PTO000029.pdf

Quantum optomechanics

Markus Aspelmeyer, Pierre Meystre, and Keith Schwab

Citation: Phys. Today 65(7), 29 (2012); doi: 10.1063/PT.3.1640

View online: http://dx.doi.org/10.1063/PT.3.1640

View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v65/i7

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July 2012

Physics Today

29

Aided by optical cavities

and superconducting

circuits,

researchers are coaxing

ever-larger objects to wiggle,

shake, and flex in ways that

are distinctly quantum

mechanical.

Markus Aspelmeyer, Pierre Meystre, and Keith Schwab

optomechanics

μm

Quantum

Give me a place to stand and with a lever I will move the

whole world.

—Archimedes

O

ver two millennia ago, scholars from

antiquity had already come to under-

stand the power of simple mechanical

elements. And from that understand-

ing they formulated an enduring,

common-sense notion of the nature of reality, de-

scribed thusly in Plato’s

The Republic

The same thing cannot ever act or be

acted upon in two opposite ways, or be

two opposite things, at the same time,

in respect of the same part of itself, and

in relation to the same object.

Today researchers at the cutting edge of physics are

still exploiting simple mechanical elements as tools

with which to carefully probe our world. But unlike

their predecessors, they are preparing those ele-

ments deeply in the quantum regime and, in the

process, challenging ancient notions of reality. Iron-

ically, today’s devices, though similar in many ways

to those of antiquity, steer us to a completely differ-

ent worldview—one in which an object, possibly

even a macroscopic one, can indeed act in two ways

at the same time.

Two key developments, born of two converg-

ing perspectives on the physical world, have en-

abled the advance. From the top-down perspective,

nanoscience and the semiconductor industries have

developed advanced materials and processing tech-

niques, which in turn have given rise to ultrasensi-

tive micromechanical and nanomechanical devices.

Such devices can probe extremely tiny forces, often

with spatial resolution at atomic scales, as exempli-

fied by the recent measurements of the Casimir

force (see the article by Steve Lamoreaux, P

HYSICS

ODAY

, February 2007, page 40) and the mechanical

detection and imaging of a single electron spin (see

HYSICS

ODAY

, October 2004, page 22). From the

bottom-up perspective, quantum optics and atomic

physics have yielded an exquisite understanding of

the mechanical aspects of light–matter interaction,

including how quantum mechanics limits the

ultimate sensitivity of measurements and how

back- action—the influence a quantum measure-

ment necessarily exerts on the object being meas-

ured—can be harnessed to control quantum states

of mechanical systems.

Quantum optomechanics combines the two

perspectives: By pairing optical or microwave cavi-

ties with mechanical resonators to form a cavity

opto mechanical system, one acquires a means to

achieve quantum control over mechanical motion

or, conversely, mechanical control over optical or

microwave fields. The laws of quantum physics can

then be made to reveal themselves in the motion

of objects ranging in size from nanometers to

Markus Aspelmeyer

is a professor of physics at the University of Vienna.

Pierre Meystre

is Regents Professor of Physics and Optical Sciences at

the University of Arizona in Tucson.

Keith Schwab

is a professor of

applied physics at the California Institute of Technology in Pasadena.

VIENNA CENTER FOR QUANTUM SCIENCE AND TECHNOLOGY AND ARKITEK STUDIOS

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centi meters, from femtograms to kilograms. Cavity

opto mechanical systems hold promise as a means to

both observe and control the quantum states of

macroscopic objects and to measure feeble forces

and fields with a sensitivity, precision, and accuracy

approaching the quantum limit

(see the article by

Keith Schwab and Michael Roukes, P

HYSICS

ODAY

July 2005, page 36).

Early optomechanics

Put simply, a cavity optomechanical system is an

optical or microwave cavity that contains a mechan-

ical element, a moving part that can support collec-

tive oscillation modes whose quanta of excitation

are known as phonons. The system could be as sim-

ple as an optical cavity in which one of the end mir-

rors oscillates as if attached to a spring.

Among the first well-understood cavity opto-

mechanical systems were the early gravitational -

wave detectors developed in the late 1970s and early

1980s, with major contributions by Vladimir Bragin-

sky, Kip Thorne, Carlton Caves, William Unruh, and

others.

Such detectors are essentially giant inter -

ferometers, with each arm being a kilometers-long

optical cavity bounded by mirrors several kilo-

grams in mass (see figure 1). In theory, a ripple in

the local curvature of spacetime due to a passing

gravitational wave should alter each cavity’s optical

path length, modulate its resonance frequency, and,

in turn, alter the optical transmission to a photo -

detector. The Laser Interferometer Gravitational -

Wave Observatory, currently the gold standard of

gravitational-wave detectors, can achieve dis -

placement sensitivities as high as 10

−19

mHz

−1/2

. In

other words, it can detect a displacement of about

1/1000 of a proton radius based on a one-second

measurement.

A related approach to detecting gravitational

waves calls for using a massive, multiton cylinder

as a gravitational -wave antenna. In theory, the cylin-

der should undergo bending oscillations in the pres-

ence of a passing gravitational wave. Provided the

cylinder is integrated into a high-quality supercon-

ducting microwave cavity, that bending should de-

tectably modulate the cavity’s resonance frequency.

Although the interferometer and bar- antenna ap-

proaches to gravitational -wave detection deploy

very different technologies, both rely on the under-

lying concept that mechanical motion can be har-

nessed to modulate an electromagnetic resonance.

Thirty years after the first deep studies of the

limits of gravitational wave detectors, it’s evident

to us that Braginsky, Caves, and their contempo-

raries had two very exciting things to say: First,

gravitational-wave astronomy might be possible,

and second, so might the measurement and manip-

ulation of macroscopic objects at their quantum lim-

its.

The second message has motivated an increas-

ing number of mostly young researchers trained in

areas as diverse as solid-state physics, quantum in-

formation, and computation to look for and exploit

the quantum behavior of large mechanical objects in

tabletop experiments.

Getting to zero

Quantum effects in any system are most pro-

nounced when the influence of thermal fluctuations

can be ignored. So, ideally, a mechanical quantum

experiment would start with the mechanical ele-

ment in its quantum ground state of motion, in

which all thermal quanta have been removed. In

practice, however, one settles for cooling the ele-

ment such that for a given mechanical mode the

time-averaged number of thermal phonons, the so-

called occupation number

, is less than one. Put

another way, the mean thermal energy

should

be less than the quantum of mechanical energy

ħω

so that

≈

ħω

< 1. Here

is Boltzmann’s con-

stant,

is the reduced Planck’s constant,

is the

30

July 2012

Physics Today

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Quantum optomechanics

Mirror

Optical cavity

Laser

Beamsplitter

Optical cavity

Photodetector

Figure 1. Chasing waves. (a)

The

Laser Interferometer Gravitational -

Wave Observatory in Livingston,

Louisiana, and similar gravitational -

wave detectors were among the

first cavity optomechanical systems.

(b)

They typically consist of massive

mirrors suspended to form a pair of

optical cavities, each some kilo -

meters long. The cavities make up

the arms of a Michelson inter -

ferometer and together can detect

changes in distance as small as

−21

relative to the cavity length.

(c)

Mirrors used in the gravitational-

wave detector GEO600, located

near Sarstedt, Germany.

LIGO LABORATORY

HARALD LÜCK, MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS

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temperature, and

is the frequency of the vibra-

tional mode of the mechanical element.

Removing thermal phonons from the mechan-

ical element is a key experimental challenge. Inter-

estingly, the ideas and methods for doing so were

theoretically developed as early as the 1960s. The

main idea is to exploit the intracavity radiation pres-

sure, the force due to momentum transfer associ-

ated with photon scattering. In particular, Bragin-

sky realized that the finite time delay between a

change in position of the mechanical element and

the response of the intracavity field allows the radi-

ation field to extract work from or perform work on

the mechanical system.

The process is best illustrated for the case of a

basic Fabry–Perot resonator (see figure 2). When

both of the resonator’s mirrors are held fixed, the

optical transmission is sharply peaked near the cav-

ity resonance frequencies

pπc

, where

is the

mirror separation,

is a positive integer, and

is the

speed of light. The resonances result from the con-

structive interferences between the partial waves

propagating back and forth inside the cavity. The

higher the quality of the mirror, the more roundtrips

light takes before exiting the cavity, and the sharper

are the resonance peaks.

If one end mirror is mounted on a spring to

form a simple harmonic oscillator, a pump laser of

frequency

will be modulated by the mechanical

frequency and form sidebands with frequencies

. From a quantum mechanical perspective,

the process is analogous to the generation of Stokes

and anti-Stokes sidebands in Raman scattering: The

upper sideband is a result of pump-laser photons

acquiring energy by annihilating thermal phonons

in the mechanical element; the lower sideband re-

sults from photons depositing phonons and shed-

ding energy. The first process occurs at a rate pro-

portional to the occupation factor

of the

mechanical mode of interest; the second, at a rate

proportional to

+ 1.

By carefully detuning the frequency of the

pump field relative to a specific cavity resonance

one can resonantly enhance one of the processes. In

particular, red-detuning from the cavity resonance

enhances the upper sideband and promotes extrac-

tion of energy from the mechanical element. As long

as the up-converted photons leave the cavity suffi-

ciently fast, carrying with them their newly ac-

quired energy, the process can cool the motion of the

mechanical element to well below the temperature

of its surroundings. Although the quantum noise of

the optical source imposes a fundamental cooling

limit, it is nonetheless theoretically possible to cool

the mechanical mode arbitrarily close to the quan-

tum ground state,

= 0. Furthermore, the coherent

interaction between photons and phonons allows

manipulations in the quantum regime, as pointed

out early on by one of us (Meystre), Peter Knight,

Paolo Tombesi, and Claude Fabre.

The technique, a form of sideband cooling, was

first demonstrated in experiments by Braginsky and

by David Blair in the microwave regime as a way to

reduce noise in gravitational -wave antennas.

Since

2004, several laboratories around the world have

used the method to cool nano- and micromechanical

levers, in both the optical and microwave domains.

Today, high-quality optomechanical devices pro-

duce couplings strong enough to cool low-mass

levers—ranging from a few picograms to hundreds

of nanograms—to their ground state of motion.

The methods used in cavity opto mechanics are

in many ways analogous to the conventional laser-

cooling techniques developed for quantum informa-

tion processing with trapped ions (see the article by

Ignacio Cirac and Peter Zoller, P

HYSICS

ODAY

, March

2004, page 38). There, the collective normal-mode os-

cillations of a string of ions modify the response of

the pump laser that drives their internal state. The re-

sulting optomechanical coupling between the ions’

motional and internal degrees of freedom allows one

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July 2012

Physics Today

31

Figure 2. Quantum optomechanics: the basics. (a)

If both end mirrors

of a Fabry–Perot cavity are fixed in place, pump-laser photons having

frequency

tuned to a cavity resonance arrive at a detector with no

frequency modulation. (The total transmission to the detector is indi-

cated in the plot at right by the solid line; the cavity transmission spec-

trum is indicated by the dashed line.)

(b)

However, if one mirror is al-

lowed to oscillate harmonically, pump photons are modulated by the

oscillation frequency

: A pump beam tuned to a cavity resonance will

yield sidebands of equal amplitude at frequencies

. Each photon

in the upper sideband acquires energy by extracting a phonon from the

oscillator, and each photon in the lower sideband sheds energy by de-

positing a phonon.

(c)

By red-detuning the pump laser, one can en-

hance the upper sideband and thereby cool the oscillating mirror.

(d)

blue-detuning the pump laser, one enhances the lower sideband and

amplifies the mirror oscillations. (Figure prepared by Jonas Schmöle.)

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to cool the ions or prepare other quantum states of

interest.

However, cavity optomechanics differs in

an important and attractive aspect: Whereas conven-

tional laser cooling relies on the fixed internal reso-

nances of materials to enhance light– matter interac-

tions, cavity opto mechanical cooling allows one to

engineer the resonance-enhancing structure. That

structure could be an optical cavity with a series of

narrow resonances or a microwave cavity such as a

superconducting

circuit.

A tale of few phonons

Until recently, the pioneering developments in opto -

mechanical coupling went largely unnoticed outside

of the experimental gravitation and quantum optics

communities. Michael Roukes, who realized nearly

two decades ago that high-frequency nanoscale me-

chanical devices could be chilled to the quantum

regime, is a notable exception. Advances in materials

science and nanofabrication—particularly the rise of

nano- and microelectromechanical systems and opti-

cal microcavities—have since

opened the possibility of cou-

pling quantum optical modes

with mechanical devices in table-

top experiments.

The original

ideas of optomechanical coupling

were extended in many new di-

rections and realized in widely

varied optomechanical systems

(see figure 3). In the past year,

those efforts have culminated in

the cooling of at least three differ-

ent micro mechanical systems to

within a fraction of a phonon of

their ground state of vibrational

motion. (Here and below, unless

otherwise specified, mechanical

cooling refers to cooling of the

center-of-mass motion.)

In a NIST experiment led by

John Teufel and Ray Simmonds,

the mechanical resonator was a

circular aluminum membrane,

15 μm across and 100 nm thick,

that underwent drum-like vibra-

tions with a resonance frequency

of 10 MHz (see figure 3e). The

membrane was tightly coupled

to a superconducting microwave

cavity and chilled in a cryostat to

20 mK, at which the phonon

occupation

was about 40.

Sideband cooling was then used

to cool the membrane to

≈ 0.3.

At Caltech, Oskar Painter

and colleagues were similarly

successful using a 15-μm-long,

600-nm-wide, and 100-nm-thick

silicon beam as the opto -

mechanical system (see fig-

ure 3b).

Clamped at both ends

to a silicon wafer, the suspended

beam acts simultaneously as a

mechanical resonator and an

optical cavity. The mechanical mode of interest was

a breathing mode, a periodic widening and nar-

rowing that is most pronounced near the beam’s

midpoint and has a remarkably high quality factor

of 10

. (On average, a phonon survives 10

oscilla-

tions before being lost to the environment.) And

periodic perforations patterned into the beam cre-

ate a photonic crystal cavity that confines light to

the same region around the beam’s midpoint.

The co- localization of light and vibrational mo-

tion in such a small volume facilitates large opto-

mechanical coupling. Thus, after cryogenically

chilling the structure to 20 K, at which

≈ 100, the

researchers could use sideband cooling to remove

the remaining phonons and cool the beam to

≈ 0.8. At that point, the group was able to observe

another genuine quantum feature: Near the ground

state, a mechanical resonator is significantly more

likely to absorb phonons than to emit them, and

that asymmetry reveals itself experimentally as a

preferential sideband scattering of blue-detuned

32

July 2012

Physics Today

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Quantum optomechanics

MACRO

MICRO

NANO

−

−10

−12

−14

−16

−20

600 nm

15 μm

130 μm

30 μ

20mm

20 mm

300

30μm

30 μm

3μm

00 μm

FREQUENCY (Hz)

MASS (kg)

Figure 3. Cavity optomechanical devices

range from nanometer-sized structures of as

little as 10

atoms and 10

−20

kg to micromechanical structures of 10

atoms and 10

−11

kg to

macroscopic, centimeter-sized mirrors comprising more than 10

atoms and weighing sev-

eral kilograms. They include

(a)

gases of ultracold atoms,

(d)

micro spheres, and

(g)

micro -

scale membranes, all of which have mechanical resonances that can couple with the light

inside an optical cavity;

(b, c)

flexible, nanoscale waveguides that have both optical and

mechanical resonances;

(e)

superconducting membranes that exhibit drum-like vibrations

and can be integrated into microwave cavities;

(f )

microtoroidal waveguides having both

optical and mechanical resonances; and mechanically compliant mirrors, which can range

from the microscopic

(h)

to the macroscopic

(i, j)

and which introduce mechanical degrees

of freedom to an optical cavity when incorporated as an end mirror. (Figure prepared by

Jonas Schmöle. Images courtesy of (a) Ferdinand Brennecke, ETH Zürich; (d) the Vienna

Center for Quantum Science and Technology; (i) Christopher Wipf; and (j) LIGO Laboratory.

Other images adapted from (b) ref. 8, J. Chan et al.; (c) M. Li et al.,

Nature

456

, 80, 2008;

(e) ref. 7; (f ) ref. 10, E. Verhagen et al.; (g) J. D. Thompson et al.,

Nature

452

, 72, 2008; and

(h) G. D. Cole et al.,

Nat. Commun.

, 231, 2011.)

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light over red- detuned light. (See figure 4.)

The next challenge is to control the quantum

state of the mechanical resonator. A group led by An-

drew Cleland at the University of California, Santa

Barbara, took an important first step in that direction

by coupling an acoustic resonator to a qubit, a two-

state quantum system.

The resonator was a 300-nm-

thick sheet of aluminum nitride, 40 μm long and

20 μm wide, whose thickness oscillates at a fre-

quency of 6 GHz. At such high oscillation frequen-

cies, the phonon energy

ħω

is large, and therefore

a conventional dilution refrigerator—which can

reach temperatures near 25 mK—sufficed to cool the

resonator to an occupation factor

< 0.07.

Exploiting the piezoelectric nature of AlN, the

Santa Barbara team then coupled the resonator to the

qubit, a superconducting Josephson junction, which

could in turn detect the presence of a single phonon.

A null result meant the mechanical resonator was in

the ground state. The technique is analogous to those

that have been developed over the years in cavity

and circuit quantum electrodynamics, except that

the photons are replaced by phonons.

Not only did the resonator–qubit coupling

allow observation of the energy quantization and

other quantum features of the resonator, it also en-

abled controlled manipulation of the mechanical res-

onator at the few-phonon level. The Santa Barbara

group was able to observe a few coherent oscillations

of a single quantum exchanged between the qubit

and the resonator—a first demonstration of coherent

control over single quantum excitations in a micro-

mechanical resonator. Recent experiments at Caltech

and Harvard University and in Grenoble, France,

have made important further steps by coupling me-

chanical devices to a variety of other qubits.

Single-quantum or few-quanta control can

occur only in a strong-coupling regime, where en-

ergy is exchanged between the mechanical res-

onator and the qubit or optical mode with very little

dissipation; loss of photons and phonons to the en-

vironment must be minimal. That regime has now

been reached with several micromechanical devices

in addition to the one used in the Santa Barbara ex-

periment.

Eventually, such strong optomechanical

coupling will allow high-fidelity transfer of quan-

tum states between light and mechanical systems. It

should even be possible to generate entanglement

between photons and phonons. Conversely, phonon

fields can be mapped onto an optical mode to take

advantage of the reliable, high- efficiency detection

schemes available in optics.

Promise in the field

The lure of quantum optomechanics goes far

beyond simply adding another class of objects—

mechanical resonators—to the list of “tamed” quan-

tum systems. Rather, the promise is that just as

we’ve learned to couple mechanical elements with

the photons in an optical cavity, we can functional-

ize those same elements to couple with, say, the

spins in a magnetic material or the charges at a con-

ducting surface. That way, a mechanical element

would serve as a universal transducer, an interme-

diary between otherwise incompatible systems. Fly-

ing photons could be linked with stationary, non -

optical qubits, for instance. Only recently, a group

led by Philipp Treutlein at the University of Basel,

Switzerland, has demonstrated a hybrid opto -

mechanical system coupling ultracold atoms to a

micromechanical membrane. Such hybrid quantum

systems may be important in classical and quantum

information processing, for which the ability to con-

vert information from one form to another is crucial.

In fact, several laboratories are now working to cou-

ple single mechanical elements to both optical and

microwave frequency resonators, with the goal of

connecting superconducting microwave circuits

and qubits to optical fields.

The connection between optomechanics and

atomic physics is particularly interesting. Not only

did laser cooling of atoms inspire and enable the

rapid progress in quantum optomechanics, it also

led to the discovery of other phenomena such as

optomechanically induced transparency, the ana-

logue to electromagnetically induced transparency.

The effect, which exploits optical interference be-

tween a mechanical resonator’s excitation paths to

control the cavity transmission, may allow storage

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July 2012

Physics Today

33

AMPLI

TUDE

AMPLI

TUDE

ω

ω

ω

−

ω

ω

ω

−2

−1

ω

NORMALIZED

FREQUENCY

Figure 4. Quantum signatures

near the ground state.

(a)

The

nanoscale beam shown here undergoes breathing-mode oscilla-

tions—successive expansions and contractions—that are strongest

near the center, as indicated in the simulated image at top. The perfo-

rations along the beam’s length form a photonic cavity that confines

light to the same region, as indicated in the bottom image.

(b)

A laser

field at an appropriately detuned frequency

can be coupled to the

waveguide via a tapered optical fiber and used to cool the breathing-

mode oscillations to near the ground state.

(c)

At that point, red-

detuned photons are less likely to extract phonons and shift upward

in frequency (blue arrow) than are blue-detuned photons to create

phonons and shift downward (red arrow). (Here,

is the cavity reso-

nance frequency and

is the breathing-mode frequency.)

(d)

The

asymmetry is detectable in experiments: With roughly three phonons

residing in the beam, the upper sideband (blue) generated from a red-

detuned laser is significantly smaller than the lower sideband (red)

generated from an equivalently blue-detuned laser. (Panels a–c

courtesy of Oskar Painter and colleagues. Panel d adapted from

ref. 8, H. Safavi-Naeini et al.)

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of light in arrays of cavity

optomechanical devices.

Atomic systems can

also serve as mechanical

elements in cavity opto -

mechanical systems.

After

trapping 10

ultracold ru-

bidium atoms inside an op-

tical cavity, Dan Stamper-

Kurn and colleagues at the

University of California,

Berkeley, coupled the cloud

of atoms with a pump laser

that was detuned by the res-

onance frequency of the

atoms’ collective motion,

about 40 kHz. By blue-

detuning the pump laser so

that it deposited phonons

into the cloud, the group

was able to observe the

back-action exerted on the

atoms by the quantum fluc-

tuations of the optical field.

The generation and ma-

nipulation of mechanical

quantum states is also a

challenging and important

goal for quantum metrology and sensing applica-

tions. To date, the force sensi tivity of atomic force

microscopes and other classical mechanical devices

already exceeds 10

−18

NHz

−1/2

. In other words, in less

than a second, one can measure a force as small as

the gravitational attraction between a person in Los

Angeles and another in New York. Still, we haven’t

reached the ultimate limit. Current implementa-

tions suffer from thermal noise and, eventually,

from noise associated with quantum uncertainty.

Fortunately, quantum physics provides a way

around thermal and quantum noise by way of what

are known as quantum nondemolition measure-

ments. Such measurements, first posited in the 1970s

by Braginsky and coworkers, typically call for gen-

erating a squeezed state—that is, confining the un-

wanted but unavoidable quantum noise to a variable

that is complementary to the variable of interest.

Heisenberg’s uncertainty principle states that

certain pairs of physical properties—say, the ampli-

tude and phase of an electromagnetic wave—cannot

simultaneously be known with arbitrary precision.

But a measurement of the wave’s amplitude can be

performed in such a way that most of the uncertainty

is carried by the phase, or vice versa. Such squeezed

states of light have recently been shown to enhance

the sensitivity of gravitational -wave detectors

(see

HYSICS

ODAY

, November 2011, page 11).

In principle, it is also possible to prepare mechan-

ical squeezed states in which nearly all of the quantum

uncertainty is confined to either the position or the mo-

mentum. In fact, the classical squeezing of micro -

mechanical oscillators below the thermal noise limit

was first demonstrated several years ago by Daniel

Rugar and colleagues at IBM.

However, squeezing

below the standard quantum limit—the precision limit

for the case when quantum uncertainty is distributed

evenly among complemen-

tary properties—has yet to be

achieved. A number of strate-

gies based on Braginsky’s orig-

inal schemes, which can be

readily implemented in opto -

mechanical systems, are being

actively pursued.

Macroscale quantum

mechanics

Although still speculative,

micromechanical oscillators

could offer a route to new

tests of quantum theory at

unprecedented size and

mass scales. Since funda-

mental particles behave

quantum mechanically, one

would by induction expect

that large collections of par-

ticles should also behave

quantum mechanically. But

that conclusion certainly

seems contrary to our every-

day classical experience

with ordinary matter. Even

large quantum conden-

sates—a cupful of superfluid helium, for in-

stance—which do display quantum properties

such as frictionless, quantized flow, do not display

macroscopic superposition states.

To explain the so-called quantum measure-

ment problem, also notoriously known as

Schrödinger’s cat, some theorists propose that

standard quantum mechanics breaks down for

macroscopic objects in such a way that their super-

position is forbidden. In one such theory, gravita-

tion, which is always unshieldable, ultimately

causes massive objects to decohere, or transition

from quantum to classical behavior. In another the-

ory, objects couple to a stochastic background field

that localizes the object at a rate that scales with the

number of particles.

Quantum optomechanics offers a promising

way to produce spatial superpositions in massive

objects such as mechanical levers or quartz nano -

spheres and to directly test theories of how they de-

cohere.

Ongoing work in that direction builds on

optical-trapping and optical-cooling techniques

originally proposed by Arthur Ashkin

and should

eventually allow a single trapped particle to be pre-

pared in a quantum superposition of two distinct

center-of-mass states.

Approaching the problem from the opposite

direction—from the bottom up—researchers in

Vienna used conventional molecular-beam tech-

niques to produce matter-wave interferences with

large, 430-atom molecules.

Ultimately, it may be

possible to conduct similar quantum experiments

with even more massive mechanical systems. A

group led by Nergis Mavalvala of MIT recently

took a first step in that direction by cooling a

kilogram-size oscillator to within about 200

phonons of the quantum ground state.

34

July 2012

Physics Today

www.physicstoday.org

Quantum optomechanics

Today’s devices,

though similar in

many ways to those

of antiquity, steer us

to a

completely

different worldview—

one in which an

object, possibly

even a macroscopic

one, can indeed act

in two ways at the

same time.

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Macroscale mechanical quantum experi-

ments will have to overcome a number of daunt-

ing technical issues. Some of those issues—

identifying gravity’s role in decoherence, for

example—might be resolved by conducting ex-

periments in free fall, perhaps aboard a satellite.

(Last year’s Caltech experiment, in which a

phonon occupancy of less than one was achieved

at a bath temperature of 20 K, shows that ground-

state cooling is now within the range of commer-

cial cryocoolers that can be flown on satellites.)

We are confident that coming experiments will

lead to a more profound understanding of quan-

tum mechanics, establish limits to its validity, or

confirm what we, and likely many others, be-

lieve—that technical issues such as environmen-

tal decoherence, and not the appearance of new

physical principles, establish the transition from

the quantum world to the classical. We have never

been so close to being able to truly address those

profound questions and to challenge Plato’s com-

monsense notion of reality in the laboratory.

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■

July 2012

Physics Today

35

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