900 research outputs found
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
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
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
A simple proof of the unconditional security of quantum key distribution
Quantum key distribution is the most well-known application of quantum
cryptography. Previous proposed proofs of security of quantum key distribution
contain various technical subtleties. Here, a conceptually simpler proof of
security of quantum key distribution is presented. The new insight is the
invariance of the error rate of a teleportation channel: We show that the error
rate of a teleportation channel is independent of the signals being
transmitted. This is because the non-trivial error patterns are permuted under
teleportation. This new insight is combined with the recently proposed quantum
to classical reduction theorem. Our result shows that assuming that Alice and
Bob have fault-tolerant quantum computers, quantum key distribution can be made
unconditionally secure over arbitrarily long distances even against the most
general type of eavesdropping attacks and in the presence of all types of
noises.Comment: 13 pages, extended abstract. Comments will be appreciate
Conditional q-Entropies and Quantum Separability: A Numerical Exploration
We revisit the relationship between quantum separability and the sign of the
relative q-entropies of composite quantum systems. The q-entropies depend on
the density matrix eigenvalues p_i through the quantity omega_q = sum_i p_i^q.
Renyi's and Tsallis' measures constitute particular instances of these
entropies. We perform a systematic numerical survey of the space of mixed
states of two-qubit systems in order to determine, as a function of the degree
of mixture, and for different values of the entropic parameter q, the volume in
state space occupied by those states characterized by positive values of the
relative entropy. Similar calculations are performed for qubit-qutrit systems
and for composite systems described by Hilbert spaces of larger dimensionality.
We pay particular attention to the limit case q --> infinity. Our numerical
results indicate that, as the dimensionalities of both subsystems increase,
composite quantum systems tend, as far as their relative q-entropies are
concerned, to behave in a classical way
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
Practical Decoy State for Quantum Key Distribution
Decoy states have recently been proposed as a useful method for substantially
improving the performance of quantum key distribution. Here, we present a
general theory of the decoy state protocol based on only two decoy states and
one signal state. We perform optimization on the choice of intensities of the
two decoy states and the signal state. Our result shows that a decoy state
protocol with only two types of decoy states--the vacuum and a weak decoy
state--asymptotically approaches the theoretical limit of the most general type
of decoy state protocols (with an infinite number of decoy states). We also
present a one-decoy-state protocol. Moreover, we provide estimations on the
effects of statistical fluctuations and suggest that, even for long distance
(larger than 100km) QKD, our two-decoy-state protocol can be implemented with
only a few hours of experimental data. In conclusion, decoy state quantum key
distribution is highly practical.Comment: 31 pages. 6 figures. Preprint forma
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.
- …