Quantum cryptography Records with Higher-Dimensional Photons

Date:
September 22, 2021
Source:
Vienna University of Technology
Summary:
A new and much faster quantum cryptography protocol has been
developed: Usually, quantum cryptography is done with photons
that can be in two different states. Using eight different states,
cryptographic keys can be generated much faster and with much more
robustness against interference.



FULL STORY ========================================================================== Quantum cryptography is one of the most promising quantum technologies
of our time: Exactly the same information is generated at two different locations, and the laws of quantum physics guarantee that no third
party can intercept this information. This creates a code with which information can be perfectly encrypted.


==========================================================================
The team of Prof. Marcus Huber from the Atomic Institute of TU Wien
developed a new type of quantum cryptography protocol, which has now
been tested in practice in cooperation with Chinese research groups:
While up to now one normally used photons that can be in two different
states, the situation here is more complicated: Eight different paths
can be taken by each of the photons.

As the team has now been able to show, this makes the generation
of the quantum cryptographic key faster and also significantly more
robust against interference. The results have now been published in the scientific journal Physical Review Letters.

Two states, two dimensions "There are many different ways of
using photons to transmit information," says Marcus Huber. "Often,
experiments focus on their photons' polarisation. For example, whether
they oscillate horizontally or vertically -- or whether they are in
a quantum-mechanical superposition state in which, in a sense, they
assume both states simultaneously. Similar to how you can describe a
point on a two-dimensional plane with two coordinates, the state of
the photon can be represented as a point in a two-dimensional space."
But a photon can also carry information independently of the direction
of polarization. One can, for example, use the information about which
path the photon is currently travelling on. This is exactly what has now
been exploited: "A laser beam generates photon pairs in a special kind
of crystal. There are eight different points in the crystal where this
can happen," explains Marcus Huber. Depending on the point at which the
photon pair was created, each of the two photons can move along eight
different paths -- or along several paths at the same time, which is
also permitted according to the laws of quantum theory.

These two photons can be directed to completely different places and
analysed there. One of the eight possibilities is measured, completely
at random -- but as the two photons are quantum-physically entangled,
the same result is always obtained at both places. Whoever is standing
at the first measuring device knows what another person is currently
detecting at the second measuring device -- and no one else in the
universe can get hold of this information.



========================================================================== Eight states, eight dimensions "The fact that we use eight possible paths
here, and not two different polarisation directions as it is usually the
case, makes a big difference," says Marcus Huber. "The space of possible quantum states becomes much larger.

The photon can no longer be described by a point in two dimensions, mathematically it now exists in eight dimensions." This has several advantages: First, it allows more information to be generated: At 8307
bits per second and over 2.5 bits per photon pair, a new record has
been set in entanglement-based quantum cryptography key generation. And secondly, it can be shown that this makes the process less susceptible
to interference.

"With all quantum technologies, you have to deal with the problem of decoherence," says Marcus Huber. "No quantum system can be perfectly
shielded from disturbances. But if it comes into contact with
disturbances, then it can lose its quantum properties very easily: The
quantum entanglements are destroyed." Higher-dimensional quantum states, however, are less likely to lose their entanglement even in the presence
of disturbances.

Moreover, sophisticated quantum error-correction mechanisms can be
used to compensate for the influence of external perturbations. "In
the experiments, additional light was switched on in the laboratory to deliberately cause disturbances -- and the protocol still worked," says
Marcus Huber. "But only if we actually used eight different paths. We were
able to show that with a mere two-dimensional encoding a cryptographic
key can no longer be generated in this case." In principle, it should
be possible to improve the new, faster and more reliable quantum
cryptography protocol further by using additional degrees of freedom
or an even larger number of different paths. "However, this not only
increases the space of possible states, it also becomes increasingly
difficult at some point to read out the states correctly," says Marcus
Huber. "We seem to have found a good compromise here, at least within
the range of what is currently technically possible."


========================================================================== Quantum cryptography is one of the most promising quantum technologies
of our time: Exactly the same information is generated at two different locations, and the laws of quantum physics guarantee that no third
party can intercept this information. This creates a code with which information can be perfectly encrypted.

The team of Prof. Marcus Huber from the Atomic Institute of TU Wien
developed a new type of quantum cryptography protocol, which has now
been tested in practice in cooperation with Chinese research groups:
While up to now one normally used photons that can be in two different
states, the situation here is more complicated: Eight different paths
can be taken by each of the photons.

As the team has now been able to show, this makes the generation of
the quantum cryptographic key faster and also significantly more
robust against interference. The results have now been published
in the scientific journal "Physical Review Letters." Two states,
two dimensions "There are many different ways of using photons
to transmit information," says Marcus Huber. "Often, experiments
focus on their photons' polarisation. For example, whether they
oscillate horizontally or vertically -- or whether they are in a quantum-mechanical superposition state in which, in a sense, they
assume both states simultaneously. Similar to how you can describe a
point on a two-dimensional plane with two coordinates, the state of
the photon can be represented as a point in a two-dimensional space."
But a photon can also carry information independently of the direction
of polarization. One can, for example, use the information about which
path the photon is currently travelling on. This is exactly what has now
been exploited: "A laser beam generates photon pairs in a special kind
of crystal. There are eight different points in the crystal where this
can happen," explains Marcus Huber. Depending on the point at which the
photon pair was created, each of the two photons can move along eight
different paths -- or along several paths at the same time, which is
also permitted according to the laws of quantum theory.

These two photons can be directed to completely different places and
analysed there. One of the eight possibilities is measured, completely
at random -- but as the two photons are quantum-physically entangled,
the same result is always obtained at both places. Whoever is standing
at the first measuring device knows what another person is currently
detecting at the second measuring device -- and no one else in the
universe can get hold of this information.

Eight states, eight dimensions "The fact that we use eight possible paths
here, and not two different polarisation directions as it is usually the
case, makes a big difference," says Marcus Huber. "The space of possible quantum states becomes much larger.

The photon can no longer be described by a point in two dimensions, mathematically it now exists in eight dimensions." This has several advantages: First, it allows more information to be generated: At 8307
bits per second and over 2.5 bits per photon pair, a new record has
been set in entanglement-based quantum cryptography key generation. And secondly, it can be shown that this makes the process less susceptible
to interference.

"With all quantum technologies, you have to deal with the problem of decoherence," says Marcus Huber. "No quantum system can be perfectly
shielded from disturbances. But if it comes into contact with
disturbances, then it can lose its quantum properties very easily: The
quantum entanglements are destroyed." Higher-dimensional quantum states, however, are less likely to lose their entanglement even in the presence
of disturbances.

Moreover, sophisticated quantum error-correction mechanisms can be
used to compensate for the influence of external perturbations. "In
the experiments, additional light was switched on in the laboratory
to deliberately cause disturbances -- and the protocol still worked,"
says Marcus Huber. "But only if we actually used eight different
paths. We were able to show that with a mere two-dimensional encoding a cryptographic key can no longer be generated in this case." In principle,
it should be possible to improve the new, faster and more reliable
quantum cryptography protocol further by using additional degrees
of freedom or an even larger number of different paths. "However,
this not only increases the space of possible states, it also becomes increasingly difficult at some point to read out the states correctly,"
says Marcus Huber. "We seem to have found a good compromise here,
at least within the range of what is currently technically possible." ========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Xiao-Min Hu, Chao Zhang, Yu Guo, Fang-Xiang Wang, Wen-Bo Xing,
Cen-Xiao
Huang, Bi-Heng Liu, Yun-Feng Huang, Chuan-Feng Li, Guang-Can
Guo, Xiaoqin Gao, Matej Pivoluska, Marcus Huber. Pathways
for Entanglement-Based Quantum Communication in the Face
of High Noise. Physical Review Letters, 2021; 127 (11) DOI:
10.1103/PhysRevLett.127.110505 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210922121850.htm

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