Science - USA (2020-09-25)

1556 25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 sciencemag.org SCIENCE

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SCIENCE

Y

ou might call Jeffrey Shapiro the re-

luctant godfather of quantum radar.

Twelve years ago, the electrical engi-

neer at the Massachusetts Institute

of Technology (MIT) helped develop

the key concept underlying this

scheme to dramatically increase radar’s

sensitivity. But even he doesn’t think the

technology will work. “There’s just a lot of

problems that make it hard for me to be-

lieve that this system is going to be of any

use,” Shapiro says. So he is both bemused

and dismayed by the attention other re-

searchers and funding agencies continue

to lavish on it.

A mini–arms race is unfolding in the sup-

posed field, initiated by press reports in 2016

that China had built a quantum radar—

potentially threatening the ability of

stealthy military aircraft to hide in plain

sight from conventional radars. “I started

working on this because there were gov-

ernment people coming to me and saying,

‘There are reports of quantum

radar coming out of China. Is

this real?’” says Christopher

Wilson, a physicist at the Univer-

sity of Waterloo in Canada. His

group and others have demon-

strated elements of a quantum

radar scheme, but only in limited

experiments that a nonquantum

system can still match.

The quantum radar story be-

gan in 2008, when Seth Lloyd,

a quantum engineer at MIT,

unveiled his concept of quan-

tum illumination (Science,

12 September 2008, p. 1433).

Lloyd argued that you could

more easily detect an object

against a bright background

if, instead of merely reflect-

ing light off it, you exploited a

quantum connection between

particles called entanglement.

Every photon has a frequency

that determines its energy.

Quantum theory says, weirdly,

that a photon can have multiple

frequencies at once—until it’s

measured and “collapses” ran-

domly to one frequency or an-

other. Even weirder, two such

photons can then be entangled so that

their frequencies, although uncertain, are

correlated: They are sure to be identical

whenever they’re measured.

Lloyd calculated that an observer could

more easily pick out an object by generating

entangled pairs, shining one photon toward

the object, keeping the other, and then mea-

suring the retained and returning photons

together in a particular way. Essentially, the

entanglement correlations would make it

harder to mistake a background photon for

one reflected off a target. The signal to noise

ratio would scale with the amount of entan-

glement: The more frequencies spanned by

each photon in an entangled pair, the stron-

ger the signal.

Lloyd’s calculation relied on a highly

idealized form of entanglement. So that

same year, he, Shapiro, and colleagues re-

did it for the real entangled light pulses

that experimenters can generate with a

special crystal that converts a single higher

frequency pulse to two entangled pulses at

lower frequencies. The pulses have no defi-

nite number of photons—just an average

number—and they are “noisy,” like radio

static. But thanks to the entanglement, the

noise in the two pulses is highly correlated.

The researchers compared the sensitivity

of a detector relying on the entangled pulses

with a conventional one sending out single

pulses of laser light, also known as coherent

states. They found that the quantum effects

boosted the signal-to-noise ratio by just a

factor of four, less than they hoped for. “We

were slightly disappointed,” says Si-Hui Tan,

now a quantum information theorist at Hori-

zon Quantum Computing. “Coherent states,

they’re just so damned good!” she says.

Still, the calculation gave experimenters

a target. In 2015, researchers at MIT dem-

onstrated quantum illumination at optical

frequencies, realizing a 20% increase in sig-

nal to noise. But that experiment had a ma-

jor limitation. The whole idea was to detect

an object against a bright background, but

there’s very little optical background at room

temperature—your surroundings don’t glow

visibly. So the MIT team had to generate ar-

tificial background light.

Things are different in the

microwave band, where radar

works, says Johannes Fink, an

experimental physicist at the In-

stitute of Science and Technology

Austria. At room temperature,

microwaves stream from every-

thing, even the air. “People are

interested in the microwave be-

cause the background is always

present,” he says. Stealth tech-

nologies hide military planes by

suppressing their reflectivity at

microwave frequencies so that

the glow of the surroundings

masks the plane’s reflections.

Quantum illumination seemed

to promise a way to defeat

stealth technologies. However,

demonstrating the scheme with

microwaves has proved daunt-

ing. Physicists can generate pairs

of entangled microwave pulses

from single ones using, instead of

a crystal, a gizmo called a Joseph-

son parametric converter. But

that device only works at temper-

atures near absolute zero, which

requires working within cryo-

stats cooled with liquid helium.

By Adrian Cho

PHYSICS

The short, strange life of quantum radar

In spite of military interest, quantum mechanics won’t defeat stealth technologies

Target

1

2

3

Cryostat

Signal pulse

Returning pulse

Retained pulse

A visionary scheme

Some researchers hope to improve the

ability of radar to spot a target against

background radiation by exploiting

a quantum connection between

microwave pulses.

NEWS | IN DEPTH

1 Generator

Creates pairs of

microwave pulses

that are entangled,

meaning their noise

is highly correlated.

One pulse is sent

toward the target.

2 Delay line

Maintains the other

microwave pulse so it

can be measured later.

3 Detector

Measures the returning microwave pulse

in concert with the retained pulse to probe

the correlations between the two and pick

out the target with greater efficiency.