Quantum noise places a fundamental limit on the per photon sensitivity attainable in optical measurements. This limit is of particular importance in biological measurements, where the optical power must be constrained to avoid damage to the specimen. By using non-classically correlated light, we demonstrated that the quantum limit can be surpassed in biological measurements. Quantum enhanced microrheology was performed within yeast cells by tracking naturally occurring lipid granules with sensitivity 2.4 dB beyond the quantum noise limit. The viscoelastic properties of the cytoplasm could thereby be determined with a 64% improved measurement rate. This demonstration paves the way to apply quantum resources broadly in a biological context

A cavity optomechanical magnetometer is demonstrated. The magnetic field induced expansion of a magnetostrictive material is resonantly transduced onto the physical structure of a highly compliant optical microresonator, and read-out optically with ultra-high sensitivity. A peak magnetic field sensitivity of 400 nT Hz −1/2 is achieved, with theoretical modeling predicting the possibility of sensitivities below 1 pT Hz −1/2 . This chipbased magnetometer combines high-sensitivity and large dynamic range with small size and room temperature operation.Ultra-low field magnetometers are essential components for a wide range of practical applications including geology, mineral exploration, archaeology, defence and medicine [1]. The field is dominated by superconducting quantum interference devices (SQUIDs) operating at cryogenic temperatures [2]. Magnetometers capable of room temperature operation offer significant advantages both in terms of operational costs and range of applications. The state-of-the-art are magnetostrictive magnetometers with sensitivities in the range of fT Hz −1/2 [3, 4], and atomic magnetometers which achieve impressive sensitivities as low as 160 aT Hz −1/2 [5] but with limited dynamic range due to the nonlinear Zeeman effect [2,6]. Recently, significant effort has been made to miniaturize room temperature magnetometers. However both atomic and magnetostrictive magnetometers remain generally limited to millimeter or centimeter size scales. Smaller microscale magnetometers have many potential applications in biology, medicine, and condensed matter physics [7,8]. A particularly important application is magnetic resonance imaging, where by placing the magnetometer in close proximity to the sample both sensitivity and resolution may be enhanced [9], potentially enabling detection of nuclear spin noise [10], imaging of neural networks [7], and advances in areas of medicine such as magneto-cardiography[1, 6] and magneto-encephalography [11].In the past few years, rapid progress has been achieved on NV center based magnetometers. They combine sensitivities as low as 4 nT Hz −1/2 with room temperature operation, optical readout and nanoscale size [12] and are predicted theoretically to reach the fT Hz −1/2 range [13]. This has allowed three-dimensional magnetic field imaging at the micro scale using ensembles of NV-centers [7], and magnetic resonance [14] and field imaging[13] at the nanoscale using single NV centers. In spite of these extraordinary achievements applications are hampered by fabrication issues and the intricacy of the read-out schemes [15]. Furthermore miniaturization is limitied by the bulky read-out optics, the magnetic field coils for state preparation and the microwave excitation device [7].In this letter we present the concept of a cavity optomechanical field sensor which combines room temperature operation and high sensitivity with large dynamic range and small size. The sensor leverages results from the emergent field of cavity optomechanics where ultra-sensitive force and positi...

Chapter 10 described the development of an optical tweezers apparatus with quantum enhanced sensitivity. This chapter applies this device to biophysical experiments. The thermal motion of lipid particles within a living yeast cell was characterized with quantum enhanced precision, and from this the mechanical properties of the cellular cytoplasm could be inferred. The use of squeezed light improved the particle tracking precision by 2.4 dB, which improved the precision with which the α parameter could be determined by 22 %. This demonstrated for the first time that quantum correlated light could be used to surpass the quantum noise limit in biological measurements. This experiment was described in the following publication [18]. MicrorheologyThis chapter describes quantum enhanced microrheology measurements of the cytoplasm within a living yeast cell. In microrheology experiments, the viscoelasticity of a fluid is determined from its influence on the motion of an embedded particle [6,10]. This can involve measuring the mechanical response to either an applied force or the thermal force, which continually pushes the particle in random directions. To infer useful information from the thermal diffusion of the particle, the key parameter of interest is generally the mean squared displacement (MSD). The MSD of a free particle undergoing thermal motion is defined aswhere τ is the delay between measurements. The MSD thus characterizes the average distance that a particle will move over a given time range τ , and provided the MSD is dominated by thermal motion, has the form (11.2)

We implement a cavity opto-electromechanical system integrating electrical actuation capabilities of nanoelectromechanical devices with ultrasensitive mechanical transduction achieved via intra-cavity optomechanical coupling. Electrical gradient forces as large as 0.40 µN are realized, with simultaneous mechanical transduction sensitivity of 1.5×10−18 m Hz −1/2 representing a three orders of magnitude improvement over any nanoelectromechanical system to date. Opto-electromechanical feedback cooling is demonstrated, exhibiting strong squashing of the in-loop transduction signal. Out-of-loop transduction provides accurate temperature calibration even in the critical paradigm where measurement backaction induces opto-mechanical correlations.Mechanical oscillators are predicted to exhibit striking quantum behavior [1]; enabling experimental tests of longstanding scientific problems such as quantum gravity [2] and quantum nonlinear dynamics [3][4][5], as well as farreaching applications in metrology [6] and quantum information systems [7]. Rapid progress towards this quantum regime is underway in both cavity optomechanical systems (COMS) [8] and nanoelectromechanical systems (NEMS) [9,10]. COMS enable ultrasensitive transduction of the mechanical motion, presenting a solution to the key challenge of resolving the oscillators quantum zero-point fluctuations [11]. To date, however, mechanical actuation in COMS has been achieved via radiation pressure [11][12][13], which is inherently weak and severely constrained in the quantum regime by heating from intra-cavity optical absorption [11]. The electrical actuation of NEMS, by comparison, can be orders of magnitude stronger and is far less prone to heating [10,14,15]; providing access to nonlinear mechanical behavior [3], as well as greater scope for quantum control and cooling [10,14,16].Recently, a non-dissipative electrical actuation technique using localized gradient forces has been developed for dielectric NEMS [16]. In this Letter we report a cavity optoelectromechanical system (COEMS) which extends this technique to COMS based on silica microtoroids on a silicon chip. The microtoroid structure integrates high quality optical and mechanical resonances; while the dielectric nature of silica is naturally suited to gradient force actuation [16]. Electrical gradient forces as large a 0.40 µN are achieved, enabling strong mechanical actuation without observable heating effects. Simultaneously, ultrasensitive optical transduction is implemented at the level of 1.5 × 10 −18 m Hz −1/2 close to the mechanical zero-point motion and surpassing the current state-of-the-art in NEMS by three orders of magnitude [17].Electo-mechanical actuation and opto-mechanical transduction, when combined within a feedback loop, allow immediate control of the state of the mechanical oscillator; with the capacity to facilitate, for example, feedback cooling or heating [12], electro-optic spring effects [18], and phonon lasing [19]. Here, feedback cooling is implemented as a demonstration. All pr...