112 research outputs found
Breaking a quantum key distribution system through a timing side channel
The security of quantum key distribution relies on the validity of quantum
mechanics as a description of nature and on the non-existence of leaky degrees
of freedom in the practical implementations. We experimentally demonstrate how,
in some implementations, timing information revealed during public discussion
between the communicating parties can be used by an eavesdropper to
undetectably access a significant portion of the ``secret'' key.Comment: 6 pages, 4 figures. Added additional references and extended
analysis. Identical to published versio
Nonlinear photon-atom coupling with 4Pi microscopy
Implementing nonlinear interactions between single photons and single atoms
is at the forefront of optical physics. Motivated by the prospects of
deterministic all-optical quantum logic, many efforts are currently underway to
find suitable experimental techniques. Focusing the incident photons onto the
atom with a lens yielded promising results, but is limited by diffraction to
moderate interaction strengths. However, techniques to exceed the diffraction
limit are known from high-resolution imaging. In this work, we adapt a
super-resolution imaging technique, 4Pi microscopy, to efficiently couple light
to a single atom. We observe 36.6(3)% extinction of the incident field, and a
modified photon statistics of the transmitted field -- indicating nonlinear
interaction at the single-photon level.Comment: 8 pages, 8 figure
Clock synchronization by remote detection of correlated photon pairs
We present an algorithm to detect the time and frequency difference of
independent clocks based on observation of time-correlated photon pairs. This
enables remote coincidence identification in entanglement-based quantum key
distribution schemes without dedicated coincidence hardware, pulsed sources
with a timing structure or very stable reference clocks. We discuss the method
for typical operating conditions, and show that the requirement in reference
clock accuracy can be relaxed by about 5 orders of magnitude in comparison with
previous schemes.Comment: 14 pages, 6 figure
Secure communication with single-photon two-qubit states
We propose a cryptographic scheme that is deterministic: Alice sends single
photons to Bob, and each and every photon detected supplies one key bit -- no
photon is wasted. This is in marked contrast to other schemes in which a random
process decides whether the next photon sent will contribute to the key or not.
The determinism is achieved by preparing the photons in two-qubit states,
rather than the one-qubit states used in conventional schemes. In particular,
we consider the realistic situation in which one qubit is the photon
polarization, the other a spatial alternative. Further, we show how one can
exploit the deterministic nature for direct secure communication, that is:
without the need for establishing a shared key first.Comment: 9 pages, 1 figure, 3 tables; final versio
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