We have observed condensation of fermionic atom pairs in the BCS-BEC crossover regime. A trapped gas of fermionic 40K atoms is evaporatively cooled to quantum degeneracy and then a magnetic-field Feshbach resonance is used to control the atom-atom interactions. The location of this resonance is precisely determined from low-density measurements of molecule dissociation. In order to search for condensation on either side of the resonance, we introduce a technique that pairwise projects fermionic atoms onto molecules; this enables us to measure the momentum distribution of fermionic atom pairs. The transition to condensation of fermionic atom pairs is mapped out as a function of the initial atom gas temperature T compared to the Fermi temperature T(F) for magnetic-field detunings on both the BCS and BEC sides of the resonance.

The realization of superfluidity in a dilute gas of fermionic atoms, analogous to superconductivity in metals, represents a long-standing goal of ultracold gas research. In such a fermionic superfluid, it should be possible to adjust the interaction strength and tune the system continuously between two limits: a Bardeen-Cooper-Schrieffer (BCS)-type superfluid (involving correlated atom pairs in momentum space) and a Bose-Einstein condensate (BEC), in which spatially local pairs of atoms are bound together. This crossover between BCS-type superfluidity and the BEC limit has long been of theoretical interest, motivated in part by the discovery of high-temperature superconductors. In atomic Fermi gas experiments superfluidity has not yet been demonstrated; however, long-lived molecules consisting of locally paired fermions have been reversibly created. Here we report the direct observation of a molecular Bose-Einstein condensate created solely by adjusting the interaction strength in an ultracold Fermi gas of atoms. This state of matter represents one extreme of the predicted BCS-BEC continuum.

Following the realization of Bose-Einstein condensates in atomic gases, an experimental challenge is the production of molecular gases in the quantum regime. A promising approach is to create the molecular gas directly from an ultracold atomic gas; for example, bosonic atoms in a Bose-Einstein condensate have been coupled to electronic ground-state molecules through photoassociation or a magnetic field Feshbach resonance. The availability of atomic Fermi gases offers the prospect of coupling fermionic atoms to bosonic molecules, thus altering the quantum statistics of the system. Such a coupling would be closely related to the pairing mechanism in a fermionic superfluid, predicted to occur near a Feshbach resonance. Here we report the creation and quantitative characterization of ultracold 40K2 molecules. Starting with a quantum degenerate Fermi gas of atoms at a temperature of less than 150 nK, we scan the system over a Feshbach resonance to create adiabatically more than 250,000 trapped molecules; these can be converted back to atoms by reversing the scan. The small binding energy of the molecules is controlled by detuning the magnetic field away from the Feshbach resonance, and can be varied over a wide range. We directly detect these weakly bound molecules through their radio-frequency photodissociation spectra; these probe the molecular wavefunction, and yield binding energies that are consistent with theory.

Converting low-frequency electrical signals into much higher frequency optical signals has enabled modern communications networks to leverage both the strengths of microfabricated electrical circuits and optical fiber transmission, allowing information networks to grow in size and complexity. A microwave-to-optical converter in a quantum information network could provide similar gains by linking quantum processors via low-loss optical fibers and enabling a large-scale quantum network. However, no current technology can convert low-frequency microwave signals into high-frequency optical signals while preserving their fragile quantum state. For this demanding application, a converter must provide a near-unitary transformation between different frequencies; that is, the ideal transformation is reversible, coherent, and lossless. Here we demonstrate a converter that reversibly, coherently, and efficiently links the microwave and optical portions of the electromagnetic spectrum. We use our converter to transfer classical signals between microwave and optical light with conversion efficiencies of ∼10%, and achieve performance sufficient to transfer quantum states if the device were further precooled from its current 4 kelvin operating temperature to below 40 millikelvin. The converter uses a mechanically compliant membrane to interface optical light with superconducting microwave circuitry, and this unique combination of technologies may provide a way to link distant nodes of a quantum information network. IntroductionModern communication networks manipulate information at several gigahertz with microprocessors and distribute information at hundreds of terahertz via optical fibers. A similar frequency dichotomy is developing in quantum information processing. Superconducting qubits operating at several gigahertz have recently emerged as promising high-fidelity and intrinsically scalable quantum processors [1][2][3]. Conversely, optical frequencies provide access to low-loss transmission [4] and long-lived quantum-compatible storage [5,6]. Converting information between gigahertz-frequency "microwave light" that can be deftly manipulated and terahertzfrequency "optical light" that can be efficiently distributed will enable small-scale quantum systems [7][8][9] to be combined into larger, fully-functional quantum networks [10,11]. But no current technology can transform information between these vastly different frequencies while preserving the fragile quantum state of the information. For this demanding application, a frequency converter must provide a near-unitary transformation between microwave light and optical light; that is, the ideal transformation is reversible, coherent, and lossless.Certain nonlinear materials provide a link between microwave and optical light, and these are commonly used in electro-optic modulators (EOMs) for just this purpose. While EOMs might be capable of reversible frequency conversion [12,13], such conversion has not yet been demonstrated, and even optimized EOMs [14,15] have predicted ...

We have created spatial dark solitons in two-component Bose-Einstein condensates in which the soliton exists in one of the condensate components and the soliton nodal plane is filled with the second component. The filled solitons are stable for hundreds of milliseconds. The filling can be selectively removed, making the soliton more susceptible to dynamical instabilities. For a condensate in a spherically symmetric potential, these instabilities cause the dark soliton to decay into stable vortex rings. We have imaged the resulting vortex rings.

Recently, remarkable advances have been made in coupling a number of high-Q modes of nano-mechanical systems to high-finesse optical cavities, with the goal of reaching regimes in which quantum behavior can be observed and leveraged toward new applications. To reach this regime, the coupling between these systems and their thermal environments must be minimized. Here we propose a novel approach to this problem, in which optically levitating a nano-mechanical system can greatly reduce its thermal contact, while simultaneously eliminating dissipation arising from clamping. Through the long coherence times allowed, this approach potentially opens the door to ground-state cooling and coherent manipulation of a single mesoscopic mechanical system or entanglement generation between spatially separate systems, even in room-temperature environments. As an example, we show that these goals should be achievable when the mechanical mode consists of the center-of-mass motion of a levitated nanosphere.ne of the most intriguing questions associated with quantum theory is whether effects such as quantum coherence and entanglement can be observed at mesoscopic or macroscopic scales. As a first step toward resolving this question, recently much effort has been directed toward quantum state preparation of high-Q modes of nano-and micro-mechanical oscillators-in particular, cooling such modes to their quantum ground state (1). Reaching a regime in which quantum properties such as entanglement (2) emerge is not only of fundamental interest but could lead to new applications in fields such as ultrasensitive detection (3, 4) and quantum information science (5, 6). To reach this regime, it is critical that the thermalization and decoherence rates of these systems be minimized by reducing the coupling to their thermal reservoirs. Thus far, this has necessitated the use of cryogenic operating environments. From an engineering standpoint, it would also be desirable to reduce the dissipation and thermalization rates of these systems through their clamping and material supports (7), so that these rates might approach their fundamental material limits (8).Here we propose a unique approach toward this problem, wherein the material supports are completely eliminated by optically levitating (9) a nano-mechanical system inside a FabryPerot optical cavity. Indeed, since the pioneering work of Ashkin on optical trapping of dielectric particles (9) (in the classical domain), it has been realized that levitation under good vacuum conditions can lead to extremely low mechanical damping rates (10, 11). We show that such an approach should also facilitate the emergence of quantum behavior even in room-temperature environments, when the particles are of subwavelength scale such that the effects of recoil heating due to scattered photons become small. As a specific example, we show that the center-ofmass (CM) motion of a levitated nanosphere can be optically self-cooled (12-14) to the ground state starting from room temperature. This system constitutes ...

In recent years microfabricated microwave cavities have been extremely successful in a wide variety of detector applications. In this article we focus this technology on the challenge of quantum-limited displacement detection of a macroscopic object. We measure the displacement of a nanomechanical beam by capacitively coupling its position to the resonant frequency of a superconducting transmission-line microwave cavity. With our device we realize near state-of-the-art mechanical force sensitivity (3 aN/ √ Hz) and thus add to only a handful of techniques able to measure thermomechanical motion at 10's of milliKelvin temperatures. Our measurement imprecision reaches a promising 30 times the expected imprecision at the standard quantum limit, and we quantify our ability to extract measurement backaction from our results as well as elucidate the important steps that will be required to progress towards the full quantum limit with this new detector.The advent of micro and nanomechanical resonators has brought the long-standing goal of exploring quantum effects such as superposition and entanglement of macroscopic objects closer to reality. With this ability one could experimentally study decoherence of superposition states thus elucidating questions about the interface between the quantum and classical worlds. Micro and nanomechanical resonators have hastened progress towards macroscopic quantum limits by providing high-frequency, small-dissipation, yet lowmass resonators. Still it remains a challenge to freeze out the thermomechanical motion of these objects to leave only zeropoint fluctuations δx zp and, equally importantly, to detect motion at this level. The problem of detection at the quantum limit is in itself intriguing. As the imprecision of any detector is decreased, measurement backaction emerges to enforce the Heisenberg constraint, which for continuous displacement detection is S im x S ba F ≥ 2 . Here S im x and S ba F are respectively the displacement imprecision and backaction force spectral densities. In fact, at the minimum allowed total position uncertainty, referred to as the standard quantum limit (SQL), the measurement imprecision and backaction must together contribute an uncertainty equal to the zero-point fluctuations.A widely used displacement detector that, in principle, is capable of reaching the SQL is an optical cavity interferometer with a moving mirror [1]. Physically, at the SQL ones knowledge of the mirror position is limited equally by shot noise in the output signal and by motion of the mirror due to quantum fluctuations in the intracavity radiation pressure. This limit has long been of interest in quantum optics and in the gravitational wave detection community. Optical cavities generally outperform all other displacement detectors with regard to measurement imprecision; they can achieve shot-noise limited position sensitivity as low as ∼ 10. Still reaching the SQL has historically been a challenge due to the inaccessibility of quantum backaction effects [3,4]. Recent experimental...

We have measured a p-wave Feshbach resonance in a single-component, ultracold Fermi gas of 40K atoms. We have used this resonance to enhance the normally suppressed p-wave collision cross section to values larger than the background s-wave cross section between 40K atoms in different spin states. In addition to the modification of two-body elastic processes, the resonance dramatically enhances three-body inelastic collisional loss.