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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Entanglement-enhanced coherent detection of radio-frequency waveforms

Ferran Martin Ciurana, Giorgio Colangelo, L. Slodička, R. J. Sewell, Morgan W. Mitchell

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Summary form only given. We demonstrate detection of radio-frequency fields and arbitrarily-shaped radio-frequency waveform components beyond the projection noise limit (PNL). Our work employs a measure-evolve-measure (MEM) sequence, in which a first stroboscopic quantum non-demolition (QND) measurement M1produces a state with reduced projection noise, a period of free evolution accumulates signal, and a second stroboscopic QND measurement M2detects the change relative to the first measurement. The MEM scheme allows signal to coherently accumulate prior to readout of the atomic state. Overcoming the PNL improves the sensitivity in a magnetic field measurement [1, 2]. Our method allows quantification of entanglement-enhanced arbitrarily-shaped signal components and includes RF magnetometry as a special case. We demonstrate a sensitivity-volume figure of merit comparable to the best RF magnetometers at these frequencies [3].

DOI
10.1109/cleoe-eqec.2017.8087390

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Summary form only given. We demonstrate detection of radio-frequency fields and arbitrarily-shaped radio-frequency waveform components beyond the projection noise limit (PNL). Our work employs a measure-evolve-measure (MEM) sequence, in which a first stroboscopic quantum non-demolition (QND) measurement M1produces a state with reduced projection noise, a period of free evolution accumulates signal, and a second stroboscopic QND measurement M2detects the change relative to the first measurement. The MEM scheme allows signal to coherently accumulate prior to readout of the atomic state. Overcoming the PNL improves the sensitivity in a magnetic field measurement [1, 2]. Our method allows quantification of entanglement-enhanced arbitrarily-shaped signal components and includes RF magnetometry as a special case. We demonstrate a sensitivity-volume figure of merit comparable to the best RF magnetometers at these frequencies [3].
Optical magnetometers use magnetically-sensitive atomic ensembles and optical read-out to detect the amplitude of magnetic fields. They have become the most sensitive instruments for measuring low-frequency magnetic fields surpassing competing technologies like superconducting quantum interface devices (SQUIDs), and find applications in a variety of fields ranging from medicine, biology and geophysics, as well as tests of fundamental physics. However, their fundamental sensitivity is bounded by quantum mechanical behavior of the atoms, which gives rise to the standard quantum limit (SQL). As many instruments are approaching this fundamental limit, it becomes necessary to explore ways to overcome the SQL. Quantum metrology studies strategies to increase the sensitivity beyond the SQL by means of quantum engineering the atomic states. In this thesis, we investigate the quantum enhanced detection of time varying radio-frequency magnetic fields using a cold atomic ensemble of 87Rb atoms held in an optical dipole trap. We first theoretically develop a new measurement technique based on stroboscopic back-action evading measurements that takes advantage of the atomic coherence. This measurement scheme is suitable for the detection of arbitrarily-chosen components of radio-frequency waveforms and includes radio-frequency magnetometry as a special case. Experimentally, we demonstrate the capabilities of this technique using a linearly chirped waveform as a test case. As a first experiment, we demonstrate the selective response of the method in the coherently accumulated signal by the atoms. For this, we dispersively probe the atoms via Faraday rotation and non-destructively measure the induced magnetization. In the last part of the thesis we demonstrate quantum enhanced magnetic field detection. In a measure-evolve-measure (MEM) sequence, a first stroboscopic quantum non-demolition (QND) measurement produces a state with reduced projection noise, followed by a period of free evolution where the atoms accumulate signal. A second QND measurement detects the change relative to the first measurement. We demonstrate entanglement-enhanced sensing of sinusoidal and linearly chirped waveforms, with metrologically-relevant noise reduction of _2m =0.84(8) and _2m =0.80(3), respectively. We achieve volume-adjusted sensitivity of _B p V _ 3.96 fT p cm3/Hz, comparable to the best radio-frequency magnetometers. Els magnetòmetres òptics utilitzen conjunts d’àtoms sensibles magnèticament i lectura òptica per detectar l’amplitud de camps magnètics. S’han convertit en els instruments més sensibles per mesurar camps magnètics de baixa freqüència, superant tecnologies rivals com els dispositius superconductors d’interfície quàntica (SQUID), alhora que troben aplicacions en camps tan diversos com la medicina, la biologia, la geofísica, fins a proves de física fonamental. No obstant això, la seva sensibilitat està fonamental limitada pel comportament quàntic dels àtoms que dóna lloc al límit quàntic estàndard (SQL). Donat que molts instruments s’aproximen a aquest límit fonamental, es imprescindible explorar maneres de superar-ho. La metrologia quàntica estudia estratègies per augmentar la sensibilitat més enllà del SQL mitjançant enginys quàntics en l’estat dels àtoms. En aquesta tesi investiguem la detecció de camps magnètics de freqüència de ràdio amb un conjunt d’atòms freds de 87Rb mantinguts en una trampa de dipol òptic. En primer lloc, desenvolupem teòricament una nova tècnica de mesura basada en mesures estroboscòpiques que eviten la contra-acció de la mesura alhora que permet aprofitar la coherència atòmica. Aquest esquema de mesura és compatible amb la detecció de components elegits arbitràriament de formes d’ona de freqüència de ràdio i inclou la magnetometria de radiofreqüència com a cas especial. Demostrem experimentalment les capacitats d’aquesta tècnica utilitzant una forma d’ona de prova amb canvi de freqüència lineal. Al primer experiment, demostrem la resposta selectiva del mètode en la senyal coherent acumulada pels àtoms. Per aconseguir-ho, provem de forma dispersiva els àtoms amb la sonda Faraday i mesurem no destructivament oscil·lació de Rabi de la població atòmica entre els subnivells magnètics dels nivells hiperfins de l’àtom. A la darrera part de la tesi, demostrem la detecció de l’amplitud de camps magnètics amb precisió millorada gràcies a efectes quàntics. En una seqüència de mesura-evolució-mesura (MEM), una primera mesura quàntica, estroboscòpica i no destructiva (QND) produeix un estat amb soroll de projecció atòmic reduït. El segueix un període d’evolució lliure on els àtoms s’acumulen senyal. Una segona mesura QND detecta el canvi relatiu a la primera mesura. Demostrem detecció millorada per entrellaçament quàntic per a formes d’ona sinusoïdals i amb canvi de freqüència lineal, amb reduccions de soroll metrològicament rellevants ξ2m =0.84(8) i ξ2m =0.80(3), respectivament. Aconseguim una sensibilitat ajustada pel volum B p V 3.96 fT p cm3/Hz, comparable als millors magnetòmetres operant a freqüència de ràdio.
We demonstrate a new technique for detecting the amplitude of arbitrarily chosen components of radio-frequency waveforms based on stroboscopic backaction evading measurements. We combine quantum nondemolition measurements and stroboscopic probing to detect waveform components with magnetic sensitivity beyond the standard quantum limit. Using an ensemble of 1.5×10^{6} cold rubidium atoms, we demonstrate entanglement-enhanced sensing of sinusoidal and linearly chirped waveforms, with 1.0(2) and 0.8(3) dB metrologically relevant noise reduction, respectively. We achieve volume-adjusted sensitivity of δBsqrt[V]≈3.96 fTsqrt[cm^{3}/Hz], comparable to the best rf magnetometers.
We study experimentally the fundamental limits of sensitivity of an atomic radio-frequency magnetometer. First, we apply an optimal sequence of state preparation, evolution, and the backaction evading measurement to achieve a nearly projection noise limited sensitivity. We furthermore experimentally demonstrate that Einstein-Podolsky-Rosen entanglement of atoms generated by a measurement enhances the sensitivity to pulsed magnetic fields. We demonstrate this quantum limited sensing in a magnetometer utilizing a truly macroscopic ensemble of 1.5x10(12) atoms which allows us to achieve subfemtotesla/square root(Hz) sensitivity.
Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.
Magnetic induction tomography (MIT) is a sensing protocol exploring conductive objects via their response to radio-frequency magnetic fields. MIT is used in nondestructive testing ranging from geophysics to medical applications. Atomic magnetometers, employed as MIT sensors, allow for significant improvement of the MIT sensitivity and for exploring its quantum limits. Here, we propose and verify a quantum-enhanced version of the atomic MIT by combining it with conditional spin squeezing and stroboscopic backaction evasion. We use this quantum enhancement to demonstrate sensitivity beyond the standard quantum limits of one-dimensional quantum MIT detecting a conductive sample.