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arXiv:1801.04811 · Physical Review Applied

Optical Amplification of Spin Noise Spectroscopy via Homodyne Detection

Pavel Sterin, J. Wiegand, J. Hübner, M. Oestreich

Published Mar 5, 2018• 20 citations
arXivHTMLPDF

Abstract

With quantum information processing in mind, homodyne optical amplification tremendously enhances the sensitivity of spin noise spectroscopy applied to delicate few- and single-spin systems. In realizing the allied experimental techniques, the authors are able to boost the low-frequency spin noise signal of a few remaining impurities in an isotopically enriched Rb vapor above the electronic noise level dictated by the photodetector system. This proof-of-principle experiment advances semiconductor spin research and facilitates higher-order spin noise measurements on semiconductor qubits, such as single (In,Ga)As quantum dots.

DOI
10.1103/physrevapplied.9.034003

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With quantum information processing in mind, homodyne optical amplification tremendously enhances the sensitivity of spin noise spectroscopy applied to delicate few- and single-spin systems. In realizing the allied experimental techniques, the authors are able to boost the low-frequency spin noise signal of a few remaining impurities in an isotopically enriched Rb vapor above the electronic noise level dictated by the photodetector system. This proof-of-principle experiment advances semiconductor spin research and facilitates higher-order spin noise measurements on semiconductor qubits, such as single (In,Ga)As quantum dots.
Optical homodyne (single-frequency) measurement is a key technology in optical quantum information processing and key distribution, since it can realize high-speed quantum measurement with a high signal-to-noise ratio. Researchers show that photodiodes have an intrinsic noise mechanism in the photodetection process, due to distributed photocarrier generation, and the excess noise cause an effective optical loss in homodyne detection at high frequencies. They evaluate this excess loss theoretically and experimentally, showing that it depends on the photodiode's material, and suggesting further requirements for high-efficiency, high-speed hardware.
We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45${}^{\ensuremath{\circ}}$ polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk nnn:GaAs in the spectral range 835--918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.
Spin noise spectroscopy has become a widespread technique to extract information on spin dynamics in atomic and solid-state systems, in a potentially nonperturbative way. Here we experimentally demonstrate a new approach in spin noise spectroscopy, based on the detection of single photons. Because of the large spin-dependent polarization rotations provided by a deterministically coupled quantum dot-micropillar device, giant spin noise signals induced by a single-hole spin are extracted in the form of photon-photon cross-correlations. Ultimately, such a technique can be extended to an ultrafast regime probing mechanisms down to few tens of picoseconds.
Spontaneous fluctuations of the magnetization of a spin system in thermodynamic equilibrium (spin noise) manifest themselves as noise in the Faraday rotation of probe light. We show that the correlation properties of this noise over the optical spectrum can provide clear information about the composition of the spin system that is largely inaccessible for conventional linear optics. Such optical spectroscopy of spin noise, e.g., allows us to clearly distinguish between optical transitions associated with different spin subsystems, to resolve optical transitions that are unresolvable in the usual optical spectra, to unambiguously distinguish between homogeneously and inhomogeneously broadened optical bands, and to evaluate the degree of inhomogeneous broadening. These new possibilities are illustrated by theoretical calculations and by experiments on paramagnets with different degrees of inhomogeneous broadening of optical transitions [atomic vapors of 41K and singly charged (In,Ga)As quantum dots].
This study demonstrates a significant enhancement in ultra-low-frequency (ULF) electric field sensitivity using Rydberg atoms via an optical chopping amplification (OCA) technique. Conventional Rydberg-based ULF measurements are fundamentally limited by 1/f noise, which severely degrades sensitivity. Our approach modulates the coupling laser with an optical chopper before the vapor cell, inducing periodic Rydberg excitation at the chopping frequency. The photodetector (PD) output signal is demodulated by a lock-in amplifier (LIA) using the optical chopper's signal as the reference. This process effectively improves the signal-to-noise ratio (SNR) by shifting the 1/f noise to a higher frequency band where it can be filtered out. The OCA technique enhanced sensitivity by 19.1 dB for the frequency 7 Hz, which is down to 49.1 uV/cm/rt(Hz). For the frequency range from 10Hz to 1kHz, it also enhanced nearly 7dB. This OCA method for enhancing the sensitivity of Rydberg atoms in ULF electric field measurements enables the Rydberg sensor's detection range to span the entire spectrum from low frequency (LF) to ULF, thereby significantly broadening its application potential.
We develop a transient photoinduced Kerr rotation spectroscopy technique using a heterodyne detection scheme to study spin dynamics of microscopic quantum states in solids, such as single quantum dots and spin helixes. The use of the heterodyne beat note signal generated by the interference of the frequency-shifted probe and reference pulses realizes the Kerr rotation measurements in combination with micro-spectroscopy, even when the probe pulse propagates collinearly with the strong pump pulse, which resonantly excites the probing state. In addition, the interference gives an optical amplification of the Kerr signal, which provides a clear observation of the photoinduced spin dynamics by the weak probe intensity. Here, we present results of Kerr rotation measurements for a single quantum dot exciton, which shows a maximum rotation angle of few µrad.