One + One is Not Two

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Vol. 10 | Spring 2013 www.onr.navy.mil/innovate

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INNOVATIONOffice Of Naval ReseaRch vOl. 10 | spRiNg 2013


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www.onr.navy.mil/innovate Vol. 10 | Spring 2013

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shared private keys that appears immune

to eavesdropping. It has been commercially

available for several years now. In the

Informac Phenomena Group at Naval Research

Laboratory (NRL), our view on QIS is thattechnologies like quantum key distribuon

represent a small fracon of the areas true

potenal. Our posion is not merely intended

to help safeguard against overinvestment. It is

also grounded in the belief that within the realm

of Informaon Technology, changing the basic

unit of informaon from classical to quantum

could be a 21st century version of spling the

atom. Only when the central idea in QIS is taken

seriously, that it is an enrely new paradigm

of informaon, and only when we learn how

to naturally apply its laws, do we expect to see

it have a revoluonary and lasng impact on

Informaon Technology.

At the center of this new paradigm lies the

qubit: a revoluonary challenge to the classical

bit. A classical bit can assume either the value

0 or 1. A quantum bit, or qubit, is capable of

being 0, 1 or anything in between 0 and 1.

Unlike classical bits, qubits cannot necessarily

+ is noTInnovaon is somemes the result of

re-examining certain fundamental beliefs about

the way the world works. To illustrate, consider

a few of the simple quesons one can ask about

informaon:

(a.) Can informaon always be copied?

(b.) Given two idencal pieces of

informaon, can we read each one and

get dierent results?

(c.) Is it possible to have complete

informaon about the whole, but no

informaon about any of the parts?

Convenonal thought would have us believe

that the answers to these quesons are (a) yes,

(b) no and (c) no. In Quantum Informaon

Science (QIS), though, the answers are just

the opposite: (a) no, (b) yes and (c) yes. While

each of these properes stands in contrast to

our everyday experience, perhaps the most

counterintuive aspect of quantum informaon

is that collecvely they have the potenal to

radically improve the way we transfer, process

and acquire informaon, including but not

limited to the way we communicate, compute

and perform sensing.

Currently though, the most developed

technology oered by QIS is quantum key

distribuon, which provides a way to generate

Keye Martin, Ph.D.

Research Mathematician

Naval Research Laboratory

In Quantum Informaon Science, as in the work above,

the whole can be more than the sum of its parts. (Used

with permission of the arst, specially commissioned for

this arcle)


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Vol. 10 | Spring 2013 www.onr.navy.mil/innovate

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be copied, and while this poses a denite

challenge when trying to perform certain tasks,

it can also help prevent an eavesdropper from

listening in on quantum communicaon. To

read a qubit, one performs a measurement

on it which yields a classical bit. The result of a

measurement is in general nondeterminisc, somore oen than not we can read two idencal

qubits and obtain dierent results. This might

seem to be completely problemachowever,

if we use a qubit to store informaon and an

aacker manages to steal it, the chances are

good that they will not be able to read the

informaon we want kept secure.

One of the most fascinang properes of

quantum informaon is entanglement. When

two qubits are entangled, measuring one

changes the other, no maer how far apartthe qubits are from each other. In the case of

maximal entanglement (Figure 1), it is possible

to teleport a qubit from one place to another:

the qubit is transferred from point A to point

B without ever travelling through space. With

maximal entanglement, we have complete

informaon about the whole without having

any informaon about the parts i.e. the whole

(2) is more than the sum of its parts (1+1).

A major problem in QIS is dealing with the

environment that a qubit evolves in. This

might be ber, freespace or even water. The

environment is not merely a canvas on to which

a qubit is paintedthe two interact with each

other, oen resulng in noise. Or they may be

entangled, which can cause informaon to be

unwillingly leaked into the environment. Though

usually in smaller doses, even the curvature of

spaceme itself (gravity) can cause noise, which

requires taking relavisc eects into accountimagine a qubit on a satellite orbing Earth for

instance. A major thrust of our groups research

at NRL is to invesgate mathemacal models for

all of these situaons and use them to develop

methods for opmizing performance in a noisy

environment. This research in turn suggests

the possibility of a number of new Naval

technologies. Let us now consider one such

example: using entanglement to enable stealthy

vehicle navigaon under the arcc shelf.

This is a dicult problem because (1) compassesand gyrocompasses are less reliable in higher

latudes, (2) GPS signals cannot penetrate

water and ice, and (3) the shape and thickness

of the ice shelf is variable. To avoid a potenally

catastrophic collision, an underwater arcc

vehicle might use an acve sonar array to

detect its proximity to the ice. In a combat

environment, however, the use of navigaon

sonar could give away the posion of the

vehicle. Our research on QIS has shown that

the operaonal requirements of stealthy

underwater arcc navigaon are tailor-made forentangled qubits. Heres how it works.

Figure 1. With a maximally

entangled state, we have

complete informaon

about the combined

system without having

any informaon about the

individual subsystems.

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Figure 2. By using entangled qubits in place of sonar, it is possible to perform stealthy underwater arcc

navigaon.

target 437 meters away in clear oceanic waters

(Jerlov Type I). For the same signal-to-noise

rao, the detecon probability of a quantum

sensor is 6 mes beer than a laser and 8

mes beer than sonar. Two of the majorchallenges in making this system a reality are

building beer photodetectors and faster

sources of entanglement than presently exist.

In a similar way, we have used entanglement

to design various approaches to radar, lidar,

magnetometry and gravimetry.

The revoluonary potenal of QIS stems

from sciencally re-examining the nature of

informaon itself. But we should not stop with

QIS. There may be other, as of yet undiscovered,

paradigms of informaon. By abstracng the

essenal properes of classical and quantum

informaon, it may be possible to use common

mathemacal structure as a guide in discovering

some of these.n

First, we replace the navigaon sonar array

with a collecon of low brightness quantum

sensors. Then an entangled pair of qubits is

produced; one qubit is kept within the sensor

while the other is emied towards a region ofspace. Aerward, entanglement correlaons

are used to disnguish between noise qubits

and those signal qubits that are bounced back

to the detector due to the presence of a target

(e.g., the ice shelf, the boom, or another

vehicle) (Figure 2). Calculaons performed at

NRL suggest that if this system were built, it

would be capable of operang in low brightness

levels using a small number of qubits and in

an extremely noisy environment, making the

existence of the sensor praccally invisible

to those without access to the entanglementcorrelaons: our quantum sensor hides the

signal qubits in environmental noise, and

entanglement is the key to detect them.

Specically, assuming a signal-to-noise rao

of 2 10-6, a quantum sensor can detect a


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