical frequency combs when pumped below and above threshold. In the configuration where the system is under threshold, the pump field is the unique oscillating mode inside the resonator, and triggers the phenomenon of spontaneous four-wave mixing, where two photons from the pump are symmetrically up- and down-converted in the Fourier domain. This phenomenon can only be understood and analyzed from a fully quantum perspective as a consequence of the coupling between …
Beam alignment is an important practical aspect of the application of squeezed states of light. Misalignments in the detection of squeezed light result in a reduction of the observable squeezing level. In the case of squeezed vacuum fields that contain only very few photons, special measures must be taken in order to sense and control the alignment of the essentially dark beam. The GEO600 gravitational wave detector employs a squeezed vacuum source to improve its detection sensitivity beyond the limits set by classical quantum shot noise. Here, we present our design and implementation of an al…
Squeezed states are a versatile class of quantum states with applications ranging from quantum computing to high-precision detection. We propose a method for generating tunable squeezed states of light with multiple modes encoded in frequency bins. Our method uses custom-engineered spontaneous parametric downconversion pumped by a pulse-shaped pump field. The multi-mode squeezed states are generated in a single pass and can be tuned in real time by adjusting the properties of the pump field. Exploring new quantum states of light, encoded in new degrees of freedom, can be a fruitful path toward…
Weak squeezed vacuum light, especially resonant to the atomic transition, plays an important role in quantum storage and generation of various quantum sources. However, the general homodyne detection (HD) cannot determine weak squeezing due to the low signal to noise ratio and the limited resolution of the HD system. Here we provide an alternative method based on photon statistics measurement to determine the weak squeezing of the squeezed vacuum light generated from an optical parametric oscillator working far below the threshold. The approach is established the relationship between the squee…
Decoherence remains one of the most serious challenges to the implementation of quantum technology. It appears as a result of the transformation over time of a quantum superposition state into a classical mixture due to the quantum system interacting with the environment. Since quantum systems are never completely isolated from their environment, decoherence therefore cannot be avoided in realistic situations. Decoherence has been extensively studied, mostly theoretically, because it has many important implications in quantum technology, such as in the fields of quantum information processing,…
We theoretically investigate the phase sensitivity with parity detection on a Mach-Zehnder interferometer with a coherent state combined with a photon-added squeezed vacuum state. When the phase shift approaches zero, the squeezed vacuum state is indeed the optimal state within a constraint on the average number of photons. However, when the phase shift to be estimated slightly deviates from zero, the optimal state is neither the squeezed vacuum state nor the photon-subtracted squeezed vacuum state, but the photon-added squeezed vacuum state when they carry many photons. Finally, we show that …
Squeezed states are a primary resource for continuous-variable (CV) quantum information processing. To implement CV protocols in a scalable and robust way, it is desirable to generate and manipulate squeezed states using an integrated photonics platform. In this Letter, we demonstrate the generation of quadrature-phase squeezed states in the radio-frequency carrier sideband using a small-footprint silicon-nitride microresonator with a dual-pumped four-wave-mixing process. We record a squeezed noise level of 1.34 dB ($\pm$0.16 dB) below the photocurrent shot noise, which corresponds to 3.09 dB …
We report generation and measurement of a squeezed vacuum from a semi-monolithic Fabry-Perot optical parametric oscillator (OPO) up to 100 MHz at 1550 nm. The output coupler of the OPO is a flat surface of a nonlinear crystal with partially reflecting coating, which enables direct coupling with waveguide modules. Using the OPO, we observed 6.2dB of squeezing at 2 MHz and 3.0 dB of squeezing at 100 MHz. The OPO operates at the optimal wavelength to minimize propagation losses in silica waveguides and looks towards solving a bottleneck of downsizing these experiments: that of coupling between a …
We show that two-mode squeezed vacuum-like states may be engineered in the Bohm-Madelung formalism by adequately choosing the phase of the wavefunction. The difference between our wavefunction and the one of the squeezed vacuum states is given precisely by the phase we choose.
We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our r…
We propose a method for building a squeezed vacuum state laser with zero diffusion, which results from the introduction of the reservoir engineering technique into the laser theory. As well as the reservoir engineering, our squeezed vacuum laser demands the construction of an effective atom-field interaction. And by building an isomorphism between the cavity field operators in the effective and the Jaynes-Cummings Hamiltonians, we derive the equations of our effective laser directly from the conventional laser theory. Our method, which is less susceptible to errors than reservoir engineering, …
