J. G. E. Harris

Strong dispersive coupling of a high-finesse cavity to a micromechanical membrane.

Macroscopic mechanical objects and electromagnetic degrees of freedom can couple to each other through radiation pressure. Optomechanical systems in which this coupling is sufficiently strong are predicted to show quantum effects and are a topic of considerable interest. Devices in this regime would offer new types of control over the quantum state of both light and matter, and would provide a new arena in which to explore the boundary between quantum and classical physics. Experiments so far have achieved sufficient optomechanical coupling to laser-cool mechanical devices, but have not yet reached the quantum regime. The outstanding technical challenge in this field is integrating sensitive micromechanical elements (which must be small, light and flexible) into high-finesse cavities (which are typically rigid and massive) without compromising the mechanical or optical properties of either. A second, and more fundamental, challenge is to read out the mechanical element’s energy eigenstate. Displacement measurements (no matter how sensitive) cannot determine an oscillator’s energy eigenstate, and measurements coupling to quantities other than displacement have been difficult to realize in practice. Here we present an optomechanical system that has the potential to resolve both of these challenges. We demonstrate a cavity which is detuned by the motion of a 50-nm-thick dielectric membrane placed between two macroscopic, rigid, high-finesse mirrors. This approach segregates optical and mechanical functionality to physically distinct structures and avoids compromising either. It also allows for direct measurement of the square of the membrane’s displacement, and thus in principle the membrane’s energy eigenstate. We estimate that it should be practical to use this scheme to observe quantum jumps of a mechanical system, an important goal in the field of quantum measurement.

Persistent currents in normal metal rings.

Normally Persistent In superconductors, currents are expected to flow persistently without dissipation. Quantum mechanics predicts that such persistent currents should also exist in normal mesoscopic metal rings. However, the predicted effect is small, which has made the detection of these currents difficult. Bleszynski-Jayich et al. (p. 272; see the Perspective by Birge) have developed a sensitive technique based on a nanomechanical resonator. An array of aluminum rings on the end of a resonator was fabricated to monitor the shift in frequency of the resonator as the rings were threaded with quanta of magnetic-field flux, setting up currents in the rings. In agreement with a theoretical scenario put forward over a decade ago, the results could be described with a model based on non-interacting electrons. A nanomechanical resonator is used to detect weak persistent currents that flow in resistive metal rings Quantum mechanics predicts that the equilibrium state of a resistive metal ring will contain a dissipationless current. This persistent current has been the focus of considerable theoretical and experimental work, but its basic properties remain a topic of controversy. The main experimental challenges in studying persistent currents have been the small signals they produce and their exceptional sensitivity to their environment. We have developed a technique for detecting persistent currents that allows us to measure the persistent current in metal rings over a wide range of temperatures, ring sizes, and magnetic fields. Measurements of both a single ring and arrays of rings agree well with calculations based on a model of non-interacting electrons.

Coherent Sensing of a Mechanical Resonator with a Single-Spin Qubit

The spin of a nitrogen vacancy defect in diamond is used to sense the motion a magnetized microresonator. Mechanical systems can be influenced by a wide variety of small forces, ranging from gravitational to optical, electrical, and magnetic. When mechanical resonators are scaled down to nanometer-scale dimensions, these forces can be harnessed to enable coupling to individual quantum systems. We demonstrate that the coherent evolution of a single electronic spin associated with a nitrogen vacancy center in diamond can be coupled to the motion of a magnetized mechanical resonator. Coherent manipulation of the spin is used to sense driven and Brownian motion of the resonator under ambient conditions with a precision below 6 picometers. With future improvements, this technique could be used to detect mechanical zero-point fluctuations, realize strong spin-phonon coupling at a single quantum level, and implement quantum spin transducers. Quantum Mechanical Coupling Observing the induced patterns of iron filings as a magnet is moved nearby, is a mainstay experiment of elementary science kits. Scaling down to the motion of the magnet and the size of the “sensing” particles enters the realm of quantum nanomechanics, where the motion of the vibrating system is quantized. That motion, however, is difficult to observe and manipulate. Kolkowitz et al. (p. 1636, published online 23 February; see the Perspective by Treutlein) coupled the single-mode vibration of a magnetized nanomechanical resonator to the quantum mechanical two-level spin system associated with the nitrogen vacancy center in diamond. The evolution of the spin degree of freedom was directly mapped to the mechanical motion, providing the opportunity to probe minute mechanical motion that would otherwise be undetectable.

Strong and tunable nonlinear optomechanical coupling in a low-loss system

An optical cavity coupled to a micrometre-sized mechanical resonator offers the opportunity to see quantum effects in relatively large structures. It is now shown that a variety of coupling mechanisms enable investigation of these fascinating systems in a number of different ways.

Dynamical Multistability Induced by Radiation Pressure in High-Finesse Micromechanical Optical Cavities

We analyze the nonlinear dynamics of a high-finesse optical cavity in which one mirror is mounted on a flexible mechanical element. We find that this system is governed by an array of dynamical attractors, which arise from phase locking between the mechanical oscillations of the mirror and the ringing of the light intensity in the cavity. We develop an analytical theory to map out the diagram of attractors in parameter space, derive the slow amplitude dynamics of the system, including thermal fluctuations, and suggest a scheme for exploiting the dynamical multistability in the measurement of small displacements.

High quality mechanical and optical properties of commercial silicon nitride membranes

We have measured the optical and mechanical loss of commercial silicon nitride membranes. We find that 50nm thick, 1mm2 membranes have mechanical Q>106 at 293K, and Q>107 at 300mK, well above what has been observed in devices with comparable dimensions. The near-IR optical loss at 293K is less than 2×10−4. This combination of properties make these membranes attractive candidates for studying quantum effects in optomechanical systems.

Fiber-cavity-based optomechanical device

We describe an optomechanical device consisting of a fiber-based optical cavity containing a silicon nitride membrane. In comparison with typical free-space cavities, the fiber-cavitys small mode size (10 μm waist, 80 μm length) allows the use of smaller, lighter membranes and increases the cavity-membrane linear coupling to 3 GHz/nm and the quadratic coupling to 20 GHz/nm2. This device is also intrinsically fiber-coupled and uses glass ferrules for passive alignment. These improvements will greatly simplify the use of optomechanical systems, particularly in cryogenic settings. At room temperature, we expect these devices to be able to detect the shot noise of radiation pressure.

Optically mediated hybridization between two mechanical modes.

In this Letter we study a system consisting of two nearly degenerate mechanical modes that couple to a single mode of an optical cavity. We show that this coupling leads to nearly complete (99.5%) hybridization of the two mechanical modes into a bright mode that experiences strong optomechanical interactions and a dark mode that experiences almost no optomechanical interactions. We use this hybridization to transfer energy between the mechanical modes with 40% efficiency.

Excess spin and the dynamics of antiferromagnetic ferritin

Temperature-dependent magnetization measurements on a series of synthetic ferritin proteins containing from 100 to 3000 Fe(III) ions are used to determine the uncompensated moment of these antiferromagnetic particles. The results are compared with recent theories of macroscopic quantum coherence which explicitly include the effect of this excess moment. The scaling of the excess moment with protein size is consistent with a simple model of finite-size effects and sublattice noncompensation.

Integrated micromechanical cantilever magnetometry of Ga1−xMnxAs

We have developed a technique for fabricating submicron GaAs micromechanical cantilevers into which lithographically patterned samples grown by molecular beam epitaxy or evaporative deposition are integrated. The torque sensitivity of the 100-nm-thick cantilevers makes them ideal for torsional magnetometry of nanometer-scale, anisotropic samples. We present measurements on samples of the ferromagnetic semiconductor Ga1−xMnxAs at temperatures from 350 mK to 65 K and in fields from 0 to 8 T. By measuring the shift in the resonant frequency of the cantilevers, we demonstrate a moment sensitivity of 3×106 μB at 0.1 T, an improvement of nearly five orders of magnitude upon existing torsional magnetometers.