Session J27: Focus Session: Nano/Optomechanics I

Nanomechanics and superconducting qubits for quantum information

There has been tremendous progress in the capabilities of superconducting quantum circuits, both for fundamental quantum science as well as for applications in quantum information. Superconducting qubits are based on the Josephson junction, which provides the fundamental inductive nonlinearity that affords full quantum control of otherwise quite simple electrical circuits. I will outline how a superconducting qubit can be used to measure and control the quantum state of a nanomechanical system [1], completely control multi-photon states in superconducting resonators [2,3], factor the number 15 using a von Neumann-style computing architecture [4,5], and possibly allow the transfer of a GHz-frequency quantum state to an optical signal.\\[4pt] [1] A.D. O'Connell et al., ``Quantum ground state and single-phonon control of a mechanical resonator,'' Nature 464, 697-703 (2010)\\[0pt] [2] M. Hofheinz et al., ``Generation of Fock states in a superconducting quantum circuit,'' Nature 454, 310-314 (2008)\\[0pt] [3] M. Hofheinz et al., ``Synthesizing arbitrary quantum states in a superconducting resonator,'' Nature 459, 546-549 (2009)\\[0pt] [4] M. Mariantoni et al., ``Implementing the quantum von Neumann architecture with superconducting circuits,'' Science 334, 61 (2011) \\[0pt] [5] E. Lucero et al., ``Computing prime factors with a Josephson phase qubit quantum processor,'' Nature Physics 8, 719 (2012)   

Observation of optical quantum measurement backaction on a mechanical resonator

Quantum mechanics provides an inextricable link between measurement and backaction on the subsequent dynamics of a system. Here we continuously monitor the position of a membrane microresonator in a cavity optomechanical system. We observe a fluctuating backaction force on the resonator which rises with measurement strength in accordance with the minimum allowed by the Heisenberg position-momentum uncertainty limit. For our optically-based position measurement the backaction takes the form of a fluctuating radiation pressure due to optical shot noise. We demonstrate radiation pressure shot noise that is comparable to in magnitude to thermal fluctuations at frequencies near the mechanical resonance. Additionally, we observe temporal correlations between fluctuations in the radiation force and resonator position, which we interpret as a non-demolition measurement of the intracavity photon field fluctuations. We will also discuss possible methods to lower the technical noise floor in all measurement quadratures.   

Quantum optics experiments with micromechanical oscillators

Mechanical oscillators coupled to optical fields via the radiation pressure force have been of great interest lately as they allow for quantum experiments with macroscopic systems. Recent experiments have shown ground-state preparation and measurement of such resonators via sideband-resolved laser cooling. We will discuss our recent work that aims at achieving quantum control over nanoscale optomechanical crystal devices, both using strong coherent optical beams as well as single photons.   

Optomechanical Coupling Between Membrane Modes

In an optomechanical device, radiation pressure couples optical power to mechanical motion. While typical experiments couple a single optical cavity to a single mechanical resonance, there has been increasing theoretical and experimental interest in multi-mode systems where there is coupling between multiple mechanical resonances and/or multiple optical cavity modes. We report on a device consisting of a dielectric SiN membrane located inside a high finesse fiber-cavity, where two nearly-degenerate mechanical modes couple to a single cavity mode. We observe that the original mechanical modes can experience a large coupling that is mediated by intracavity field. This causes the mechanical eigenmodes of the system to depend strongly on the radiation pressure and change from the original mechanical modes to a symmetric and antisymmetric combination of the original modes. The symmetric/antisymmetric modes are also known as ``dark'' and ``bright'' modes, as they have very different coupling to the cavity. In the quantum regime, this effective interaction between mechanical modes would open up the possibility of state transfer between multiple mechanical modes.   

Gain-enhanced optical cooling in cavity optomechanics

We study the optical cooling of the mechanical motion of the resonator mirror in a cavity-optomechanical system that contains an optical gain medium. We find that the optical damping caused by radiation pressure force is vanishingly small if the active medium is pumped incoherently above its lasing threshold. In addition, we find that the spontaneous emission of the active medium always tends to increase the final effective temperature of the mechanical motion. In the presence of an additional seeding signal, i.e. a coherent drive of fixed frequency within the width of the gain curve however, we find that the cooling rate can be enhanced significantly with respect to that of a passive cavity. We attribute this effect to a reduced effective optical damping in the presence of incoherent pumping.   

Novel cooling mechanisms in optomechanical systems

We present here our theoretical work on unconventional cooling mechanisms in optomechanical systems. In particular our classical and quantum theory of photothermal cooling [1] and our more recent work on cooling of a mechanical oscillator in cavity QED systems [2].\\[4pt] [1]J. Restrepo, J. Gabelli, C. Ciuti and I. Favero, Comptes Rendus Physique,12, 860-870 (2011). doi:10.1016/j.crhy.2011.02.005 (arXiv:1011.3911)\\[0pt] [2] J. Restrepo, I. Favero, C. Ciuti. in preparation.   

Optical measurement of the thermal motion of a micromechanical resonator and its modal interaction by sideband actuation scheme

We present measurement of the thermal motion of a micromechanical resonator and excitation of flexural mode by sideband actuation. Doubly-clamped micromechanical resonators are fabricated from high-stress silicon nitride on SiO2/Si substrate and patterned with standard e-beam lithographic techniques. Optical measurement of resonant response of micromechanical resonator reveals its fundamental flexural mode of thermal motion at approximately 3.4 MHz ($f_{o})$ with quality factor up to 180,000 and higher modes at room temperature in moderate vacuum. With fundamental and higher flexural modes of thermal motion and sideband actuation scheme, we also observe amplitude increase in flexural mode of thermal motion with blue-detuned sideband pumping.   

Optomechanics in a Fiber-Cavity

In an optical displacement measurement, the quantum back-action is radiation pressure shot noise (RPSN), which is the Poissonian noise in the momentum transferred by reflecting photons. In an attempt to measure RPSN at room temperature, we have made an optomechanical device consisting of a fiber-based optical cavity containing a silicon nitiride membrane. In comparison with typical free-space cavities, the fiber-cavity's small mode size (10 micron waist, 60 micron length) allows the use of smaller, lighter membranes and increases the cavity-membrane linear coupling to 3 GHz/nm. This device is also intrinsically fiber-coupled and uses v-grooves for passive alignment; these improvements greatly simplify the use of optomechanical devices. Based on the parameters demonstrated by this device, we expect it to be able to detect RPSN at room temperature. The increased coupling in this system also makes it an excellent testbed for investigating optomechanical coupling between mechanical modes, and for demonstrating quadratic coupling between a single mechanical mode and the cavity.   

Robust entanglement via optomechanical dark mode: adiabatic scheme

Entanglement is a powerful resource for studying quantum effects in macroscopic objects and for quantum information processing. Here, we show that robust entanglement between cavity modes with distinct frequencies can be generated via a mechanical dark mode in an optomechanical quantum interface. Due to quantum interference, the effect of the mechanical noise is cancelled in a way that is similar to the electromagnetically induced transparency. We derive the entanglement in the strong coupling regime by solving the quantum Langevin equation using a perturbation theory approach. The entanglement in the adiabatic scheme is then compared with the entanglement in the stationary state scheme. Given the robust entanglement schemes and our previous schemes on quantum wave length conversion, the optomechanical interface hence forms an effective building block for a quantum network.   

Development of a dispersive read-out technique for quantum measurements of nanomechanical resonators

Over the last decade, there has been an active effort to prepare and measure mechanical structures in the quantum regime for the purpose of sensing weak forces and for studying fundamental topics in quantum mechanics such as quantum measurement, entanglement and decoherence in new macroscopic limits. One promsing tool for such studies is the qubit-coupled mechanical resonator. In this work we discuss some of our first results towards the development of a nanoelectromechanical system that integrates a charge-type superconducting qubit as a detector to probe the number-states of a nanomechanical mode. In our system the qubit-coupled nanoresonator is embedded in a superconducting microwave resonator (SMR); the SMR then serves to perform spectroscopic measurements of the qubit to infer the number-state statistics of the nanoresonator in a manner analogous to dispersive measurement techniques used in circuit and cavity QED to probe the number-states of electromagnetic cavities. We will discuss the design and measurement of our latest generation devices and the prospects for achieving single-phonon measurement resolution with this system.   

Fabricating Micro-Optomechanical Resonators Using High-Stress Si$_{3}$N$_{4}$

Optomechanical systems have been highly researched as a platform for testing macroscopic quantum effects and quantum decoherence. However, the required optical and mechanical properties are difficult to achieve. Increasing the tensile stress of a device is known to correlate with higher mechanical frequency and quality factor. We discuss fabrication of monolithic optomechanical devices using dielectric mirrors and high-stress stoichiometric Si$_{3}$N$_{4}$. We also present preliminary data on their mechanical and optical properties.   

Coherent optical wavelength conversion via cavity-optomechanics

In this talk we theoretically propose and experimentally demonstrate coherent wavelength conversion of optical photons using photon-phonon translation in a cavity-optomechanical system. Our system is an engineered silicon optomechanical crystal nanocavity supporting a $4$~GHz localized phonon mode, optical signals in a $1.5$~MHz bandwidth are coherently converted over a $11.2$~THz frequency span between one cavity mode at wavelength $1460$~nm and a second cavity mode at $1545$~nm with a 93\% internal (2\% external) peak efficiency. The thermal and quantum limiting noise involved in the conversion process is also analyzed, and in terms of an equivalent photon number signal level are found to correspond to an internal noise level of only $6$ and $4 \times 10^{-3}$ quanta, respectively [1].\\[4pt] [1] J.~T.~Hill, A.~H.~Safavi-Naeini, J.~Chan, O.~Painter, arXiv:1206.0704 (2012).   

Fast readout of carbon nanotube mechanical resonators

We perform fast readout measurements of carbon nanotube mechanical resonators. Using an electronic mixing scheme, we can detect the amplitude of the mechanical motion with an intermediate frequency (IF) of 46 MHz and a timeconstant of 1 us, up to 5 orders of magnitude faster than before. Previous measurements suffered from a low bandwidth due to the combination of the high resistance of the carbon nanotube and a large stray capacitance. We have increased the bandwidth significantly by using a high-impedance, close-proximity HEMT amplifier. The increased bandwidth should allow us to observe the nanotube's thermal motion and its transient response, approaching the regime of real-time detection of the carbon nanotube's mechanical motion.