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

The Einstein-Podolsky-Rosen (EPR) paradox [1] established a link between entanglement [2,3] and nonlocality in quantum mechanics [4]. EPR steering [5][6][7][8] is the nonlocality associated with the EPR paradox and has traditionally only been investigated between two parties [9][10][11][12][13][14][15]. Here, we present the first experimental observations of multipartite EPR steering, and of the genuine tripartite continuous variable entanglement of three mesoscopic optical systems [16][17][18]. We explore different linear optics networks -each one with optimised asymmetries -that create multipartite steerable states containing different numbers of quantised optical modes (qumodes). By introducing asymmetric loss on a 7-qumode state, we characterize 8 regimes of directional steering, showing that N + 1 regimes exist for an N -qumode state. Further, we reveal the directional monogamy of steering, and experimentally demonstrate continuous variable one-sided semi device-independent quantum secret sharing [19]. Our methods establish principles for the development of multiparty quantum communication protocols with asymmetric observers, and can be extended to qubits, whether photonic [12][13][14][15][16]20], atomic [21], superconducting [22], or otherwise.Schrödinger introduced the term "steering" to describe the nonlocality apparent in the EPR paradox, and pointed out that these states involve a quantum property called "entanglement" [2,5]. Wiseman et al [6,7] have formalised the meaning of steering in terms of violations of local hidden state models, and revealed that the EPR paradox is a manifestation of quantum steering. In simple terms, quantum steering dictates that measurements made by one observer can apparently "steer" (alter) the state of another observer at a different location.The observation of multipartite EPR steering has not been possible until recently as the framework necessary to understand the concept has only just been developed [6][7][8]23]. The motivation to expand this framework arises from considerations of real-world quantum networks, such as the quantum internet [24], for which security and privacy are of paramount importance. Here, we expand on the theoretical framework and derive optimised criteria to detect multipartite EPR steering using linear optical circuits. The criteria involve the canonical position and momentum observables, which are realised in our experiment as highly efficient quadrature phase amplitude measurements. Following the criteria, we present the first experimental investigation of multipartite EPR steering, including demonstration of directional monogamy relations which give bounds on the way steering is distributed among the different parties. Further, we demonstrate the principle of one-sided deviceindependent quantum secret sharing and in doing so confirm for the first time the continuous variable genuine tripartite entanglement of three optical modes. For bipartite EPR states, there are 3 different regimes: 2-way, 1-way, and no-way steering [25,26]. In general, for...

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)

Entanglement between large numbers of quantum modes is the quintessential resource for future technologies such as the quantum internet. Conventionally, the generation of multimode entanglement in optics requires complex layouts of beamsplitters and phase shifters in order to transform the input modes into entangled modes. Here we report the highly versatile and efficient generation of various multimode entangled states with the ability to switch between different linear optics networks in real time. By defining our modes to be combinations of different spatial regions of one beam, we may use just one pair of multi-pixel detectors in order to measure multiple entangled modes. We programme virtual networks that are fully equivalent to the physical linear optics networks they are emulating. We present results for N=2 up to N=8 entangled modes here, including N=2, 3, 4 cluster states. Our approach introduces the highly sought after attributes of flexibility and scalability to multimode entanglement.

Free space propagation and conventional optical systems such as lenses and mirrors all perform spatial unitary transforms. However, the subset of transforms available through these conventional systems is limited in scope. We present here a unitary programmable mode converter (UPMC) capable of performing any spatial unitary transform of the light field. It is based on a succession of reflections on programmable deformable mirrors and free space propagation. We first show theoretically that a UPMC without limitations on resources can perform perfectly any transform. We then build an experimental implementation of the UPMC and show that, even when limited to three reflections on an array of 12 pixels, the UPMC is capable of performing single mode tranforms with an efficiency greater than 80% for the first four modes of the transverse electromagnetic basis.

Nonlocal correlations, a longstanding foundational topic in quantum information, have recently found application as a resource for cryptographic tasks where not all devices are trusted, for example, in settings with a highly secure central hub, such as a bank or government department, and less secure satellite stations, which are inherently more vulnerable to hardware "hacking" attacks. The asymmetric phenomena of Einstein-Podolsky-Rosen (EPR) steering plays a key role in one-sided device-independent (1sDI) quantum key distribution (QKD) protocols. In the context of continuous-variable (CV) QKD schemes utilizing Gaussian states and measurements, we identify all protocols that can be 1sDI and their maximum loss tolerance. Surprisingly, this includes a protocol that uses only coherent states. We also establish a direct link between the relevant EPR steering inequality and the secret key rate, further strengthening the relationship between these asymmetric notions of nonlocality and device independence. We experimentally implement both entanglement-based and coherent-state protocols, and measure the correlations necessary for 1sDI key distribution up to an applied loss equivalent to 7.5 and 3.5 km of optical fiber transmission, respectively. We also engage in detailed modeling to understand the limits of our current experiment and the potential for further improvements. The new protocols we uncover apply the cheap and efficient hardware of CV-QKD systems in a significantly more secure setting.

An integrated optical chip is used for generating, manipulating, and detecting squeezed vacuum and two-mode entanglement.

We report both sub-diffraction-limited quantum metrology and quantum enhanced spatial resolution for the first time in a biological context. Nanoparticles are tracked with quantum correlated light as they diffuse through an extended region of a living cell in a quantum enhanced photonic force microscope. This allows spatial structure within the cell to be mapped at length scales down to 10 nm. Control experiments in water show a 14% resolution enhancement compared to experiments with coherent light. Our results confirm the longstanding prediction that quantum correlated light can enhance spatial resolution at the nanoscale and in biology. Combined with state-of-the-art quantum light sources, this technique provides a path towards an order of magnitude improvement in resolution over similar classical imaging techniques.