The amazing world of quantum mechanics may revolutionize
the way
logisticians compute support requirements and communicate on
the battlefield.
Editor’s note: Innovative developments in science and
technology will change how the Army is deployed and sustained.
This is the first in a series of articles written by members
of the Army Logistics Innovation Agency’s Futures Group
that survey some of the most promising possibilities.
We live in a time of unprecedented technological advances that
hold profound logistics implications for our Army. The journey
to fascinating, powerful, and novel computation and communication
capabilities, in particular, will lead to new scientific and
technological developments with many benefits to Army logistics.
“Quantum Computation and Communication” is one
of five themes for future logistics innovation identified
by the Army Logistics Innovation Agency (LIA) at Fort Belvoir,
Virginia. The others are “Prediction and Cooperation,” “Energy-on-Demand,” “Designer
Materials,” and “Telepresence.” Each of
these themes describes plausible future advances in technology
and business processes that may improve logistics effectiveness
significantly. They also depict future conditions under which
logistics functions will be significantly improved and logistics
requirements radically reduced. Together, the themes offer
an advanced look at some amazing possibilities for Army logistics.
Our goal in this and subsequent articles in Army Logistician is to explain the science underlying these themes in plain
language while outlining the possibilities
they offer for future logistics. In this article, we examine the salient features
of quantum computation and quantum communication. We will explore the quantum
world and explain how quantum mechanics (QM) forms the bedrock of these emerging
technologies. The effects of QM are counterintuitive and require that we rethink
our everyday view of how the world operates. Relating the effects of QM to
quantum computation and quantum communication will give you
an appreciation of how these
technologies will benefit and revolutionize logistics. Admittedly, this is
a challenging subject. The terminology and concepts will be new to many readers.
But we feel it is important to realize that, over the course of a 20-year career,
today’s sergeants and second lieutenants undoubtedly will be affected
by developments in this and related scientific fields.
We should mention that LTA serves a unique role in the Army logistics community.
As “scouts” for advanced business processes and technology for
the Army’s Deputy Chief of Staff, G–4, LTA looks for opportunities
to inform the logistics community about research efforts of potential value
to logisticians. Articles like this provide information that can contribute
to the development of a vision for future logistics capabilities, policies,
and plans.
Technology for a New Logistics Environment
Future Army logisticians will have to manage a range of logistics
functions across an end-to-end logistics enterprise and will
need tools that permit effective decisionmaking and rapid,
dynamic planning.
In an increasingly complex environment, we must consider new
ways to model and analyze diverse and dynamic processes that
exist at globally distributed locations. We must be open to
the most efficient methods of modeling interrelated phenomena
among the intelligence, operational, and logistics domains.
Decreased cycle time, increased situational awareness, and
secure transmission of real-time logistics information are
just some of the benefits that quantum computation and quantum
communication will offer to Army logistics. These exciting
new fields of science will harness the fundamental laws of
physics to dramatically improve the acquisition, transmission,
and processing of logistics information. The goal is unprecedented
computation capabilities and secure communications for complete
battlefield dominance.
Emerging joint warfighting concepts require that
Army logisticians be completely integrated into the
joint fight. To realize fully many of the capabilities prescribed
in the Joint Logistics (Distribution) Joint Integrating Concept,
Army logisticians must perform their missions with unprecedented
levels of connectivity and joint interdependence. Quantum computation
and quantum communication promise to provide those capabilities.
Quantum computers would be capable of computing at speeds far
exceeding those of conventional computers and performing
calculations that are too large for conventional computers
to complete in a reasonable time. Likewise, quantum communication
devices would allow for real-time, highly secure transfer of
information with near-zero latency. [“Latency” refers
to the time lag encountered in an end-to-end communication.
Humans can detect time lags of about 16 milliseconds and greater. “Near-zero
latency” refers to a lag of less than 16 milliseconds.]
The
Quantum World
Before 1900, the laws of classical physics
did an excellent job of describing large, slow-moving particles,
but they
could not explain the behavior of subatomic particles such
as electrons, protons, neutrons, photons, and quarks. It
was not until the development of QM that the behavior of
such particles could
be explained.
Today, QM is the most satisfactory theory
available for explaining life in the quantum world. QM,
however, is notoriously
difficult to understand because it requires a complete revision
in our concept of a “particle.” While the features
of QM presented here may seem bizarre, it is important to
remember that a host of astonishing, practical applications
have resulted from QM. Some everyday examples include the
laser, the processors in a computer, and the many forms of
medical imaging in use today. With this in mind, let’s
try to understand those features of QM that are most relevant
to quantum computation and quantum communication.
What is a subatomic particle? The
development of QM followed a number of surprising observations
that could not be explained
by classical physics. One observation in particular—the
photoelectric effect—led Albert Einstein to
suggest that light exists in discrete packets of energy called “photons.” Before
1900, light always was described as a wave. After Einstein,
light was described as consisting of little “quanta” of
energy. Today, physicists accept the fact that light behaves
as both a wave and a particle—it has “wave-particle
duality.” In fact, it is the wave-particle duality
of light that allows night-vision goggles to operate. Light
exhibits wave-like properties when passing through the goggles’ lens
but particle-like properties when it hits the goggles’ internal
sensor.
In the years following Einstein’s suggestion, suitably
designed experiments demonstrated that electrons also exhibit
wave-particle duality. At the time, QM’s proposition
that electrons had wave-particle duality was quite disconcerting.
Didn’t electrons have mass? Weren’t they little
point-like things that orbit the nucleus of an atom like
planets orbiting the sun? Perhaps even more disconcerting
was the fact that wave-particle duality applies to all subatomic
particles. Quantum particles were not the tangible particles
that we were used to. QM rendered our conventional images
of particles obsolete.
Where is that particle? If a subatomic particle is not really
a particle in the classical sense, then how can we say where
it is? In QM, the physical properties of subatomic particles
are not evident until someone decides to measure them. This
is rather disquieting. It implies that physicists can predict
the possible outcome of a particular measurement but cannot,
with absolute certainty, ensure the outcome of that measurement.
The
definite location of an electron, for example, cannot be
given until one measures its location. Before a measurement
is made, the particle is actually in several possible locations,
so several possible outcomes to its whereabouts are possible
and the best one can do is give the probability that it will
be “over here” or “over there.” In
the language of QM, the particle is said to be in a “superposition
of states.”
Immediately after a measurement is made, we can say with
certainty where the particle is; but until the particle is
measured, it does not have a position. This feature of QM
is very important to both quantum computation and communication.
As we will see shortly, the fact that measuring a subatomic
particle forces it to take a value is the feature
that allows us to know if a quantum communication has been
compromised.
|
|
| When photons
travel, they vibrate, and the direction in which
they vibrate is called “polarization.” In
figure A, photons can be forced to vibrate in certain
directions (vertically, diagonally, or horizontally)
by passing them through a filter known as a Polaroid.
Figure B portrays quantum entanglement, a concept
of fundamental importance for quantum communication.
Two photons have become entangled. They then no longer
have individual quantum states; they are interrelated. |
|
Where is that particle going? Isaac Newton is
famous for, among other things, the laws of motion governing
everyday objects. For example, given the position and momentum
of an object, we can determine where that object is going
using Newton’s laws of motion. In QM, the position and
momentum of a subatomic particle cannot be measured with very
high accuracy. Simply trying to measure the position of an
electron will alter its state. If we measure its position with
great accuracy, we alter its momentum; if we measure its momentum
with great accuracy, we alter its position. These features
are embodied in what is called the “Heisenberg Uncertainty
Principle.” This principle, along with wave-particle
duality and super-position of states, is crucial to our discussion
of quantum computation and communication.
Quantum entanglement. Quantum entanglement is actually a useful feature of
QM; it is a resource at our disposal. To explain what it is, suppose that
we have two photons that have become interrelated. If a measurement of one
photon instantly influences the other photon—even if the photons
are very far apart and isolated from each other—then these two
photons are “entangled.”
As an example, let’s introduce two characters, Alice and Bob. When photons
travel, they vibrate, and the direction in which they vibrate is called “polarization.” (See
figure A above) Now, let’s consider that Alice and Bob share entangled
(and perfectly correlated) polarized photons. Alice (let’s put her on
Earth) has one photon, and Bob (let’s put him on the Moon) has the other
photon. It is important to realize that, while these entangled photons are
polarized, neither Alice nor Bob knows what the polarizations are. If Alice
then proceeds to measure the polarization of her photon with a polarizer and
finds that it is vertically polarized, then Bob’s photon must be horizontally
polarized. (See figure B above.) Such quantum entanglement, which Einstein
called “spooky,” is of fundamental importance for quantum communication
and, in particular, for quantum teleportation.
The main characters. The stage
is now set. Quantization (subdivision into quantum particles), wave-particle
duality, superposition of states, uncertainty, and entanglement are the only
features of QM that we need to consider in order to explore quantum computation
and communication. It is important to realize that these quantum effects are
not a result of shortcomings in QM but that they are inherent in nature; they
reflect the essence of our world.
Quantum Computation
We begin our discussion of quantum computation with an example
adapted from Dr. Lov Kumar Grover, the inventor of the Grover
quantum search algorithm.
Assume that you are trying to solve a crossword puzzle and
you have the following: _ _ r _ n h _. (The solution is “piranha.”)
In an effort to solve the puzzle, you check an online dictionary
with 1,000,000 alphabetically arranged words and then develop
a program to search the dictionary automatically for a fit
to the puzzle. Your program typically solves the puzzle after
looking through 500,000 words. This is really the best one
can do using a conventional computer [a computer that measures
data in bits (0s and 1s)].
What would happen if we used the multiple states available
to a photon or other quantum particle rather than the 0s and
1s of a conventional computer? What if we used a quantum computer
(a computer with data that can be in multiple states at the
same time)? If we had a quantum computer with an online dictionary,
it would be possible to carry out multiple computations simultaneously.
A quantum computer, using quantum search algorithms, could
complete the search for an answer to our puzzle after 1,000
words.
In the future, database searches will occur at speeds faster
than possible with even the most powerful conventional computer
in existence today. This kind of revolutionary computing power
would be a tremendous aid to the exploitation of massive supply,
maintenance, and transportation databases and holds great potential
for improving the responsiveness of highly complex logistics
systems. So, how does a quantum computer work?
Superposition of states at work. Recall
that subatomic particles, until measured, are in a superposition
of states. Let’s
see how this applies to quantum computation. The memory in
a conventional
computer is made up of bits. A bit is a binary digit. For
example, the binary number 1001011 is 7 bits long. (A byte
is a collection of bits—almost always 8 bits.)
A bit can be either a 0 or a 1 but not both. A bit is like
a switch;
it can either be “on” or “off.” A conventional
computer can do three things: it can set a bit to 0, it can
set a bit to 1, or it can look at a bit and use it to decide
what value to give to some other bit. In a nutshell, a conventional
computer operates by interpreting bits and figuring out what
to do with them.
A quantum computer uses qubits (quantum bits.) Quantum computers
operate by observing the state of qubits. A qubit can be a
0, a 1, a combination of the two, or it can represent a number
that is somewhere between 0 and 1; that is, it can be in a
superposition of states. Now, imagine for a moment that you
have 200 qubits; this represents a quantum superposition of
as many as 2200 states. Each one of these states is equivalent
to a conventional computer’s single list of 200 1s and
0s. A quantum computer would simultaneously operate on all
2200 states in the process of doing a computation. In the same
amount of time it takes a conventional computer to operate
on one state, a quantum computer can operate on 2200 states.
To perform a calculation in the same amount of time as a quantum
computer, a conventional computer would need 1060 processors. This amount of computing power
is staggering; we certainly would not use such a computer for
word-processing or email. Dr. Peter W. Shor of AT&T’s
Bell Laboratories has provided an application of a quantum
computer, rapidly factoring very large numbers in a matter
of seconds. Currently, the premier application of quantum
computing is in the area of cryptology, which is the art of
making and breaking ciphers (secret codes) that are used to
encrypt messages. Virtually all encryption methods in use
today can be decrypted using quantum algorithms. However, many
exciting potential applications of quantum computers remain
to be discovered.
The state of quantum computation today. Current quantum computers
can perform simple operations on 2 and 3 qubits. This level
of computational power, however, does not rival the conventional
computer. The challenges to creating a quantum computer of
any value are primarily in the areas of “decoherence” and
error correction.
If a qubit comes into contact with its environment (for example,
becomes entangled), then its quantum state will decay into
a mixed state and the qubit is said to “decohere.” Decoherence
destroys the efficiency of a quantum computer. This interaction
of quantum states of a particle with the environment is typically
referred to as “noise.” Conventional computers
correct for noise through error correction codes that store
the bits with redundancy (the bits are copied and later recovered).
The states of a quantum particle cannot be copied, and, even
if they could, we would not know if an error occurred without
making a measurement (which we cannot do without altering the
state of the particle). Nevertheless, enough progress is being
made in error correction so that we can realistically predict
(although it is a precarious endeavor to predict technology
breakthroughs) that, within a decade, error-free quantum computation
on a small scale will be achieved.
Quantum Communication
Quantum communication aims to provide secure communication
mechanisms by using the features of QM. Some questions that
quantum communication seeks to answer are: What novel ways
are there to achieve secret communications? Is it possible
to improve the efficiency of sending conventional communications?
Is it possible to communicate information to distant locations
without ever transporting the actual information? These questions
are best answered within the realms of quantum teleportation
and quantum cryptography, which are subsets of quantum communication.
Quantum Teleportation
According to classical physics, teleportation is defined as “the
disembodied transport of matter through space.” It is
perhaps unrealistic to think that an object can be disintegrated
in one place, transmitted, and then perfectly reconstructed
in another place; the process of teleportation conjures up
memories of the “transporter” in the television
series Star Trek. QM, however, makes teleportation, in a sense,
plausible.
Let’s take this periodical as an example. At the fundamental
level, Army Logistician consists of particles such as electrons.
According to QM, all “fundamental” particles of
the same kind are identical. The electrons in this periodical
are identical to the electrons in any other periodical. What
are not identical, and what distinguishes this issue of Army
Logistician from another magazine, in terms of QM, are the
quantum states of the electrons that collectively make up each
periodical. QM implies that, to transport matter from one location
to another, it is not necessary to actually transport the
particles that make up the matter in question. It is sufficient
to recreate, in the other location, the quantum states of
all the particles that make up the piece of matter. So, teleporting
the quantum states of all the particles that make up this periodical
would create a replica in another location. In other words,
this Army Logistician would have been teleported to the other
location.
However, to re-create the quantum state of a particle, one
has to measure the particle’s quantum state with great
accuracy. We know from our earlier discussion that this is
not possible; nature simply will not allow it (recall the Heisenberg
Uncertainty Principle). Even if it were possible, re-creating
the quantum states of the particles that make up Army Logistician would
require a phenomenal amount of information—so
much information that it would take much longer to transmit
the information than to physically send the periodical by airmail
to the remote location in question.
|
|
Quantum
teleportation does not involve the actual transportation
of particles; it is based on recreating the quantum
states of all the particles that make up a piece
of matter in another
location. This is demonstrated at left. In figure
A, Alice and Bob share an entangled pair of photons.
Using this entangled pair of photons, they can teleport
another photon (Alice’s Photon 3), or, equivalently,
the state of that photon, between them. To do this,
Alice entangles her Photon 1
with her Photon 3. Figure B shows that Alice has
succeeded in teleporting the state of her Photon
3 to Bob, so they no longer share an entangled pair
of photons. |
|
Let’s imagine for the moment that we can determine the
quantum state of a particle without measuring it. This is
possible if we use entangled pairs of particles. (See chart
above.) Let’s assume, again, that Alice and Bob share
entangled polarized photons (Alice has photon 1 and Bob has
photon 2) and that neither Alice nor Bob knows what those polarizations
are. Alice also has another photon (let’s call this
photon 3) with an unknown polarization that she would like
to teleport
to Bob. Remember, Alice cannot measure the polarization of
her photons. If she does, she will cause them to assume specific
polarizations. But if Alice performs a measurement on her
photons jointly, then she will cause Bob’s photon to correlate
with the joint measurement she makes. Bob’s photon then
will be in the same quantum state (that is, have the same polarization)
as Alice’s photon 3. In other words, the state of Alice’s
photon 3 will have been teleported to Bob’s photon
2.
Some comments are in order. First, teleporting the state of
a photon is completely equivalent to teleporting a photon.
Thus, entanglement provides a means of communication. Second,
during the process of Alice’s measurement of photon
3 (in conjunction with photon 1), photon 3 became entangled
with
photon 1 and, as a result, photon 3 lost its original state.
In a sense, it forgot what state it was in. This does not
matter too much to Alice since she succeeded in teleporting
the state
of photon 3 to Bob. However, it is important to understand
that, if we try to teleport an object from one location to
another, the state of the original object will be destroyed
in the process.
Classical Cryptography
Cryptography is the art of sending messages in disguised form.
Message encryption is accomplished using an “encryption
algorithm,” or “key.” Some of the earliest
keys in existence consisted of what are known as substitution
algorithms. The Caesar Cipher (allegedly used by Julius Caesar),
which consisted of encrypting a message by shifting letters,
is an example of a substitution algorithm. The size of the
shift is the key, and that is what needs to be kept secret.
This is where the problem of key distribution arises. The key
must be shared between the sender and the receiver in order
for the receiver to decipher the message. How can Alice send
the key to Bob without an eavesdropper (Eve) monitoring the
communication? Without a key, a classically encrypted message
must be decrypted using cryptanalysis.
Today, the problem of key distribution is solved and cryptosystems
are nearly impossible to crack with conventional computers.
The reason is simple: while it is fairly easy to multiply two
large numbers, it is very difficult to factor very large numbers
because factoring a large number requires a lot of computing
power. Our discussion of quantum computation, however, indicates
that this might not always be the case. Is there a better
way to encrypt messages? Is there a way for Alice and Bob to
communicate and know if their communication has been compromised?
Classical cryptography has no way of addressing these questions.
Quantum Cryptography: Secure Tansmission
Quantum cryptography is a means of communicating securely
using physical objects such as photons. One of the central
issues in cryptography, as we have noted, is key distribution.
Using mathematics in classical cryptography circumvents the
problem of key distribution, but the coming of quantum computers
puts classical cryptography in jeopardy. Another way to circumvent
the key distribution problem is to use the effects of QM to
create quantum cryptography. In quantum cryptography, the
security of a communication is guaranteed by the uncertainty
principle and by the fact that performing a measurement on
a quantum particle alters its state. One way to achieve quantum
key distribution is to use polarized photons: bits of information
are encoded using polarization.
To demonstrate communication using polarized photons, let’s
turn to Alice and Bob again. Alice wants to send an encrypted
message consisting of 0s and 1s to Bob and, at the same time,
wants to outwit Eve. (See chart below.) She can use a scheme
(
)
in which she represents a 0 with a horizontal photon “—” and
a 1 with a vertical photon “|,” or she can use
a scheme
(
)
where she represents a 0 with “\” and a 1 with “/.” To
send a binary message, Alice sends an unpredictable series
of polarized photons using either the scheme
or the scheme.
|
|
| Quantum
cryptography uses physical objects such as photons
to communicate securely. In the illustration above,
Alice randomly polarizes photons in one of four possible
positions (the photon’s polarization) to transmit
a “key” to Bob. Alice represents a ‘0’ by
a horizontal or left diagonal polarization and a ‘1’ by
a vertical or right diagonal polarization. Bob uses
his Polaroid filter to “read” Alice’s
photons. If he chooses the correct polarization,
he can read the bit. After Alice sends her message
to Bob, they speak over a normal communications channel
to determine what key to use for further encryption. |
|
In order to intercept the message, Eve needs to identify the
polarization of each photon. As she sees a photon coming,
she will orient her Polaroid filter. However, every time she performs
the wrong measurement, she alters the photon’s state.
Alice, after sending her photons to Bob, picks up the phone
and tells
Bob what scheme she used but not the polarization she used
for each photon. In this way, Alice and Bob can monitor the
communication for disturbances and will be able to determine
if eavesdropping occurred. Bob also will know which photons
he measured correctly simply by knowing the scheme that Alice
used. In this way, they will have created a secure key that
they can use to encrypt further messages. Eve cannot intercept
a photon, and she cannot measure the quantum state of that
photon with great accuracy and then re-emit it without introducing
some error into the communication.
The uncertainty principle guarantees secure communication
between two parties. Today, there are point-to-point commercial
quantum cryptography devices on the market. Cost, the lack
of dedicated fiberoptic lines (for sending the photons),
and the nonexistence of single-photon sources are the factors
currently limiting quantum cryptography. The use of quantum
cryptography over networks is anticipated
in the next few years, and long-haul secure quantum communication
(such as by satellite optical communications) is anticipated
to occur within the
next decade.
The range and reliability of quantum cryptography devices
currently are a concern. Scientists and engineers may be
able to overcome these problems by using devices such as
quantum repeaters and by employing quantum error-correction
techniques. These devices would enable quantum signals to
be restored at distant locations without reading, and hence
altering, the quantum information. Quantum cryptography
may be applied to achieve completely secure communications
involving sensitive logistics plans in support of widely
distributed, and, in some cases, potentially vulnerable forces
on future nonlinear battlefields.
Future Benefits for Army Logistics
Assuming that scientists can overcome several of the challenges
described in this article, it is possible that, within the
next decade, we will be able to identify a number of practical
applications for Army logistics planners. For example, breaking
through the scalability barrier (that is, solving the problem
of decoherence) may lead to applications with tremendous
military utility for solving seemingly impossible problems
inherent in complex systems and chaotic environments. Quantum
computation and quantum communication would permit precise
logistics support to future Army forces and allow for an enormously
high level of control over an integrated deployment, distribution,
and sustainment system, so that Soldiers deployed anywhere
on the globe would receive exactly the right support at the
right time and at the right place.
The future battlespace will be populated by an increasing
number of unmanned or robotic systems and equipment, which
will place enormous demands on logisticians and their planning
systems. In such an environment, with apparently intractable
problem sets, quantum computation and communication may serve
as enabling technologies for other remarkable future capabilities.
In our next article, for instance, we will describe the idea
of telepresence (remote presence in the battlespace). In the
area of telepresence, there may be a role for emerging quantum
technologies such as quantum robots. Such robots, operating
within a distributed quantum computing system, could allow
small-scale sensors and actuators to be controlled remotely
over great distances. This would truly revolutionize the
concept of “sense and respond” logistics.
In fact, the speed of quantum computation, properly harnessed,
may enable future logisticians to meet, or preempt, real-time
requirements rapidly. Rather than relying on logistics estimates
that are based on historical data processed at today’s
speeds, computational and communication speeds attained from
quantum systems may replace “best guesses” with
real-time, actionable
knowledge. Functioning within a common logistics operating
environment, such radically capable systems truly would change
the conduct of warfare.
The goal is to remain alert to the advances in quantum information
science so that we are better prepared to exploit potential
capabilities to solve intractable computational challenges
impacting Army logistics. In particular, we are concerned with
the application of these capabilities to solve increasingly
dynamic logistics planning and simulation requirements. As
we monitor advances, we may indeed see new classes of simulations
that allow for accurately predicting logistics requirements
and outcomes.
We hope that this article and the others that follow in Army
Logistician will give logisticians and acquisition personnel
a basic understanding of the possibilities, and the limitations,
of emerging scientific areas. The intent is to pique interest
among the logistics community and allow a glimpse of future
concepts and technologies that young logisticians will inevitably
encounter in their careers. The Quantum Computation
and Communication theme is perhaps the most
difficult to grasp conceptually and technically. Subsequent
articles will be less complex, but the ideas and scientific
areas discussed will be just as significant in their far-reaching
implications. The next two articles in this series will address
the themes of Telepresence and Designer Materials. Like the
other themes, they offer the potential of improved capabilities
for deployment and sustainment and a wide range of prospective
solutions to the challenges facing logisticians.
ALOG
Dr. Keith Aliberti is a research physicist
in the Sensors and Electron Devices Directorate at the Army
Research Laboratory
at Adelphi, Maryland. He currently serves as the laboratory’s
liaison officer to the Army Logistics Innovation Agency at
Fort Belvoir, Virginia. He has a B.S. degree in physics from
Rensselaer Polytechnic Institute and M.S. and Ph.D. degrees
from the State University of New York at Albany.
Thomas L. Bruen is a logistics management specialist at the
Army Logistics Innovation Agency at Fort Belvoir, Virginia.
He has a bachelor’s degree in engineering from the U.S.
Military Academy and is a graduate of the Army Management
Staff College’s Sustaining Base Leadership Management
Program.
