The spectral purity of an oscillator is central to many applications, such as detecting gravity waves, defining the second, ground-state cooling and quantum manipulation of nanomechanical objects, and quantum computation. Recent proposals suggest that laser oscillators which use very narrow optical transitions in atoms can be orders of magnitude more spectrally pure than present lasers. Lasers of this high spectral purity are predicted to operate deep in the 'bad-cavity', or superradiant, regime, where the bare atomic linewidth is much less than the cavity linewidth. Here we demonstrate a Raman superradiant laser source in which spontaneous synchronization of more than one million rubidium-87 atomic dipoles is continuously sustained by less than 0.2 photons on average inside the optical cavity. By operating at low intracavity photon number, we demonstrate isolation of the collective atomic dipole from the environment by a factor of more than ten thousand, as characterized by cavity frequency pulling measurements. The emitted light has a frequency linewidth, measured relative to the Raman dressing laser, that is less than that of single-particle decoherence linewidths and more than ten thousand times less than the quantum linewidth limit typically applied to 'good-cavity' optical lasers, for which the cavity linewidth is much less than the atomic linewidth. These results demonstrate several key predictions for future superradiant lasers, which could be used to improve the stability of passive atomic clocks and which may lead to new searches for physics beyond the standard model.

Many quantum information processing protocols require efficient transfer of quantum information from a flying photon to a stationary quantum system 1-3 . To transfer information, a photon must first be absorbed by the quantum system. This can be achieved, with a probability close to unity, by an atom residing in a high-finesse cavity 1 . However, it is unclear whether a photon can be absorbed effectively by an atom in a free space. Here, we report on an observation of substantial extinction of a light beam by a single 87 Rb atom through focusing light to a small spot with a single lens. The measured extinction values can be directly compared to the predictions of existing free-space photon-atom coupling models 4-6 . Our result should open a new perspective on processing quantum information carried by light using atoms, in particular for experiments that require strong absorption of single photons by an atom in free space.Strong interaction between light and matter is essential for successful operation of many quantum information protocols such as quantum networking 1,2 , entanglement swapping between two distant atoms 3,7,8 and implementation of elementary quantum gates 9 . These protocols consider quantum states of localized carriers (nodes), such as atoms, ions or even atomic ensembles, that exchange information through a quantum channel with the help of 'flying' qubits (photons). The quantum channels can be implemented via well-defined photonic modes that couple the nodes with high efficiency. For example, in the original proposal for quantum networks 1 , atoms were placed in high-finesse cavities that not only provide a strong interaction between a photon and an atom, but also ensure that most of the spontaneously emitted photons are collected into the same mode. Experimental advances in atom-photon cavity quantum electrodynamics indeed enabled the information exchange between an atom and single photons in this configuration to be carried out with high efficiency 10-14 . However, scaling such a scheme to many localized nodes is experimentally difficult, because managing the losses and coupling of the intracavity field of high-Q cavities to propagating modes of flying qubits is already quite challenging.In an attempt to avoid the complications connected with cavities, an interface between stationary and flying qubits in a simpler free-space configuration could be considered, where the quantum channel is defined, for example, by a Gaussian mode of a single-mode optical fibre, and a single atom is strongly coupled to this mode with the help of a large-numerical-aperture lens. Indeed, the common model describing the interaction of a monochromatic plane wave with a two-level atom predicts a scattering cross-section of σ = 3l 2 /2π. This area is close to a diffraction-limited spot size of a lens with a large numerical aperture (NA), hence suggesting a high coupling efficiency for such a system. Coupling efficiency here refers to the absorption probability of a flying photon by a stationary quantum system. For a fr...

We use the vacuum Rabi splitting to perform quantum nondemolition (QND) measurements that prepare a conditionally spin-squeezed state of a collective atomic psuedo-spin. We infer a 3.4(6) dB improvement in quantum phase estimation relative to the standard quantum limit for a coherent spin state composed of uncorrelated atoms. The measured collective spin is composed of the twolevel clock states of nearly 10 6 87 Rb atoms confined inside a low finesse F = 710 optical cavity. This technique may improve atomic sensor precision and/or bandwidth, and may lead to more precise tests of fundamental physics.PACS numbers: 42.50.Pq, 42.50.Dv, 37.30.+i, Large ensembles of uncorrelated atoms are extensively used as precise sensors of time, rotation, and gravity, and for tests of fundamental physics [1][2][3][4]. The quantum nature of the sensors imposes a limit on their ultimate precision. Larger ensembles of N atoms can be used to average the quantum noise as 1/ √ N , a scaling known as the standard quantum limit. However, the ensemble size is limited by both technical constraints and atom-atom collisions-a fundamental distinction from photon-based sensors. Learning to prepare entangled states of large ensembles with noise properties below the standard quantum limit will be key to extending both the precision [5] and/or bandwidth [6] of atomic sensors. More broadly, the generation and application of entanglement to solve problems is a core goal of quantum information science being pursued in both atomic and solid state systems.In this Letter, we utilize the tools of cavity-QED to prepare an entangled ensemble with a 3.4(6) dB improvement in spectroscopic sensitivity over the standard quantum limit. The method does not require single particle addressability and is applied to a spectroscopically large ensemble of N = 7 × 10 5 atoms using a single < 200 µs operation. The gain in sensitivity is spectroscopically equivalent to the enhancement obtained had we created > 10 5 pairs of maximally entangled qubits, demonstrating the power of a top-down approach for entangling large ensembles. The probing of atomic populations via the vacuum Rabi splitting is also of broad interest for nondestructively reading out a wide variety of both atomic and solid state qubits.The large ensemble size is a crucial component. Entangled states of cold, neutral atoms are unlikely to impact the future of quantum sensors and tests of fundamental physics unless the techniques for generating the states are demonstrated to work for the 10 4 to 10 7 neutral atom ensembles typically used in primary frequency standards [7] and atom interferometers [2,4].The approach described here allows quantum-noise limited readout of a sensor with < 0.2 photon recoils/atom, producing little heating of the atomic ensemble. Applied to a state-of-the-art optical lattice clock, the resulting enhanced measurement rates will suppress the dominant aliasing of the local oscillator noise [1,8].The gain in spectroscopic sensitivity demonstrated here is far from the fundamental Hei...

Collective measurements can project a system into an entangled state with enhanced sensitivity for measuring a quantum phase, but measurement backaction has limited previous efforts to only modest improvements. Here we use a collective measurement to produce and directly observe, with no background subtraction, an entangled, spin-squeezed state with phase resolution improved in variance by a factor of 10.5(1.5), or 10.2(6) dB, compared to the initially unentangled ensemble of N = 4.8 × 10 5 87 Rb atoms. The measurement uses a cavity-enhanced probe of an optical cycling transition to mitigate back-action associated with state-changing transitions induced by the probe. This work establishes collective measurements as a powerful technique for generating entanglement for precision measurement, with potential impacts in biological sensing, communication, navigation, and tests of fundamental physics.A defining characteristic of quantum mechanics is the ability of a measurement to change the state of the system being measured. For example, a measurement of a system in a super-1 arXiv:1310.3177v1 [quant-ph] 11 Oct 2013 position of two states causes the system to project, or collapse, into one of the two discrete states. Measurements performed on an ensemble, however, can project the ensemble into an entangled state when only collective quantities are measured. For instance, here we measure a cavity field that is entangled with the total number of spin-1/2 atoms in spin up (Fig. 1A). Any information about the spin-state of a single atom that leaks to the environment due to imperfections in the collective measurement reduces entanglement due to collapse of individual atoms.Such collective or joint measurements arise in a wide range of applications, including quantum teleportation (1), quantum information protocols (2), studies of strongly-correlated quantum systems (3), Dicke superradiance (4), and entanglement generation in optical (5), solid state (6) and atomic systems (7).Entanglement generated by a collective measurement can be used to overcome the fundamental quantum randomness that limits a diverse set of precision measurements (8). Atomic sensors in particular are nearly or already limited by quantum noise, so entanglement-enhanced metrology would improve some of the most precise measurements of external fields (9), rotations (10), and time (11), and will advance searches for new physics (12). Atomic sensors encode their information in a quantum phase θ, whose value is estimated by measuring the population of atoms in different quantum states. Quantum projection noise (13) for an ensemble of N independent atoms limits the uncertainty in the estimate of θ to a variance ∆θ 2 ≥ ∆θ 2 SQL = 1/N , a limit known as the standard quantum limit (SQL) for a coherent spin state (CSS). Entanglement can be used to bypass this limitation in atomic sensors, as well as in microwave (14) and optical (15) fields.A collective measurement that both resolves the quantum noise that appears in θ and induces sufficiently small measuremen...