YSM Issue 96.2

COUNTERPOINT
QUANTUM
BY MATTHEW DOBRE
CRYPTOGRAPHY
Encrypting data in holograms composed of corkscrews of light
In 1933, Nobel Prize-winning chemist Sir William
Bragg said, “Light brings us the news of the
Universe.” Though it would take decades before
scientists better understood the properties of light,
researchers today are devising ways to utilize this
resource to store information by uniting information
theory and quantum mechanics, a subfield of physics
that governs the behavior of photons, or light particles.
Among the general public, anything that includes the
word “quantum” elicits a degree of awe. Even Einstein
was troubled by one of the theoretical implications of
quantum mechanics, which he called “spooky action
at a distance.” Today, Einstein’s boogeyman is called
entanglement. When two particles are entangled, their
quantum states, which may include properties like
geometric orientation, angular momentum, and energy,
are inextricably linked. A change in the state of one
particle will immediately affect its twin. For example,
consider a pair of entangled mittens: one is given to a
lab assistant named Bob, and the other to his partner,
Alice. No attempt is made to observe either mitten. Bob
is then sent to a neighboring solar system. Up to this
point, nobody in the universe knows the handedness of
either Alice’s or Bob’s mittens. In fact, by the postulates
of quantum mechanics, their handedness will be
indeterminate until they are observed. If this experiment
was repeated many times, it would reveal that when
Alice observes her mitten, she will have a right-handed
mitten half the time, and a left-handed mitten the other
half. Eerily, Bob’s mitten will instantaneously acquire
the opposite handedness, even though he’s two million
light years away from Alice.
These same principles of entanglement have been
used to send data over thin air. In 2015, a research
team in Vienna transmitted information by changing
the relative angular momentum, or “twistiness,” of
entangled pairs of photons. In quantum mechanics,
angular momentum can only take on certain values
called discrete quantities, separated by a constant ‘step
size’. These entangled particles stored information by
acting analogously to computer bits, which can only
have a value of zero or one. In the experiment, each twist
served as an alternative channel of communication,
enabling ultra-high-speed data transmission.
A group of physicists from the Beijing Institute of
Technology (BIT) recently expanded on this research
to create an unprecedented storage system. Instead of
transmitting information via distinct light channels,
they stored several unique data sets at a time by creating
holograms made of entangled photon pairs. Though
holograms seem to be a quintessential element of
science fiction, they already have applications in
everyday life. Used as a security feature in credit
cards, passports, and banknotes, holograms are stacks
of slightly misaligned images that are difficult for a
computer to read. The physicists generated holograms
made of twisted, entangled photon pairs by using
β-barium borate crystals to manipulate the relative
angular momentum between the particles, creating
corkscrew patterns of light that resemble rotini pasta.
To prove their setup worked, the researchers encoded
words in the holograms and used twisted light to
reconstruct the data by measuring the coincidence
between the particles’ angular momentums.
By increasing the number and diversity of twists
in a single hologram, the system securely encrypted
information since the odds of guessing the degree and
direction of the twisted light’s path became very low. For
instance, seven distinct twists, which is a relatively small
system, led to millions of potential combinations.
Many quantum technologies are sensitive to noise.
However, the researchers asserted that entanglement
can diminish the effect of classical noise sources, which
include mechanical vibrations and thermal fluctuations.
Thus, the decrypted output was of a higher quality
compared to other similar systems.
This technology, however, is still in its infancy.
Utilizing six forms of entangled light, the group was
able to decode an image of the acronym “BIT” by
measuring the orbital angular momentum states of the
particles over the course of twenty minutes, which is far
from practical. Fortunately, the group is optimistic that
these difficulties can be overcome with improvements
in quantum technology. Though holographic data
storage is still in a nascent stage, today’s scientists
are utilizing techniques that the founders of modern
physics laid the groundwork for—but may not have
even understood. ■
38 Yale Scientific Magazine May 2023 www.yalescientific.org