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Science

Quantum Squeeze Wrings Uncertainty From Atom Waves

Daniel Clery

Published Mar 14, 1997• 2 citations
Publisher

Abstract

Quantum “noise” gets in the way of ultraprecise measurements of any physical quantity, such as a particle's location or a wave's frequency. In this issue, however, a team of physicists reports finding a way around this quantum limit for the wavelike vibrations of atoms in solids called phonons. By delivering ultrafast pulses of light to a sample of potassium tantalate, they squeezed quantum noise out of the phonons at certain times while making them noisier at others.

DOI
10.1126/science.275.5306.1566

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Quantum “noise” gets in the way of ultraprecise measurements of any physical quantity, such as a particle's location or a wave's frequency. In this issue, however, a team of physicists reports finding a way around this quantum limit for the wavelike vibrations of atoms in solids called phonons. By delivering ultrafast pulses of light to a sample of potassium tantalate, they squeezed quantum noise out of the phonons at certain times while making them noisier at others.
Twelve years ago, Richart Slusher and his colleagues at AT&T Bell Labs produced light whose noise was below the vacuum quantum fluctuations, at least in part of the signal. Since then, researchers have been trying to squeeze the uncertainties out of other systems as well. So far they have succeeded in quieting a classical mechanical oscillator and both classical and nonclassical states of a vibrating, trapped ion. Now comes a report of squeezed phonons: By striking a crystal with a femtosecond laser pulse, a group at the University of Michigan believes it has excited an acoustic mode whose variance falls below the standard quantum limit. So far, the noise has been reduced by only 0.01% (the earliest experiments on optical squeezing yielded 20%), but just the concept of squeezed phonons has intrigued many observers.
Summary form only given. In high-precision atomic experiments such as atomic interferometry, the accuracy is ultimately limited by the so-called quantum projection noise, due to the fact that the atoms are not in an eigenstate of the measured quantity. In an ensemble of N independent atoms, fluctuations proportional to N/sup -1/2/ result from this effect. A few years ago, it was shown that using squeezed atomic states would allow to reduce these fluctuations, in the same way as squeezed states of light allow to obtain noise reduction in measurements of the electromagnetic field.
For the first time, physicists have performed an experiment confirming that thermodynamic processes are irreversible in a quantum system—meaning that, even on the quantum level, you can't put a broken egg back into its shell. The results have implications for understanding thermodynamics in quantum systems and, in turn, designing quantum computers and other quantum information technologies. [11] Disorder, or entropy, in a microscopic quantum system has been measured by an international group of physicists. The team hopes that the feat will shed light on the of time: the observation that time always marches towards the future. The experiment involved continually flipping the spin of carbon atoms with an oscillating magnetic field and links the emergence of the arrow of time to quantum fluctuations between one atomic spin state and another. [10] Mark M. Wilde, Assistant Professor at Louisiana State University, has improved this theorem in a way that allows for understanding how quantum measurements can be approximately reversed under certain circumstances. The new results allow for understanding how quantum information that has been lost during a measurement can be nearly recovered, which has potential implications for a variety of quantum technologies. [9] Today, we are capable of measuring the position of an object with unprecedented accuracy, but quantum physics and the Heisenberg uncertainty principle place fundamental limits on our ability to measure. Noise that arises as a result of the quantum nature of the fields used to make those measurements imposes what is called the standard quantum limit. This same limit influences both the ultrasensitive measurements in nanoscale devices and the kilometer-scale gravitational wave detector at LIGO. Because of this troublesome background noise, we can never know an object's exact location, but a recent study provides a solution for rerouting some of that noise away from the measurement. [8] The accelerating electrons explain not only the Maxwell Equations and the Special Relativity, but the Heisenberg Uncertainty Relation, the Wave-Particle Duality and the electron’s spin also, building the Bridge between the Classical and Quantum Theories. The Planck Distribution Law of the electromagnetic oscillators explains the electron/proton mass rate and the Weak and Strong Interactions by the diffraction patterns. The Weak Interaction changes the diffraction patterns by moving the electric charge from one side to the other side of the diffraction pattern, which violates the CP and Time reversal symmetry. The diffraction patterns and the locality of the self-maintaining electromagnetic potential explains also the Quantum Entanglement, giving it as a natural part of the relativistic quantum theory.
We investigate coherent and squeezed quantum states of phonons. The latter allow the possibility of modulating the quantum fluctuations of atomic displacements below the zero-point quantum noise level of coherent states. The expectation values and quantum fluctuations of both the atomic displacement and the lattice amplitude operators are calculated in these states---in some cases analytically. We also study the possibility of squeezing quantum noise in the atomic displacement using a polariton-based approach. \textcopyright{} 1996 The American Physical Society.
Quantum uncertainty Spin I want to start this section with a description of particle spin. In addition to the usual properties of mass, charge, position and velocity, particles have a further parameter which describes their state of rotation. Physicists call this ‘spin’. We have already seen that in quantum physics it is impossible to measure simultaneously, in a precise way, both speed and position (recall the Heisenberg uncertainty relations). The same considerations apply to spin, which cannot be absolutely determined with arbitrary precision. The things that we can in principle find out are the rate of rotation (that is, the number of revolutions per second) and the component of this rotation in a given direction in space (for example, the angle of the axis of rotation relative to the direction of motion). It is important to understand that these two knowable quantities can take only certain quantised values. This is an aspect of quantum physics that we have already encountered when describing wave motion. We can use as an analogy the stable vibrations that can be sustained on a piano string: the basic note is generated when the centre of the string vibrates to its maximum extent while the two ends are (necessarily) fixed.
While it is true that quantum mechanics imposes strict limits on the precision of measurements, we can sometimes improve measurement precision without violating the uncertainty principle. The rub is that this improvement may be formidable to achieve in practice—and always comes at the expense of a loss in the precision of a complementary measurement. Last October a team at AT&T Bell Labs succeeded in reducing the noise observed from an optical cavity below the “shot noise”—the level of fluctuations associated with variations in the vacuum fields, which is the ultimate limit to accuracy that would be predicted by semiclassical models. Although the Bell Labs experiment reduced the noise below this level by only 7 to 10%, it bolstered hopes that continued progress might eventually yield reductions in noise by factors of ten. Manipulation of the quantum noise limitations might be absolutely critical to such precision measurements as detection of gravity waves, whose faint signals are likely to be near the measurement limit of laser interferometers or mechanical oscillators. The phenomenon might possibly find applications in optical communication or optical memory systems.