Quantum-mechanics free subsystem with mechanical oscillators

Quantum mechanics sets a limit for the precision of measurement of the position of an oscillator. The quantum noise associated with the measurement of a quadrature of the motion imprints a backaction on the orthogonal quadrature, which feeds back to the measured observable in the case of a continuous measurement. In a quantum backaction evading measurement, the added noise can be confined in the orthogonal quadrature. Here we show how it is possible to evade this limitation and measure an oscillator without backaction by constructing one effective oscillator from two physical oscillators. This facilitates detection of weak forces and the creation and measurement of nonclassical motional states of the oscillators. We realize the proposal using two micromechanical oscillators, and show the measurements of two collective quadratures while evading the quantum backaction by $8$ decibels on both of them, obtaining a total noise within a factor two from the full quantum limit. Moreover, by modifying the measurement we directly verify the quantum entanglement of the two oscillators by measuring the Duan quantity $1.3$ decibels below the separability bound

Revealing hidden quantum correlations in an electromechanical measurement

Under a strong quantum measurement, the motion of an oscillator is disturbed by the measurement backaction, as required by the Heisenberg uncertainty principle. When a mechanical oscillator is continuously monitored via an electromagnetic cavity, as in a cavity optomechanical measurement, the backaction is manifest by the shot noise of incoming photons that becomes imprinted onto the motion of the oscillator. Following the photons leaving the cavity, the correlations appear as squeezing of quantum noise in the emitted field. Here we observe such “ponderomotive” squeezing in the microwave domain using an electromechanical device made out of a superconducting resonator and a drumhead mechanical oscillator. Under a strong measurement, the emitted field develops complex-valued quantum correlations, which in general are not completely accessible by standard homodyne measurements. We recover these hidden correlations, using a phase-sensitive measurement scheme employing two local oscillators. The utilization of hidden correlations presents a step forward in the detection of weak forces, as it allows us to fully utilize the quantum noise reduction under the conditions of strong force sensitivity.Peer reviewe

Sideband cooling of nearly degenerate micromechanical oscillators in a multimode optomechanical system

| openaire: EC/H2020/681476/EU//QOM3D | openaire: EC/H2020/732894/EU//FETPRO HOTMultimode optomechanical systems are an emerging platform for studying fundamental aspects of matter near the quantum ground state and are useful in sensitive sensing and measurement applications. We study optomechanical cooling in a system where two nearly degenerate mechanical oscillators are coupled to a single microwave cavity. Due to an optically mediated coupling the two oscillators hybridize into a bright mode with a strong optomechanical cooling rate and a dark mode nearly decoupled from the system. We find that at high coupling, sideband cooling of the dark mode is strongly suppressed. Our results are relevant to novel optomechanical systems where multiple closely spaced modes are intrinsically present.Peer reviewe

Nonreciprocal Transport Based on Cavity Floquet Modes in Optomechanics

| openaire: EC/H2020/732894/EU//HOT | openaire: EC/H2020/742102/EU//QUENOCOBADirectional transport is obtained in various multimode systems by driving multiple, nonreciprocally interfering interactions between individual bosonic modes. However, systems sustaining the required number of modes become physically complex. In our microwave-optomechanical experiment, we show how to configure nonreciprocal transport between frequency components of a single superconducting cavity coupled to two drumhead oscillators. The frequency components are promoted to Floquet modes and generate the missing dimension to realize an isolator and a directional amplifier. A second cavity left free by this arrangement is used to cool the mechanical oscillators and bring the transduction noise close to the quantum limit. We furthermore uncover a new type of instability specific to nonreciprocal coupling. Our approach is generic and can greatly simplify quantum signal processing and the design of topological lattices from low-dimensional systems.Peer reviewe

Realization of Directional Amplification in a Microwave Optomechanical Device

| openaire: EC/FP7/615755/EU//CAVITYQPD | openaire: EC/H2020/732894/EU//HOTDirectional transmission or amplification of microwave signals is indispensable in various applications involving sensitive measurements. In this work we show experimentally how to use a generic cavity optomechanical setup to nonreciprocally amplify microwave signals above 3 GHz in one direction by 9 dB and simultaneously attenuate the transmission in the opposite direction by 21 dB. We use a device including two on-chip superconducting resonators and two metallic drumhead mechanical oscillators. Application of four microwave pump-tone frequencies allows the design of constructive or destructive interference for a signal tone depending on the propagation direction. The device can also be configured as an isolator with lossless nonreciprocal transmission and 18 dB of isolation.Peer reviewe

Quantum mechanics-free subsystem with mechanical oscillators

| openaire: EC/H2020/732894/EU//HOTQuantum mechanics sets a limit for the precision of continuous measurement of the position of an oscillator. We show how it is possible to measure an oscillator without quantum back-action of the measurement by constructing one effective oscillator from two physical oscillators. We realize such a quantum mechanics-free subsystem using two micromechanical oscillators, and show the measurements of two collective quadratures while evading the quantum back-action by 8 decibels on both of them, obtaining a total noise within a factor of 2 of the full quantum limit. This facilitates the detection of weak forces and the generation and measurement of nonclassical motional states of the oscillators. Moreover, we directly verify the quantum entanglement of the two oscillators by measuring the Duan quantity 1.4 decibels below the separability bound.Peer reviewe

Low-Noise Amplification and Frequency Conversion with a Multiport Microwave Optomechanical Device

High-gain amplifiers of electromagnetic signals operating near the quantum limit are crucial for quantum information systems and ultrasensitive quantum measurements. However, the existing techniques have a limited gain-bandwidth product and only operate with weak input signals. Here, we demonstrate a two-port optomechanical scheme for amplification and routing of microwave signals, a system that simultaneously performs high-gain amplification and frequency conversion in the quantum regime. Our amplifier, implemented in a two-cavity microwave optomechanical device, shows 41 dB of gain and has a high dynamic range, handling input signals up to 1013 photons per second, 3 orders of magnitude more than corresponding Josephson parametric amplifiers. We show that although the active medium, the mechanical resonator, is at a high temperature far from the quantum limit, only 4.6 quanta of noise is added to the input signal. Our method can be readily applied to a wide variety of optomechanical systems, including hybridoptical-microwave systems, creating a universal hub for signals at the quantum level.Peer reviewe

Unimon qubit

Funding Information: S.K., A.G., O.K., V.V., and M.M. acknowledge funding from the European Research Council under Consolidator Grant No. 681311 (QUESS) and Advanced Grant No. 101053801 (ConceptQ), European Commission through H2020 program projects QMiCS (grant agreement 820505, Quantum Flagship), the Academy of Finland through its Centers of Excellence Program (project Nos. 312300, and 336810), and Business Finland through its Quantum Technologies Industrial grant No. 41419/31/2020. S.K. and M.M. acknowledge Research Impact Foundation for grant No. 173 (CONSTI). E.H. thanks Emil Aaltonen Foundation (grant No. 220056 K) and Nokia Foundation (grant No. 20230659) for funding. We acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Micronova Nanofabrication Center and LTL infrastructure which is part of European Microkelvin Platform (EMP, No. 824109 EU Horizon 2020). We thank the whole staff at IQM and QCD Labs for their support. Especially, we acknowledge the help withthe experimental setup from Roope Kokkoniemi, code and software support from Joni Ikonen, Tuukka Hiltunen, Shan Jolin, Miikka Koistinen, Jari Rosti, Vasilii Sevriuk, and Natalia Vorobeva, and useful discussions with Brian Tarasinski. | openaire: EC/H2020/681311/EU//QUESS | openaire: EC/H2020/101053801/EU//ConceptQSuperconducting qubits seem promising for useful quantum computers, but the currently wide-spread qubit designs and techniques do not yet provide high enough performance. Here, we introduce a superconducting-qubit type, the unimon, which combines the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. In agreement with our quantum models, we measure the qubit frequency, ω01/(2π), and increased anharmonicity α/(2π) at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13 ns single-qubit gates on two qubits with (ω01, α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation seems to be dominated by dielectric losses. Thus, improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible useful quantum advantage with noisy systems.Peer reviewe