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The state of an itinerant microwave field can be coherently transferred into, stored in and retrieved from a mechanical oscillator with amplitudes at the single-quantum level, and the time to capture and retrieve the microwave state is shorter than the quantum state lifetime of the mechanical oscillator. Mechanical oscillation and quantum state storage In the last decade it has become possible to control macroscopic mechanical oscillators in such a way that they show quantum behaviour. The next step is to exploit this capability to produce useful devices for quantum information applications, in particular as storage elements for quantum states, a role for which mechanical oscillators show promise. One way of achieving this is to embed mechanical oscillators in superconducting circuits where quantum information can be processed in the form of microwave fields. Tauno Palomaki et al . now reach an important goal in this area by showing that the state of a microwave field can be coherently stored in and retrieved from a mechanical oscillator at the single-quantum level. Macroscopic mechanical oscillators have been coaxed into a regime of quantum behaviour by direct refrigeration 1 or a combination of refrigeration and laser-like cooling 2 , 3 . This result supports the idea that mechanical oscillators may perform useful functions in the processing of quantum information with superconducting circuits 4 , 5 , 6 , 7 , either by serving as a quantum memory for the ephemeral state of a microwave field or by providing a quantum interface between otherwise incompatible systems 8 , 9 , 10 , 11 , 12 , 13 , 14 . As yet, the transfer of an itinerant state or a propagating mode of a microwave field to and from a storage medium has not been demonstrated, owing to the inability to turn on and off the interaction between the microwave field and the medium sufficiently quickly. Here we demonstrate that the state of an itinerant microwave field can be coherently transferred into, stored in and retrieved from a mechanical oscillator with amplitudes at the single-quantum level. Crucially, the time to capture and to retrieve the microwave state is shorter than the quantum state lifetime of the mechanical oscillator. In this quantum regime, the mechanical oscillator can both store quantum information and enable its transfer between otherwise incompatible systems.

Micromechanical motion grounded It has been a long-standing goal in the field of cavity optomechanics to cool down a mechanical resonator to its motional quantum ground state by using light. Teufel et al . have now achieved just that with a recently developed system in which a drum-like flexible aluminium membrane is incorporated in a superconducting circuit. Ground-state cooling of a mechanical resonator was demonstrated for the first time last year in a different type of device, but the quantum states in this new device should be much longer lived, allowing direct tests of fundamental principles of quantum mechanics. As a first step, the authors perform a quantum-limited position measurement that is only a factor of about five away from the Heisenberg limit. The advent of laser cooling techniques revolutionized the study of many atomic-scale systems, fuelling progress towards quantum computing with trapped ions 1 and generating new states of matter with Bose–Einstein condensates 2 . Analogous cooling techniques 3 , 4 can provide a general and flexible method of preparing macroscopic objects in their motional ground state. Cavity optomechanical or electromechanical systems achieve sideband cooling through the strong interaction between light and motion 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 . However, entering the quantum regime—in which a system has less than a single quantum of motion—has been difficult because sideband cooling has not sufficiently overwhelmed the coupling of low-frequency mechanical systems to their hot environments. Here we demonstrate sideband cooling of an approximately 10-MHz micromechanical oscillator to the quantum ground state. This achievement required a large electromechanical interaction, which was obtained by embedding a micromechanical membrane into a superconducting microwave resonant circuit. To verify the cooling of the membrane motion to a phonon occupation of 0.34 ± 0.05 phonons, we perform a near-Heisenberg-limited position measurement 3 within (5.1 ± 0.4) h /2π, where h is Planck’s constant. Furthermore, our device exhibits strong coupling, allowing coherent exchange of microwave photons and mechanical phonons 16 . Simultaneously achieving strong coupling, ground state preparation and efficient measurement sets the stage for rapid advances in the control and detection of non-classical states of motion 17 , 18 , possibly even testing quantum theory itself in the unexplored region of larger size and mass 19 . Because mechanical oscillators can couple to light of any frequency, they could also serve as a unique intermediary for transferring quantum information between microwave and optical domains 20 .

By coupling a macroscopic mechanical oscillator to two microwave cavities, we simultaneously prepare and monitor a nonclassical steady state of mechanical motion. In each cavity, correlated radiation pressure forces induced by two coherent drives engineer the coupling between the quadratures of light and motion. We, first, demonstrate the ability to perform a continuous quantum nondemolition measurement of a single mechanical quadrature at a rate that exceeds the mechanical decoherence rate, while avoiding measurement backaction by more than 13 dB. Second, we apply this measurement technique to independently verify the preparation of a squeezed state in the mechanical oscillator, resolving quadrature fluctuations 20% below the quantum noise.

The ability to engineer nonreciprocal interactions is an essential tool in modern communication technology as well as a powerful resource for building quantum networks. Aside from large reverse isolation, a nonreciprocal device suitable for applications must also have high efficiency (low insertion loss) and low output noise. Recent theoretical and experimental studies have shown that nonreciprocal behavior can be achieved in optomechanical systems, but performance in these last two attributes has been limited. Here, we demonstrate an efficient, frequency-converting microwave isolator based on the optomechanical interactions between electromagnetic fields and a mechanically compliant vacuum-gap capacitor. We achieve simultaneous reverse isolation of more than 20 dB and insertion loss less than 1.5 dB. We characterize the nonreciprocal noise performance of the device, observing that the residual thermal noise from the mechanical environments is routed solely to the input of the isolator. Our measurements show quantitative agreement with a general coupled-mode theory. Unlike conventional isolators and circulators, these compact nonreciprocal devices do not require a static magnetic field, and they allow for dynamic control of the direction of isolation. With these advantages, similar devices could enable programmable, high-efficiency connections between disparate nodes of quantum networks, even efficiently bridging the microwave and optical domains.

Converting low-frequency electrical signals into much higher-frequency optical signals has enabled modern communication networks to leverage the strengths of both microfabricated electrical circuits and optical fibre transmission, enabling 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 through low-loss optical fibres 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. Here we demonstrate a converter that provides a bidirectional, coherent and efficient link between 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 K operating temperature to temperatures below 40 mK. An optomechanical system that converts microwaves to optical frequency light and vice versa is demonstrated. The technique achieves a conversion efficiency of approximately 10%. The results indicate that the device could work at the quantum level, up- and down-converting individual photons, if it were cooled to millikelvin temperatures. It could, therefore, form an integral part of quantum-processor networks.

Quantum states with a prolonged life The drive towards observing quantum effects in macroscopic mechanical systems could lead to new insights in quantum-limited measurements and help to test fundamental questions regarding the impossible consequences of quantum physics at a macroscopic scale. To obtain sufficiently long-lived mechanical states, the usual approach is to couple a mechanical oscillator to an electromagnetic resonance in a cavity. Teufel et al . present a new design for such a system in which a free-standing flexible aluminium membrane (like a drum) is incorporated in a cavity defined by a superconducting circuit, and which demonstrates a coupling strength that is two orders of magnitude higher than that achieved before. The approach shows the way to observing long-lived quantum states that could survive for hundreds of microseconds. There is a strong drive towards observing quantum effects in macroscopic mechanical systems, as this could lead to new insights in quantum-limited measurements as well as test fundamental questions regarding the impossible consequences of quantum physics at a macroscopic scale. To obtain sufficiently long-lived mechanical states the usual approach is to couple a mechanical oscillator to an electromagnetic resonance in a cavity. This study presents a new design for such a system where a free-standing flexible aluminium membrane (like a drum) is incorporated in a cavity defined by a superconducting circuit, and demonstrates a coupling strength that is two orders of magnitude higher than achieved before. The approach shows the way to observing long-lived quantum states that could survive for hundreds of microseconds. Demonstrating and exploiting the quantum nature of macroscopic mechanical objects would help us to investigate directly the limitations of quantum-based measurements and quantum information protocols, as well as to test long-standing questions about macroscopic quantum coherence 1 , 2 , 3 . Central to this effort is the necessity of long-lived mechanical states. Previous efforts have witnessed quantum behaviour 4 , but for a low-quality-factor mechanical system. The field of cavity optomechanics and electromechanics 5 , 6 , in which a high-quality-factor mechanical oscillator is parametrically coupled to an electromagnetic cavity resonance, provides a practical architecture for cooling, manipulation and detection of motion at the quantum level 1 . One requirement is strong coupling 7 , 8 , 9 , in which the interaction between the two systems is faster than the dissipation of energy from either system. Here, by incorporating a free-standing, flexible aluminium membrane into a lumped-element superconducting resonant cavity, we have increased the single-photon coupling strength between these two systems by more than two orders of magnitude, compared to previously obtained coupling strengths. A parametric drive tone at the difference frequency between the mechanical oscillator and the cavity resonance dramatically increases the overall coupling strength, allowing us to completely enter the quantum-enabled, strong-coupling regime. This is evidenced by a maximum normal-mode splitting of nearly six bare cavity linewidths. Spectroscopic measurements of these ‘dressed states’ are in excellent quantitative agreement with recent theoretical predictions 10 , 11 . The basic circuit architecture presented here provides a feasible path to ground-state cooling and subsequent coherent control and measurement of long-lived quantum states of mechanical motion.

When two physical systems share the quantum property of entanglement, measurements of one system appear to determine the state of the other. This peculiar property is used in optical, atomic, and electrical systems in an effort to exceed classical bounds when processing information. We extended the domain of this quantum resource by entangling the motion of a macroscopic mechanical oscillator with a propagating electrical signal and by storing one half of the entangled state in the mechanical oscillator. This result demonstrates an essential requirement for using compact and low-loss micromechanical oscillators in a quantum processor, can be extended to sense forces beyond the standard quantum limit, and may enable tests of quantum theory.

Vacuum fluctuations in a ground-state mechanical oscillator are hard to distinguish from noise, but by using the coupling with a superconducting qubit in a microwave cavity one can amplify and convert them to directly measurable real photons. Heisenberg’s uncertainty principle results in one of the strangest quantum behaviours: a mechanical oscillator can never truly be at rest. Even at a temperature of absolute zero, its position and momentum are still subject to quantum fluctuations 1 , 2 . However, direct energy detection of the oscillator in its ground state makes it seem motionless 1 , 3 , and in linear position measurements detector noise can masquerade as mechanical fluctuations 4 , 5 , 6 , 7 . Thus, how can we resolve quantum fluctuations? Here, we parametrically couple a micromechanical oscillator to a microwave cavity to prepare the system in its quantum ground state 8 , 9 and then amplify the remaining vacuum fluctuations into real energy quanta 10 . We monitor the photon/phonon-number distributions using a superconducting qubit 11 , 12 , 13 , allowing us to resolve the quantum vacuum fluctuations of the macroscopic oscillator’s motion. Our results further demonstrate the ability to control a long-lived mechanical oscillator using a non-Gaussian resource, directly enabling applications in quantum information processing and enhanced detection of displacement and forces.

Interferometers enable ultrasensitive measurement in a wide array of applications from gravitational wave searches to force microscopes. The role of quantum mechanics in the metrological limits of interferometers has a rich history, and a large number of techniques to surpass conventional limits have been proposed. In a typical measurement configuration, the trade-off between the probe’s shot noise (imprecision) and its quantum backaction results in what is known as the standard quantum limit (SQL). In this work, we investigate how quantum correlations accessed by modifying the readout of the interferometer can access physics beyond the SQL and improve displacement sensitivity. Specifically, we use an optical cavity to probe the motion of a silicon nitride membrane off mechanical resonance, as one would do in a broadband displacement or force measurement, and observe sensitivity better than the SQL dictates for our quantum efficiency. Our measurement illustrates the core idea behind a technique known as variational readout, in which the optical readout quadrature is changed as a function of frequency to improve broadband displacement detection. And, more generally, our result is a salient example of how correlations can aid sensing in the presence of backaction.

One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits). We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit |01[right-pointing angle bracket] and |10[right-pointing angle bracket] states, consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization of multiqubit gates and elementary quantum algorithms.