Macroscale entanglement and measurement

doi:10.1126/science.abh3419

GSTDTAP > 气候变化

DOI	10.1126/science.abh3419
	Macroscale entanglement and measurement
	Hoi-Kwan Lau; Aashish A. Clerk
	2021-05-07
发表期刊	Science
出版年	2021
英文摘要	Quantum mechanics governs both fundamental particles and large objects, but for the latter, a myriad of different factors conspire to mask its effects and render deviations from a purely classical description all but invisible. However, careful and controlled experiments can reveal purely quantum mechanical phenomena of large objects. On pages 622 and 625 of this issue, Kotler et al. ([ 1 ][1]) and Mercier de Lépinay et al. ([ 2 ][2]), respectively, report experimental examples of the direct observation of two effects of quantum mechanics in macroscopic objects that cannot be seen classically. Kotler et al. report direct evidence of quantum entanglement of macroscopic objects (vibrating drumhead membranes), and Mercier de Lépinay et al. detect and even evade back-action in quantum-mechanical measurements of an analogous system. A number of physical platforms have been developed to exploit quantum effects in systems much larger than a single atom or molecule. Among the largest are quantum optomechanical systems ([ 3 ][3]), in which the motion of a macroscopic mechanical resonator (such as a vibrating cantilever or membrane) is controlled with photons in an electromagnetic resonator. Both Kotler et al. and Mercier de Lépinay et al. used a superconducting aluminum resonant inductor–capacitor circuit with a microwave resonant frequency (a few gigahertz), and a thin suspended aluminum membrane with an extremely low-dissipation, drumhead-like vibrational mode in the megahertz range (see the image). These membranes have diameters of ∼10 µm and masses of ∼100 pg (∼1012 atoms). The membrane forms one plate of the capacitor in the circuit, so its motion directly modulates the resonant frequency of the circuit (see the figure, top). The number of microwave photons in the circuit acts as a force on the mechanical system. By driving the circuit with appropriate microwave voltages, the mechanical motion could be prepared, manipulated, and read out with quantum-level precision. Crucially, in each experiment, the circuit is coupled to two distinct mechanical drumhead resonators (realized as two vibrating capacitors) so that quantum correlations between different systems could be explored. Kotler et al. drove the circuit with tailored microwave pulses that strongly correlate the motion of the two vibrating drumheads at the quantum level, creating a quantum-entangled state of two macroscopic objects. Entanglement is a strictly quantum effect in which distinct objects exhibit correlations that, in a certain sense, are stronger than what is allowed classically (see the figure, top). In various nascent quantum technologies, such as computation and sensing, entanglement enables quantum advantages over purely classical devices. To verify entanglement, the authors used the circuit to measure correlations in the motion of the two vibrating drumheads. Although strong evidence of macroscopic entanglement has been previously reported in related systems ([ 4 ][4], [ 5 ][5]), the results of Kotler et al. represent an advance on several fronts. The entanglement was generated in a deterministic (nonrandom) manner, and all of the correlations needed to verify entanglement were directly measured rather than inferred. Further, the measurement efficiency was substantially higher than in previous microwave-frequency studies, so that entanglement was apparent even without the subtraction of amplifier noise from the measured data. Another purely quantum phenomena is back-action, the concept that measurement of an object must disturb it in some way. According to Heisenberg's uncertainty principle, precise observation of an object's position necessarily imparts a random “kick” that disturbs its momentum. For an oscillator whose position is being continuously monitored, measurement precision is often expressed in terms of the standard quantum limit (SQL) ([ 6 ][6]). However, back-action need not limit force measurement, and a variety of “quantum back-action–evasion” strategies have been developed. A particularly powerful one is the construction of so-called quantum mechanics–free subsystem (QMFS) ([ 7 ][7], [ 8 ][8]). A single oscillator is effectively encoded into two distinct oscillators, so that the measurement back-action can be moved into collective degrees of freedom that are dynamically uncoupled from the quantities being measured (see the figure, bottom). This general strategy allows the measurement of a narrow-band (classical) force without any fundamental quantum limit. The experiment by Mercier de Lépinay et al. explicitly demonstrated this idea using an optomechanical scheme first analyzed in ([ 9 ][9]). The unusual QMFS dynamics of the two mechanical membranes is realized by driving the circuit with four distinct microwave control tones, each at a different slightly detuned frequency. The net result is an effective single harmonic oscillator encoded in collective variables of the two physical vibrating membranes that can be measured without any degradation from quantum back-action. Indeed, they achieved a measurement precision better than the SQL. Their technique allows for a complete measurement of the effective mechanical motion without a back-action limit in both the sine and cosine quadratures of the motion. Previous work protected only one quadrature of the mechanical motion from back-action ([ 10 ][10]–[ 12 ][11]), could not measure both protected collective quadratures ([ 13 ][12]), or coupled a mechanical resonator to an atomic ensemble ([ 14 ][13]). Mercier de Lépinay et al. also used their technique to generate entanglement. By slightly breaking the conditions needed for a perfect QMFS measurement, the circuit realizes an effective autonomous measurement-plus-feedback operation that correlates and ultimately entangles the motions of the two vibrating drumheads. This approach both generated and stabilized an entangled state as long as the circuit is energized. This method is complementary to that of Kotler et al. , which does not stabilize an entangled state but has the advantage of being a true entangling quantum gate that preserves information in the initial mechanical state. Beyond demonstrating direct evidence of quantum entanglement and measurement beyond the conventional limits imposed by the quantum back-action for macroscopic objects, the advanced techniques developed by both groups could have a broader impact. The pulsed unitary entangling operation by Kotler et al. could be used as logic gates for quantum computation with continuous variables, and the collective measurement process by Mercier de Lépinay et al. could be combined with entanglement to enable new kinds of enhanced measurements. The refined microwave optomechanical devices of both groups could be used to faithfully convert quantum information between different physical platforms ([ 15 ][14]). Apart from practical applications, these experiments address how far into the macroscopic realm experiments can push the observation of distinctly quantum phenomena. ![Figure][15] Quantum effects writ large Two studies use microwave-driven membranes to demonstrate quantum-mechanical effects of macroscopic objects. GRAPHIC: C. BICKEL/ SCIENCE 1. [↵][16]1. S. Kotler et al ., Science 372, 622 (2021). [OpenUrl][17][Abstract/FREE Full Text][18] 2. [↵][19]1. L. Mercier de Lépinay, 2. C. F. Ockeloen-Korppi, 3. M. J. Woolley, 4. M. A. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/325925
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Hoi-Kwan Lau,Aashish A. Clerk. Macroscale entanglement and measurement[J]. Science,2021.
APA	Hoi-Kwan Lau,&Aashish A. Clerk.(2021).Macroscale entanglement and measurement.Science.
MLA	Hoi-Kwan Lau,et al."Macroscale entanglement and measurement".Science (2021).