Mechanical devices that exploit micro/nano-fabrication as well as integration into other ultrasensitive transducers – optical, electronic, or atomic – have opened intriguing new avenues of research. These devices provide new means for cooling, signal processing, and sensing of electric, magnetic, and gravitational fields. With many of these systems now entering the quantum regime, their possible use in quantum engineering – for state storage, transduction and conversion – is explored. Beyond engineering, mechanical oscillators also provide especially clear realizations of quantum measurement processes, and may ultimately enable new insights into fundamental limits arising from decoherence of these unprecedentedly massive quantum objects. Picture credits: Albert Schliesser/EPFL
Mechanical devices that exploit micro/nano-fabrication as well as integration into other ultrasensitive transducers – optical, electronic, or atomic – have opened intriguing new avenues of research. These devices provide new means for cooling, signal processing, and sensing of electric, magnetic, and gravitational fields. With many of these systems now entering the quantum regime, their possible use in quantum engineering – for state storage, transduction and conversion – is explored. Beyond engineering, mechanical oscillators also provide especially clear realizations of quantum measurement processes, and may ultimately enable new insights into fundamental limits arising from decoherence of these unprecedentedly massive quantum objects. Picture credits: Albert Schliesser/EPFL
The quantum realm is often portrayed as weird and nebulous, far removed from our everyday world. Subatomic particles can be in many places at once or linked across vast distances by the intimate connection known as entanglement. But researchers are now using this quantum fuzziness to forge a new set of practical tools.
In this entanglement-assisted magnetometer, lenses, and other optical devices steer a laser beam to the small glass cell filled with cesium vapor, and a photodetector above records the effect of magnetic fields on light emitted by the cesium atoms. Image credit: Ola J. Joensen (Niels Bohr Institute, Copenhagen, Denmark).
Although quantum computers gain the most headlines, a quieter revolution in the world of sensors may be just as profound. Quantum states, such as the spin of an electron, have a peculiar delicacy that is being exploited to detect tiny variations in gravity and magnetism, opening up a range of applications that could give us a clearer view of the world beneath our feet and provide deep insights into our own biology.
This has become possible thanks to continuing advances in components such as lasers and cooling systems that allow researchers and engineers to manipulate the quantum states of atoms and measure how they are affected by their environment. “We are at this sweet spot where we have quantum systems under sufficient control,” says Kai Bongs, who leads the Quantum Sensors and Metrology Hub (1), a collaboration between academics and industry centered at the University of Birmingham in the United Kingdom.
Some of the large vacuum systems and magnetic traps now used to hold cold atoms are being replaced with chip-scale devices, while researchers are developing compact, low-power lasers that could make quantum sensors much more accessible. “The early devices are still clumsy, but now we can start …
“Quantum Simulators”, as originally envisaged by Richard Feynman are highly controllable and specially engineered many‐body systems that have the potential to shed new light on open questions in condensed matter physics, high‐energy and nuclear physics. They are currently set‐up and explored in a diverse range of quantum systems ranging from ultracold quantum gases, over trapped ions and photonic systems, to superconducting and semiconductor devices. Image: The mathematical description of a phenomenon to be investigated is programmed using a series of laser pulses to perform a quantum calculation with atoms (credit: H. Ritsch).
The coupling of nanomechanical resonators to controllable quantum systems represents a growing and promising area of research. Recent advances in this field have brought to astonishing results, such as the initialization of a mechanical resonator into its quantum ground state and even the preparation of non-classical coherent states of motion. These achievements open up appealing scenarios to quantum information technologies and to the exploration of the quantum-classical boundary. From an application perspective, such quantum-mechanical hybrid systems provide a versatile and attractive tool for a variety of precision measurements, like ultra-sensitive detection of force, mass, and displacement. \n \nQuantum control over a mechanical resonator, or, conversely, the prospect of using resonator's motion for probing quantum states, both involve some tight requirements. First of all, the interaction between the mechanics and its quantum partner has to be large on the scale of the decoherence rates of the coupled systems. In most cases, the coupling mechanism has to be activated by engineering the resonator with electrodes, magnets, or mirrors, or by using tailored laser fields. For this reason, the search for a strong coupling typically competes with the requirement of overcoming decoherence effects. In addition, for quantum effects to be observable, strong coupling has to be accompanied by the preparation of zero entropy initial states, for instance by cooling the resonator into its ground state of motion. \n \nThe motivation pursued by this thesis is to contribute to the inspiring field of hybrid systems by walking through each of the aforementioned directions. We in fact demonstrate a promising system in which optically active quantum dots, embedded in fully self-assembled core-shell nanowires, are coupled to the nanowire motion. Mechanical vibrations of the nanowire modulate the quantum dot emission energy over a broad range exceeding 14 meV, by means of deformation potential coupling. In our system, therefore, both the coupling mechanism and the quantum states themselves are intrinsic to the resonator's structure. Besides revealing unusually strong, such a built-in opto-mechanical interaction produces a hybrid system whose inherent coherence is unspoiled by any functionalization or external field and whose fabrication is simpler than top-down techniques. \n \nWe further demonstrate the use of a quantum point contact as a displacement transducer to measure and control the low-temperature thermal motion of a nearby micromechanical cantilever. We show that by including the QPC in a suitable feedback loop, we are able to cool the cantilever's fundamental mechanical mode down to the level of the measurement noise, achieving a squashing of the QPC noise at high gain. Due to its off-board design, our system is particularly versatile and suitable to force-sensing applications. Since the QPC transducer is sensitive to local modifications of the nearby electric field, our approach is in principle compatible with any nanoscale resonator, without requiring any functionalization to activate the coupling. By improving the performance of the QPC as a one-dimensional conductor and the cantilever-QPC capacitive coupling, our system has the potential to achieve quantum-limited displacement resolution and ground state cooling. We then report on some ongoing attempts to overcome the current limitations of our system and couple mechanical motion to different mesoscopic transport devices. \n \nWe remark that our demonstration of an as-grown quantum-dot-in-nanowire hybrid system opens up bright prospects on future experiments, as also testified by the very recent ferment of the scientific community around this topic. We find that the opto-mechanical coupling rate is not far from the nanowire mechanical frequency. This fact makes our system particularly promising for the quantum non-demolition readout of a quantum dot state through a measurement of the nanowire position. On the other hand, we show that the coupling can be further optimized, and the nanowire mechanical heating rate reduced, thus disclosing the captivating perspective of using quantum dots to probe and control the mechanical state of a nanowire. In this context, it would be important to investigate a possible spin-oscillator coupling in our system, given the long coherence time offered by a spin state in a quantum dot. The wide range of control over the quantum dots emission energy and the ability to tune two neighboring dots into resonance pave the way to mechanically induced emitter-emitter coupling. In addition, our monolithic opto-mechanical system constitutes a good candidate for recent proposals of a tripartite hybrid system, which would integrate an optical cavity, a quantum two-level system and a mechanical resonator. The intrinsically strong coupling between the quantum dots and the nanowire motion would enhance the interaction between the mechanics and the cavity field, leading for instance to efficient cooling of the resonator to the ground state. Finally, the quantum dot sensitivity to the nanowire resonant vibration could also support the use of our system as an integrated force probe or as a nanomechanical mass sensor. In other words, the results reported in this dissertation constitute not only a relevant proof-of-principle, moreover they open up intriguing challenges for future research.
New experiments are revealing the power of large-scale quantum devices .
Entanglement is the counterintuitive idea that particles can have an intrinsic connection—a connection that endures no matter the distance between them. The phenomenon remains one of the most bizarre and least understood consequences of quantum mechanics. Measure the quantum properties of one of a pair of entangled particles, and the other changes instantaneously.
Researchers have pioneered ways to demonstrate quantum entanglement in mechanical systems. This artist’s conception of one experiment depicts an interferometer’s light field, which “carries” the entangled state. Image credit: Kavli Institute of Nanoscience, Delft University of Technology/Moritz Forsch.
Such strange phenomena typically have been relegated to the subatomic. But recently, physicists have taken entanglement and other quantum effects to new extremes by observing them in large systems including clouds of atoms, quantum drums, wires, and etched silicon chips. Device by device, they are bringing the quantum world into new territory—the macroscopic world.
This work is driving new applications. Some experimental quantum computers use loops of superconducting wire as qubits, storing quantum information. Large quantum objects have already been used to help detect gravitational waves; they could appear in next-generation devices such as ultrasensitive sensors and encryption systems. These innovations, though, reach beyond cutting-edge tech. Building bigger and bigger quantum objects even raises the possibility of exploring some of the most pervasive unsolved mysteries at the intersection between quantum and classical worlds—and between quantum mechanics and gravity.
Ever since Austrian physicist Erwin Schrodinger first described wave–particle duality 90 years ago (1), physicists have been probing the boundary between the observable, predictable macroscopic world and the one where probabilistic quantum rules dominate. In the quantum world, a particle exists as a wave representing the probabilities of its location. Once measured, however, the particle is found at a point …
Microscale and nanoscale mechanical resonators have been used in advanced technological applications, from high precision time keeping and mass sensing, to processing high frequency signals in mobile communications. In the last few decades, they have been an important part of progress in the field of quantum information and metrology and have been proposed as quantum memories or transducers for measuring or connecting different types of quantum systems. The field of cavity optomechanics and electromechanics is concerned with coupling the electromagnetic field of a resonant optical cavity or electrical circuit to mechanical motion. These systems provide potential means to control and engineer the state of a mechanical object at the quantum level. This thesis contains the description of mechanical systems in megahertz to a few hundred megahertz frequency range formed by nano-fabricating photonic, phononic, and electrical circuits on a chip. These structures are designed to provide a large radiation pressure coupling between mechanical motion and electromagnetic fields to address and manipulate motional degrees of freedom. Qualitatively novel quantum effects are expected when one takes a step beyond linear coupling and exploits higher order interactions. To that end, we integrate electrical, mechanical and photonic structures in a multimode photonic crystal structure to observe "x2-coupling", where the optical cavity frequency is coupled to the square of the mechanical displacement. Moreover, we have developed two integrated on-chip platforms based on Si3N4 and Si nanomembranes capable of interfacing superconducting qubits and optical photons and realizing reversible microwave-to-optical conversion. We employ radiation pressure to cool these mechanical resonators to their quantum ground state. Finally, we demonstrate a form of electromechanical crystal for coupling microwave photons and hypersonic phonons of frequency ωm/2π = 0.425 GHz by capacitively coupling a phononic crystal acoustic cavity to a superconducting microwave resonator. Moving to higher frequency acoustic cavities not only facilitates the integration of electromechanical circuits and nanophotonic systems capable of operation in the resolved sideband limit of optomechanics for noise-free quantum signal conversion, but it opens up the possibility of using phonons as information carriers via phononic circuits. Utilizing a two-photon resonance condition for efficient microwave pumping and phononic bandgap shield to eliminate acoustic radiation, we achieve large cooperative electromechanical coupling (C ≈ 30) and intrinsic decay time of 2.3 ms. Moreover, electrical read-out of the phonon occupancy shows that the acoustic mode thermalizes close to its quantum ground-state of motion (phonon occupancy nm=1.5) at a fridge temperature of Tf = 10 mK. We conclude by considering several designs and fabrication improvements to the hypersonic electromechanical crystals that would enable them to perform quantum conversion between the electrical and acoustic domain.