The observation of gravitational waves from binary neutron star mergers offers insights into properties of extreme nuclear matter. However, their high-frequency signals in the kHz range are often masked by quantum noise of the laser light used. Here, we propose the ``quantum expander with coherent feedback,'' a new detector design that features an additional optical cavity in the detector output and an internal squeeze operation. This approach allows us to boost the sensitivity at high frequencies while at the same time providing a compact and tunable design for signal extraction. It also allows us to tailor the sensitivity of the detector to the specific signal frequency range. We demonstrate that our design can improve the sensitivity of the high-frequency detector concept NEMO, increasing the detection rates by around 14%. Our approach promises a new level of flexibility in designing the detectors aimed at high-frequency signals.
The observation of gravitational waves from binary neutron star mergers offers insights into properties of extreme nuclear matter. However, their high-frequency signals in the kHz range are often masked by quantum noise of the laser light used. Here, we propose the ``quantum expander with coherent feedback,'' a new detector design that features an additional optical cavity in the detector output and an internal squeeze operation. This approach allows us to boost the sensitivity at high frequencies while at the same time providing a compact and tunable design for signal extraction. It also allows us to tailor the sensitivity of the detector to the specific signal frequency range. We demonstrate that our design can improve the sensitivity of the high-frequency detector concept NEMO, increasing the detection rates by around 14%. Our approach promises a new level of flexibility in designing the detectors aimed at high-frequency signals.
The quantum uncertainty of laser light limits the sensitivity of gravitational-wave observatories. Over the past 30 years, techniques for squeezing the quantum uncertainty, as well as for enhancing gravitational-wave signals with optical resonators have been invented. Resonators, however, have finite linewidths, and the high signal frequencies that are produced during the highly scientifically interesting ring-down of astrophysical compact-binary mergers still cannot be resolved. Here, we propose a purely optical approach for expanding the detection bandwidth. It uses quantum uncertainty squeezing inside one of the optical resonators, compensating for the finite resonators' linewidths while keeping the low-frequency sensitivity unchanged. This quantum expander is intended to enhance the sensitivity of future gravitational-wave detectors, and we suggest the use of this new tool in other cavity-enhanced metrological experiments.
Abstract We propose adding a nonlinear element to a long signal recycling cavity to enhance the high-frequency sensitivity (900 Hz–5 kHz) of a kilometer-scale interferometric gravitational wave detector. Using numbers for absorption and scattering losses in the detector consistent with advanced LIGO+, we demonstrate a factor of 3.5 improvement in quantum noise limited strain sensitivity in the kHz regime. Such a configuration is robust to internal losses and reduces the requirement on the amount of circulating power in the detector to achieve sensitivity comparable to future gravitational wave detectors. This proposed configuration is compatible with the existing gravitational wave detector vacuum infrastructure and could enable exploration of exotic science, such as observing the merger phase of binary neutron stars, which in turn may provide constraints on the neutron star equation of state.
The dawn of gravitational-wave astronomy has begun in 2015 with the historic detection of a binary black hole merger. Several more detections followed in the years after. Among them, the detectors observed the inspiral of a neutron star binary. This merger was also observed in a broad spectrum of electromagnetic counterparts. This multi-messenger observation demonstrated that gravitational-wave astronomy is invaluable for understanding the Universe.
Current detectors, however, are able to see only a small part of binaries’ inspirals. A typical signal from an inspiral of a binary neutron star appears in the detector at low frequencies a few minutes before the merger. During the inspiral, the frequency of the signal increases, faster for higher frequencies. It quickly passes through the detection band of the observatory. The merger itself and post-merger oscillations often remain inaccessible due to the reduced sensitivity of the detector to high-frequency signals. An increase in the low-frequency sensitivity will allow to detect signals significantly earlier, extracting more information about the binary. This will allow to precisely locate the source on the sky for the follow-up multi-messenger observations. An increase in the high-frequency sensitivity will give a possibility to observe the merger and post-merger signals, gaining insight into the physics of ultra-dense quantum matter.
The limitations to the sensitivity arise from quantum nature of light used to sense the displacement of mirrors caused by gravitational waves. At low frequencies, quantum fluctuations in the amplitude of the light field cause random forces on the mirrors, which mask the signal from gravitational waves. At high frequencies, quantum fluctuations in the phase of the light field cause measurement noise on the detectors. The detectors use optical cavities to enhance the signal within the bandwidth of the cavities. At frequencies outside this bandwidth the signal is getting suppressed. Signals from the binary neutron star mergers typically have frequencies outside the detection bandwidth.
In this thesis I study the fundamental limitations on the sensitivity of detectors, in the quest of a broadband observatory. I discuss a speedmeter approach to increasing the low-frequency sensitivity, and propose two ways of achieving it: with quantum-entangled light, and in a novel ring cavity topology of the detector, which I test experimentally. I further study the fundamental limits arising from the finite bandwidth of the cavities, and demonstrate experimentally how this limit can be overcome. I propose a concept of quantum expander that allows to increase the detection bandwidth by using quantum correlations, generated directly inside the detector’s cavities. I investigate, how quantum correlations, controlled by internal squeezing, can help to tailor the sensitivity of the detector for the optimal performance at selected frequencies. Finally, I show that, although the benefit from application of squeezed light in metrology is limited by quantum decoherence, its impact on the sensitivity can be reduced by exploiting quantum correlations created with internal squeezing.
Quantum nature of light is a nuisance, which prevents us from seeing gravitational waves, but also a powerful tool, if properly applied. Quantum correlations hidden in the noise can be exploited for the benefit of the detectors, as I show in various scenarios throughout the thesis.
Quantum noise can be tamed, and not only in gravitational-wave detectors. All cavity-enhanced quantum metrological experiments can use quantum noise and underlying correlations as the resource for tailoring their sensitivity.
2015 begann mit der erstmaligen Beobachtung der Verschmelzung zweier schwarzer Locher das Zeitalter der Gravitationswellenastronomie. In den darauffolgenden Jahren gab es viele weitere Detektionen. Unter anderem wurde das Gravitationswellensignal der Verschmelzung zweier Neutronensterne detektiert. Parallel zur Messung der Gravitationswellen wurde auch das elektromagnetische Spektrum dieses Ereignisses beobachtet. Diese Multi-Messenger Beobachtung hat gezeigt, dass Gravitationswellenastronomie wesentlich fur das Verstandnis des Universums ist.
Aktuell kann jedoch nur eine kleine Zahl von astronomischen Ereignissen detektiert werden, denn die Signale sind typischerweise nur wenige Minuten vor der eigentlichen Verschmelzung messbar und haben dann sehr kleine Frequenzen. Solange sich die astronomischen Objekte umkreisen, erhoht sich die Signalfrequenz und zwar mit schneller werdender Rate hin zu hoheren Frequenzen. Auf diese Art und Weise durchlauft das Signal schnell das Detektionsband des Gravitationswellendetektors und die eigentliche Verschmelzung sowie darauffolgende Oszillationssignale konnen nicht beobachtet werden, weil die Sensitivitat der Detektoren bei den dafur notigen hoheren Frequenzen nicht ausreicht. Eine Erhohung der Sensitivitat bei kleinen Frequenzen wurde eine fruhere Detektion von Signalen ermoglichen, sodass mehr Informationen uber die astronomischen Objekte gewonnen werden konnen. Auserdem wurde die Signalquelle am Himmel prazise lokalisierbar, sodass Beobachtungen durch weitere Teleskope fruhzeitig beginnen konnten. Eine hohere Sensitivitat im hochfrequenten Bereich wurde es ermoglichen, auch die eigentliche Verschmelzung zu beobachten sowie Signale nach der Verschmelzung zu messen. Dadurch konnten neue Erkenntnisse zur Physik von ultradichter Quantenmaterie gewonnen werden.
Limitiert wird die Sensitivitat, sowohl bei hohen als auch bei niedrigen Frequenzen, durch die Quanteneigenschaften des Laserlichts, welches genutzt wird, um die relativen Positionsanderungen von Spiegeln zu messen. Diese Anderungen werden durch Gravitationswellen hervorgerufen.
Bei kleinen Frequenzen sorgt die Amplitudenunscharfe des Lichts fur Schwankungen des auf die Spiegel wirkenden Strahlungsdrucks, wodurch das Gravitationswellensignal uberdeckt wird. Bei hohen Frequenzen koppelt durch die Phasenunscharfe des Lichts Rauschen in die Detektoren ein. In die Detektoren sind auserdem optische Resonatoren integriert, welche das Messsignal innerhalb der Resonatorbandbreite verstarken. Signale auserhalb der Bandbreite werden unterdruckt. Gravitationswellensignale von sich umkreisenden Neutronensternen haben in der Regel Frequenzen auserhalb dieser Detektionsbandbreite.
In dieser Arbeit befasse ich mich mit der Limitierung der Sensitivitat der Detektoren und zeige Wege auf, diese zu verbessern. Dafur diskutiere ich einen Detektor, welcher auf einer Geschwindigkeitsmessung basiert und zeige zwei Wege auf, um die Messgenauigkeit zu steigern: Zum einen mit quantenverschranktem Licht und zum anderen mit einem Umlaufresonator, also einer neuartigen Detektortopologie. Auserdem studiere ich die Sensitivitasbeschrankungen, welche durch die Resonatoren zur Signalverstarkung entstehen. Um diese Beschra nkungen zu reduzieren, schlage ich ein neuartiges Konzept vor, bei dem quantenkorreliertes Licht direkt in den Resonatoren erzeugt wird und so die Detektionsbandbreite erhoht. Ich untersuche, wie die durch internes squeezing kontrollierten Quantenkorrelationen dabei helfen konnen, die Empfindlichkeit des Detektors bei ausgewahlten Frequenzen zu optimieren. Dabei zeige ich, das der Einfluss von Dekoharenz auf durch Quetschlicht optimierte Sensitivitat durch die Nutzung von Quantenkorrelationen, die durch internes Squeezing erzeugt werden, reduziert werden kann.
Die Quantennatur des Lichts stellt ein fundamentales Limit fur die Sensitivitat der Gravitationswellendetektoren dar und verhindert die Beobachtung schwacher Signale. Sie kann aber auch als nutzliches Werkzeug eingesetzt werden, um durch Quantenkorrelationen schwa chste, im Rauschen versteckte Signale, sichtbar zu machen. Dies zeige ich fur unterschiedliche Szenarien in dieser Arbeit.
Nicht nur die Gravitationswellenastronomie kann von diesen Ansatzen profitieren. In allen Resonator gestutzten quantenmetrologischen Experimenten konnen Quantenkorrelationen genutzt werden, um die Messsensitivitat zu verbessern.
Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitationalwave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise.Here, we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of hertz to several kilohertz.In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz; in the Livingston detector, the noise reduction was a factor of 1.9 (5.8 dB).These improvements directly impact LIGO's scientific output for high-frequency sources (e.g., binary neutron star postmerger physics).The improved lowfrequency sensitivity, which boosted the detector range by 15%-18% with respect to no squeezing, corresponds to an increase in the astrophysical detection rate of up to 65%.Frequency-dependent squeezing was enabled by the addition of a 300-meter-long filter cavity to each detector as part of the LIGO Aþ upgrade.
The era of gravitational-wave astronomy has begun, with the detection of 5 confirmed binary black holes and a binary neutron star coalescence by the Advanced Laser Interferometer Gravitational- wave Observatory (aLIGO), and later with the advanced Virgo detector. These detections have already revealed a wealth of discoveries across the fields of nuclear physics, general relativity and astrophysics. The work presented in this thesis is part of the ongoing effort to improve the sensitivity of ground-based interferometric gravitational-wave detectors. The sensitivity of aLIGO, and other interferometric detectors, is broadly limited by quantum noise. Improving on the quantum noise will increase the astrophysical range of the detectors, and improve the source parameter estimation. One way to reduce quantum noise is to inject audio- band squeezed vacuum states into the detection port. This technique has been demonstrated on the initial LIGO and GEO600 detectors. A squeezed light source for aLIGO must meet stringent requirements in terms of optical loss, phase noise, and scattered light. The squeezer must produce high levels of audio-band squeezing and operate under vacuum, to take advantage of the excellent existing isolation systems and to minimise optical loss. At design sensitivity, squeezed states whose quantum noise depends on frequency will be required. We have demonstrated an ultra-stable glass-based squeezed light source, the first experiment of this kind to operate under vacuum. The squeezer cavity is constructed quasi-monolithically, with optics and nonlinear crystal oven optically contacted to a breadboard base. The cavity is designed to have extremely low length noise, and to produce high levels of audio-band squeezing. We have measured 8.6 ± 0.9 dB of squeezing and infer the generation of 14.2 ± 1.0 dB after accounting for all known losses. The squeezer has demonstrated record phase noise performance of 1.3 mradRMS, dominated by sources other than cavity length noise. This exceeds the phase-noise requirement for a squeezer for aLIGO. A copy of this squeezer is currently being installed in a squeezing- ellipse rotation experiment to demonstrate frequency-dependent squeezing for aLIGO. Lessons learnt during the construction and operation of the in-vacuum squeezer have helped inform the design of a frequency-independent squeezed light source currently being installed at the LIGO sites. Future gravitational-wave detectors will continue to use interferometric techniques, and will be limited by quantum noise for the foreseeable future. To improve on thermal noise limits and interferometer power handling, future detectors look to cryogenic silicon as a test mass material. To take advantage of the desirable properties of silicon, including low scatter and absorption, a longer operating wavelength is required. The proposed LIGO Voyager upgrade has an operating wavelength in the 2 μ m region, with the specific wavelength to be determined. LIGO Voyager will require a squeezed light source in the 2 μ m region to reach its design sensitivity. We present the design, characterisation, and results of the first squeezed light source in the 2 μ m region. Laser and detector technologies at 2 μ m are less developed than their 1064 nm counterparts, causing significant technical challenges. We have measured 4.0 ± 0.2 dB of squeez- ing at 1984 nm, limited by loss due to detector quantum efficiency. Accounting for known losses in the system, we infer the generation of 10 dB of squeezing. This is an important demonstra- tion of quantum noise reduction for future detectors, and a pathfinder technology for the design choices of LIGO Voyager. So far we have found no reason why a 2 μ m interferometer should not be feasible.
Gravitational-wave astronomy promises a radically new method of investigating the universe, one that does not detect elementary particles, but rather ripples in space-time itself. One might expect that with this radically new method, our understanding of the universe will make an equally giant leap. One of the most promising methods for the eventual detection of gravitational-waves is that of interferometric gravitational-wave detection. The sensitivity of these devices is nothing short of amazing, and their sensitivity continues to improve. The next generation of detectors are expected reach a point where increasing the laser power, previously used to increase the sensitivity, is no longer expected to provide any benefit. One of the most promising options for further increasing the sensitivity, through reduction of the quantum noise, is via the application of "squeezed states of light". Squeezed states of light are light fields for which the noise of some observable has been reduced below the quantum noise limit. These states have been shown for many years to offer increased sensitivity in interferometers and recently in interferometric gravitational-wave detectors. One of the most challenging tasks to make suitable squeezed light sources for these detectors is to produce low-frequency squeezing, corresponding to the detection band of these interferometers. This thesis details the doubly resonant travelling wave bow-tie squeezer as a source of squeezing for interferometric gravitational-wave detection. This squeezer achieves record results of squeezing in the gravitational-wave detection band. For the first time, 10 dB of shot noise suppression at 10Hz is directly observed and above 200Hz, 11.6 dB is observed. The work presented in this thesis provides evidence that further substantiates previous progress indicating that squeezed states of light are now ready for full-time integration into interferometric gravitational-wave detectors. The new cavity design, described in detail throughout this thesis, is shown to outperform previous designs and provide benefits when integrating squeezing into these detectors, in particular, isolation to backscattered light from the interferometer. The noise sources that typically lead to degradation in squeezing measurements are investigated and discussed. Additionally, a modification to the standard locking technique used to control these vacuum squeezed states, coherent locking, is presented and discussed. The modified technique reduces the required number of locking loops and provides a larger beat note from which to derive an error signal for one of the remaining loops. A squeezer using these design philosophies is then constructed and used to inject squeezing into the LIGO gravitational-wave detector. Injection of the squeezed state provides enhancement of the sensitivity of the detector at frequencies around 200Hz and above. The sensitivity seen was equivalent to approximately 2 dB of shot noise reduction. The enhancement is limited by the loss within the interferometer. Injection of the squeezed state did not show any degradation in the sensitivity of the device below 200Hz, indicating for the first time that noise couplings between the interferometer and the squeezer could be sufficiently suppressed at these frequencies even for the most sensitive detectors.