Shot Noise Level Research Articles

Optically pumped non-zero field magnetometric sensor for the magnetoencephalographic systems using intra-cavity contacted VCSELs with rhomboidal oxide current aperture

We demonstrate the possibility of using vertical-cavity surface emitting lasers with intracavity contacts and a rhomboidal oxide current aperture for creating compact optically pumped 133Cs atomic magnetometer operating in non-zero magnetic fields, which is promising to use in magnetoencephalographic systems. The magnetic resonance parameters were studied in a two-beam MX optically pumped magnetic field sensor scheme based on the effect of the magnetic resonance line narrowing at high laser pumping and high concentrations of alkali-metal atoms. The ultimate sensitivity of OPM was estimated by the ratio of the magnetic resonance steepness to the probe light shot noise level. It is shown that a magnetic sensor based on the vertical-cavity surface emitting lasers and a compact (0.125 cm3 volume) Cs vapor cell can achieve a shot noise-limited sensitivity better than 15 fT in 1 Hz bandwidth. Developed intracavity contacted VCSELs suitable for use in compact atomic magnetometers for magnetoencephalographic systems.

The Squeezed Light Source for the Advanced Virgo Detector in the Observation Run O3

From 1 April 2019 to 27 March 2020, the Advanced Virgo detector, together with the two Advanced LIGO detectors, conducted the third joint scientific observation run O3, aiming for further detections of gravitational wave signals from astrophysical sources. One of the upgrades to the Virgo detector for O3 was the implementation of the squeezed light technology to improve the detector sensitivity beyond its classical quantum shot noise limit. In this paper, we present a detailed description of the optical setup and performance of the employed squeezed light source. The squeezer was constructed as an independent, stand-alone sub-system operated in air. The generated squeezed states are tailored to exhibit high purity at intermediate squeezing levels in order to significantly reduce the interferometer shot noise level while keeping the correlated enhancement of quantum radiation pressure noise just below the actual remaining technical noise in the Advanced Virgo detector.

Redshift-space effects in voids and their impact on cosmological tests. Part I: the void size function

ABSTRACT Voids are promising cosmological probes. Nevertheless, every cosmological test based on voids must necessarily employ methods to identify them in redshift space. Therefore, redshift-space distortions (RSD) and the Alcock–Paczyński effect (AP) have an impact on the void identification process itself generating distortion patterns in observations. Using a spherical void finder, we developed a statistical and theoretical framework to describe physically the connection between the identification in real and redshift space. We found that redshift-space voids above the shot noise level have a unique real-space counterpart spanning the same region of space, they are systematically bigger and their centres are preferentially shifted along the line of sight. The expansion effect is a by-product of RSD induced by tracer dynamics at scales around the void radius, whereas the off-centring effect constitutes a different class of RSD induced at larger scales by the global dynamics of the whole region containing the void. The volume of voids is also altered by the fiducial cosmology assumed to measure distances, this is the AP change of volume. These three systematics have an impact on cosmological statistics. In this work, we focus on the void size function. We developed a theoretical framework to model these effects and tested it with a numerical simulation, recovering the statistical properties of the abundance of voids in real space. This description depends strongly on cosmology. Hence, we lay the foundations for improvements in current models of the abundance of voids in order to obtain unbiased cosmological constraints from redshift surveys.

Nanophotonic source of quadrature squeezing via self-phase modulation

Squeezed light is optical beams with variance below the shot noise level. They are a key resource for quantum technologies based on photons, and they can be used to achieve better precision measurements and improve security in quantum key distribution channels and as a fundamental resource for quantum computation. Here, we demonstrate an integrated source of squeezing based on four-wave mixing that requires a single laser pump, measuring 0.45 dB of broadband quadrature squeezing at high frequencies. We identify and verify that the current results are limited by excess noise produced in the chip and propose ways to reduce it. Calculations suggest that an improvement in the optical properties of the chip achievable with existing technology can develop scalable quantum technologies based on light.

Detecting the anisotropic astrophysical gravitational wave background in the presence of shot noise through cross-correlations

The spatial and temporal discreteness of gravitational wave sources leads to shot noise that may, in some regimes, swamp any attempts at measuring the anisotropy of the gravitational wave background. Cross-correlating a gravitational wave background map with a sufficiently dense galaxy survey can alleviate this issue, and potentially recover some of the underlying properties of the gravitational wave background. We quantify the shot noise level and we explicitly show that cross-correlating the gravitational wave background and a galaxy catalog improves the chances of a first detection of the background anisotropy with a gravitational wave observatory operating in the frequency range (10Hz,100Hz), given sufficient sensitivity.

Two-Color Quantum Correlation between Down-Conversion Beams at 1.5 and 3.3 μm from a Singly Resonant Optical Parametric Oscillator

We demonstrated a two-color quantum correlation between the down-conversion beams with a telecommunication wavelength at 1.5 μm and mid-infrared wavelength at 3.3 μm generated by a singly resonant optical parametric oscillator (SRO) operated above the pump threshold with a magnesium-oxide doped periodically-poled lithium niobate crystal in the cavity. A maximum of 1.8 dB noise reduction of the intensity difference of the twin beams was measured at the analysis frequency of 5 MHz. Based on a theoretical model for the quantum correlation between the twin beams given by a semi-classical approach, the influence of the analysis frequency and pump parameter on the quantum correlation between the twin beams was discussed theoretically and experimentally. The quantum correlation between the twin beams was degraded at the analysis frequencies above 5 MHz due to the limitation of the bandwidth of SRO cavity and was degraded at the analysis frequencies below 5 MHz due to the excess intensity noise of the pump. The two-color quantum correlated twin beams at 1.5 and 3.3 μm have potential applications in high-precision measurements beyond the shot noise level.

The atomic hydrogen content of the post-reionization Universe

ABSTRACT We present a comprehensive analysis of atomic hydrogen (H i) properties using a semi-analytical model of galaxy formation and N-body simulations covering a large cosmological volume at high resolution. We examine the H i mass function and the H i density, characterizing both their redshift evolution and their dependence on hosting halo mass. We analyse the H i content of dark matter haloes in the local Universe and up to redshift z = 5, discussing the contribution of different galaxy properties. We find that different assembly history plays a crucial role in the scatter of this relation. We propose new fitting functions useful for constructing mock H i maps with halo occupation distribution techniques. We investigate the H i clustering properties relevant for future 21 cm intensity mapping (IM) experiments, including the H i bias and the shot-noise level. The H i bias increases with redshift and it is roughly flat on the largest scales probed. The scale dependence is found at progressively larger scales with increasing redshift, apart from a dip feature at z = 0. The shot-noise values are consistent with the ones inferred by independent studies, confirming that shot noise will not be a limiting factor for IM experiments. We detail the contribution from various galaxy properties on the H i power spectrum and their relation to the halo bias. We find that H i poor satellite galaxies play an important role at the scales of the one-halo term. Finally, we present the 21 cm signal in redshift space, a fundamental prediction to be tested against data from future radio telescopes such as Square Kilometre Array.

Influence of the pump scheme on the output power and the intensity noise of a single-frequency continuous-wave laser.

The influence of the pump scheme on the intensity noise of the single-frequency continuous-wave (CW) laser is investigated in this paper, which is implemented in a single-frequency CW Nd:YVO4 1064 nm laser by comparing the traditional 808 nm pumping scheme (TPS) to the direct 888 nm pumping scheme (DPS). Under the conditions that the lasers with TPS and DPS have the same cavity structure and the cavity mirrors, as well as the same operation state including the thermal lens of the laser crystals and the mode-matching between the pump laser mode and the laser cavity mode at the laser crystals, the output power of the laser with DPS is up-to 32.0 W, which is far higher than that of 21.1 W for the laser with TPS. However, the intensity noise of the DPS laser including resonant relaxation oscillation (RRO) frequency of 809 kHz, RRO peak amplitude of 31.6 dB/Hz above the shot noise level (SNL) and the SNL cutoff frequency of 4.2 MHz, respectively, is also higher than that of 606 kHz, 20.4 dB/Hz and 2.4 MHz for the TPS laser. After further analyses, we find that the laser crystal with high doping concentration and long optical length is employed for DPS laser in order to improve the pump laser absorption efficiency, which can simultaneously increase the dipole coupling between the active atoms and the laser cavity, and then results in a high RRO frequency with a large amplitude peak as well as a high SNL cutoff frequency of the laser.

Intensity noise suppression of a high-power single-frequency CW laser by controlling the stimulated emission rate.

The intensity noise of a high-power single-frequency continuous-wave (CW) laser is very harmful for applications in precise measurements and quantum communication. By simply lengthening the length of a laser resonator to decrease the stimulated emission rate of the laser, the coupling strength of all noise sources in the resonant laser field will be reduced, and thus the intensity noise of the output laser will be prominently suppressed. Based on theoretical analyses of the laser noise spectra, we experimentally implement a low-noise high-power single-frequency laser with a 1050 mm long resonator. With the assistance of an intracavity imaging system and nonlinear second-harmonic generation, the amplitude of the resonant relaxation oscillation peak and the shot-noise level (SNL) cutoff frequency are successfully reduced to 8.6 dB/Hz above SNL and 1.0 MHz, respectively, under the output power of 16 W. The work provides an effective way to develop a high-quality laser with high output power and low intensity noise.

Modulating quantum fluctuations of scattered light in disordered media via wavefront shaping

After multiple scattering of quadrature-squeezed lights in a disordered medium, the quadrature amplitudes of the scattered modes present an excess noise above the shot-noise level [Opt. Expr. 14, 6919 (2006)]. A natural question is raised whether there exists a method of suppressing the quadrature fluctuation of the output mode. The answer is affirmative. In this work, we prove that wavefront shaping is a promising method to reduce the quantum noise of quadrature amplitudes of the scattered modes. This reduction is owing to the destructive interference of quantum noise. Specifically, when the single-mode squeezed states are considered as inputs, the quantum fluctuation can always be reduced, even below the shot-noise level. These results may have potential applications in quantum information processing, for instance, sub-wavelength imaging using the scattering superlens with squeezed-state sources.

Squeezed vacuum phase control at 2 μm.

We demonstrate phase control for vacuum-squeezed light at a 2μm wavelength, which is a necessary technology for proposed future gravitational wave observatories. The control scheme allowed examination of noise behavior at frequencies below 1kHz and indicated that squeezing below this frequency was limited by dark noise and scattered light. We directly measure 3.9±0.2 dB of squeezing from 2kHz to 80kHz and 14.2±0.3 dB of antisqueezing relative to the shot noise level. The observed maximum level of squeezing is currently limited by photodetector quantum efficiency and laser instabilities at this new wavelength for squeezed light. Accounting for all losses, we conclude the generation of 11.3dB of squeezing at the optical parametric oscillator.

Squeezed vacuum states from a whispering gallery mode resonator

Squeezed vacuum states enable optical measurements below the quantum limit and hence are a valuable resource for applications in quantum metrology and also quantum communication. However, most available sources require high pump powers in the milliwatt range and large setups, which hinders real world applications. Furthermore, degenerate operation of such systems presents a challenge. Here, we use a compact crystalline whispering gallery mode resonator made of lithium niobate as a degenerate parametric oscillator. We demonstrate about 1.4 dB noise reduction below the shot noise level for only 300 $\mu\text{W}$ of pump power in degenerate single mode operation. Furthermore, we report a record pump threshold as low as 1.35 $\mu\text{W}$. Our results show that the whispering gallery based approach presents a promising platform for a compact and efficient source for nonclassical light.

Reducing the phase noise in diode lasers.

Diode lasers are widely used in atomic physics given their narrow linewidth and wavelength tunability. Nevertheless, although they present low noise for their intensity, excessive noise in the phase limits their application in quantum optics. Looking for reduction of this phase noise, we built and characterized a ring laser, using a semiconductor-tapered amplifier as the gain medium. We were able to reduce the phase noise of a diode laser to a factor of 10 above shot-noise level, bringing it closer to a useful coherent state for applications in quantum information.

Analytical approach of reduction in bit error rate using amplitude-squeezed states of light

In optical communication, bit error rate (BER) is a real problem today. In communication with coherent light, there is a limitation in reduction in BER due to inherent statistical property of light. Noise level produced by amplitude-squeezed light is below the shot noise level which is called quantum limit of noise. So if the encoding is done by amplitude-squeezed states of light, then one can overcome the limitation significantly. In this paper, we have shown analytically that using amplitude-squeezed sates of light a massive reduction in BER is possible.

Subhertz interferometry at the quantum noise limit.

Quantum noise limited interferometry for subhertz phase measurement is of essence in precision metrology applications, such as gravitational wave detection. However, suppression of subhertz classical noises below the shot-noise level is very challenging in practice. Two-frequency interferometry has been previously proposed to avoid low-frequency noises and may be useful for squeezing-enhanced high-precision phase measurement. Here we experimentally demonstrate a subhertz interferometer at the standard quantum noise (shot-noise) limit, by use of dual-frequency coherent probe light at the input and phase-sensitive heterodyne detection to measure the output interfered light. When the interferometer is locked at a dark fringe, the noise floor of the phase variance in the interferometer reaches the shot-noise level down to Fourier frequencies below 1Hz. This work should pave a way for a realization of subhertz interferometry beyond the shot-noise limit.

Observation of Atom Number Fluctuations in a Bose-Einstein Condensate.

Fluctuations are a key property of both classical and quantum systems. While the fluctuations are well understood for many quantum systems at zero temperature, the case of an interacting quantum system at finite temperature still poses numerous challenges. Despite intense theoretical investigations of atom number fluctuations in Bose-Einstein condensates, their amplitude in experimentally relevant interacting systems is still not fully understood. Moreover, technical limitations have prevented their experimental investigation to date. Here we report the observation of these fluctuations. Our experiments are based on a stabilization technique, which allows for the preparation of ultracold thermal clouds at the shot noise level, thereby eliminating numerous technical noise sources. Furthermore, we make use of the correlations established by the evaporative cooling process to precisely determine the fluctuations and the sample temperature. This allows us to observe a telltale signature: the sudden increase in fluctuations of the condensate atom number close to the critical temperature.

Angular Diversity Aperture (ADA) Receivers for Indoor Multiple-Input Multiple-Output (MIMO) Visible Light Communications (VLC)

The angular diversity aperture (ADA) receiver is a new form of optical wireless receiver which is made up of a number of receiving elements (REs) each consisting of a photodiode (PD) located below an aperture in an opaque screen. Previous work on the use of ADA receivers for visible light communication (VLC) analyzes only REs with round apertures and photodiodes with a limited range of parameters. This paper extends the analysis to calculate the channel gains for REs with for both round and square apertures and PDs and for more parameters. Graphs show how the gain varies as a function of the relative orientation of the transmitter and receiver for ten different designs. It is shown that the directionality of an RE depends on the offset of the PD from the aperture and that the field of view depends on the distance between the aperture and the PD. Detailed theoretical analysis and extensive simulation results demonstrate the bit error rate (BER) performance of receivers based on these ten designs in a multiple-input multiple-output (MIMO) VLC application. The round and square designs are found to have very similar BERs. It is shown that, even with high levels of shot noise, good BER results can be achieved throughout a typical scenario.

The phase sensitivity of a fully quantum three-mode nonlinear interferometer

We study a nonlinear interferometer consisting of two consecutive parametric amplifiers, where all three optical fields (pump, signal and idler) are treated quantum mechanically, allowing for pump depletion and other quantum phenomena. The interaction of all three fields in the final amplifier leads to an interference pattern from which we extract the phase uncertainty. We find that the phase uncertainty oscillates around a saturation level that decreases as the mean number N of input pump photons increases. For optimal interaction strengths, we also find a phase uncertainty below the shot-noise level and obtain a Heisenberg scaling . This is in contrast to the conventional treatment within the parametric approximation, where the Heisenberg scaling is observed as a function of the number of down-converted photons inside the interferometer.

Correlated electron beam microbunching and shot-noise characterization with near- and far-field optical transition radiation

In this work we demonstrate how to analyze the dynamics of the correlated longitudinal current noise above and below the Poissonian shot-noise level in an e-beam, by calculating the emitted transition radiation (OTR) using a complex vector field formulation which is exact in the near and far fields. We simulate a non-zero emittance electron beam, uncorrelated in the transverse cross section, but with different types of longitudinal correlations, resulting in coherent optical transition radiation. We show how this method is applied on the opposite case (suppression), by tracking simulated beam particles that correlate due to quarter-plasma oscillation during drift, and show how the OTR tracks the dynamics of the electron beam noise level.

Laser Power Stabilization beyond the Shot Noise Limit Using Squeezed Light

High levels of laser power stability are necessary for high precision metrology applications. The classical limit for the achievable power stability is determined by the shot noise of the light used to generate a power control signal. Increasing the power of the detected light reduces the relative shot noise level and allows higher stabilities. However, sufficiently high power is not always available and the detection of high laser powers is challenging. Here, we demonstrate a nonclassical way to improve the achievable power stability without increasing the detected power. By the injection of a squeezed vacuum field of light we improve the classical laser power stability beyond its shot noise limit by 9.4_{-0.6}^{+0.6} dB at Fourier frequencies between 5 and 80kHz. For only 90.6 μA of detected photocurrent we achieve a relative laser power noise of 2.0_{-0.1}^{+0.1}×10^{-8}/sqrt[Hz]. This is the first demonstration of a squeezed light-enhanced laser power stabilization and its performance is equivalent to an almost tenfold increase of detected laser power in a classical scheme. The analysis reveals that the technique presented here has the potential to achieve stability levels of 4.2×10^{-10}/sqrt[Hz] with 58mA photocurrent measured on a single photodetector.