755 research outputs found
Security proof of a three-state quantum key distribution protocol without rotational symmetry
Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.Comment: 10 pages, 3 figures, 2 column
Security of quantum bit string commitment depends on the information measure
Unconditionally secure non-relativistic bit commitment is known to be
impossible in both the classical and the quantum world. However, when
committing to a string of n bits at once, how far can we stretch the quantum
limits? In this letter, we introduce a framework of quantum schemes where Alice
commits a string of n bits to Bob, in such a way that she can only cheat on a
bits and Bob can learn at most b bits of information before the reveal phase.
Our results are two-fold: we show by an explicit construction that in the
traditional approach, where the reveal and guess probabilities form the
security criteria, no good schemes can exist: a+b is at least n. If, however,
we use a more liberal criterion of security, the accessible information, we
construct schemes where a=4 log n+O(1) and b=4, which is impossible
classically. Our findings significantly extend known no-go results for quantum
bit commitment.Comment: To appear in PRL. Short version of quant-ph/0504078, long version to
appear separately. Improved security definition and result, one new lemma
that may be of independent interest. v2: added funding reference, no other
change
Large collective Lamb shift of two distant superconducting artificial atoms
Virtual photons can mediate interaction between atoms, resulting in an energy
shift known as a collective Lamb shift. Observing the collective Lamb shift is
challenging, since it can be obscured by radiative decay and direct atom-atom
interactions. Here, we place two superconducting qubits in a transmission line
terminated by a mirror, which suppresses decay. We measure a collective Lamb
shift reaching 0.8% of the qubit transition frequency and exceeding the
transition linewidth. We also show that the qubits can interact via the
transmission line even if one of them does not decay into it.Comment: 7+5 pages, 4+2 figure
Is Quantum Bit Commitment Really Possible?
We show that all proposed quantum bit commitment schemes are insecure because
the sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen type of attack and delaying her measurement until she
opens her commitment.Comment: Major revisions to include a more extensive introduction and an
example of bit commitment. Overlap with independent work by Mayers
acknowledged. More recent works by Mayers, by Lo and Chau and by Lo are also
noted. Accepted for publication in Phys. Rev. Let
Modular detergents tailor the purification and structural analysis of membrane proteins including G-protein coupled receptors
Detergents enable the purification of membrane proteins and are indispensable reagents instructural biology. Even though a large variety of detergents have been developed in the lastcentury, the challenge remains to identify guidelines that allowfine-tuning of detergents forindividual applications in membrane protein research. Addressing this challenge, here weintroduce the family of oligoglycerol detergents (OGDs). Native mass spectrometry (MS)reveals that the modular OGD architecture offers the ability to control protein purificationand to preserve interactions with native membrane lipids during purification. In addition to abroad range of bacterial membrane proteins, OGDs also enable the purification and analysisof a functional G-protein coupled receptor (GPCR). Moreover, given the modular design ofthese detergents, we anticipatefine-tuning of their properties for specific applications instructural biology. Seen from a broader perspective, this represents a significant advance forthe investigation of membrane proteins and their interactions with lipids
Probing the quantum vacuum with an artificial atom in front of a mirror
Quantum fluctuations of the vacuum are both a surprising and fundamental
phenomenon of nature. Understood as virtual photons flitting in and out of
existence, they still have a very real impact, \emph{e.g.}, in the Casimir
effects and the lifetimes of atoms. Engineering vacuum fluctuations is
therefore becoming increasingly important to emerging technologies. Here, we
shape vacuum fluctuations using a "mirror", creating regions in space where
they are suppressed. As we then effectively move an artificial atom in and out
of these regions, measuring the atomic lifetime tells us the strength of the
fluctuations. The weakest fluctuation strength we observe is 0.02 quanta, a
factor of 50 below what would be expected without the mirror, demonstrating
that we can hide the atom from the vacuum
Insecurity of Quantum Secure Computations
It had been widely claimed that quantum mechanics can protect private
information during public decision in for example the so-called two-party
secure computation. If this were the case, quantum smart-cards could prevent
fake teller machines from learning the PIN (Personal Identification Number)
from the customers' input. Although such optimism has been challenged by the
recent surprising discovery of the insecurity of the so-called quantum bit
commitment, the security of quantum two-party computation itself remains
unaddressed. Here I answer this question directly by showing that all
``one-sided'' two-party computations (which allow only one of the two parties
to learn the result) are necessarily insecure. As corollaries to my results,
quantum one-way oblivious password identification and the so-called quantum
one-out-of-two oblivious transfer are impossible. I also construct a class of
functions that cannot be computed securely in any ``two-sided'' two-party
computation. Nevertheless, quantum cryptography remains useful in key
distribution and can still provide partial security in ``quantum money''
proposed by Wiesner.Comment: The discussion on the insecurity of even non-ideal protocols has been
greatly extended. Other technical points are also clarified. Version accepted
for publication in Phys. Rev.
Implementation of two-party protocols in the noisy-storage model
The noisy-storage model allows the implementation of secure two-party
protocols under the sole assumption that no large-scale reliable quantum
storage is available to the cheating party. No quantum storage is thereby
required for the honest parties. Examples of such protocols include bit
commitment, oblivious transfer and secure identification. Here, we provide a
guideline for the practical implementation of such protocols. In particular, we
analyze security in a practical setting where the honest parties themselves are
unable to perform perfect operations and need to deal with practical problems
such as errors during transmission and detector inefficiencies. We provide
explicit security parameters for two different experimental setups using weak
coherent, and parametric down conversion sources. In addition, we analyze a
modification of the protocols based on decoy states.Comment: 41 pages, 33 figures, this is a companion paper to arXiv:0906.1030
considering practical aspects, v2: published version, title changed in
accordance with PRA guideline
Measurement-device-independent quantum key distribution
How to remove detector side channel attacks has been a notoriously hard
problem in quantum cryptography. Here, we propose a simple solution to this
problem---*measurement* device independent quantum key distribution. It not
only removes all detector side channels, but also doubles the secure distance
with conventional lasers. Our proposal can be implemented with standard optical
components with low detection efficiency and highly lossy channels. In contrast
to the previous solution of full device independent QKD, the realization of our
idea does not require detectors of near unity detection efficiency in
combination with a qubit amplifier (based on teleportation) or a quantum
non-demolition measurement of the number of photons in a pulse. Furthermore,
its key generation rate is many orders of magnitude higher than that based on
full device independent QKD. The results show that long-distance quantum
cryptography over say 200km will remain secure even with seriously flawed
detectors.Comment: 7 pages, two-column format, 4 figures in tota
Possibility, Impossibility and Cheat-Sensitivity of Quantum Bit String Commitment
Unconditionally secure non-relativistic bit commitment is known to be
impossible in both the classical and the quantum worlds. But when committing to
a string of n bits at once, how far can we stretch the quantum limits? In this
paper, we introduce a framework for quantum schemes where Alice commits a
string of n bits to Bob in such a way that she can only cheat on a bits and Bob
can learn at most b bits of information before the reveal phase. Our results
are two-fold: we show by an explicit construction that in the traditional
approach, where the reveal and guess probabilities form the security criteria,
no good schemes can exist: a+b is at least n. If, however, we use a more
liberal criterion of security, the accessible information, we construct schemes
where a=4log n+O(1) and b=4, which is impossible classically. We furthermore
present a cheat-sensitive quantum bit string commitment protocol for which we
give an explicit tradeoff between Bob's ability to gain information about the
committed string, and the probability of him being detected cheating.Comment: 10 pages, RevTex, 2 figure. v2: title change, cheat-sensitivity adde
- …