Vol.12
No.7&8,
July 1, 2012
Research Articles:
Nondemolition dispersive measurement of a superconducting qubit with
a microstrip SQUID amplifier
(pp05410552)
G.P.
Berman, D.I. Kamenev, D. Kinion, and V.I. Tsifrinovich
We have studied the possibility of a
singleshot nondemolition measurement of a superconducting qubit using
a microstrip SQUID amplifier (MSA). The Johnson noise generated by all
resistors in the MSA is taken into consideration. We show that a
singleshot nondemolition measurement is possible with six photons in
the measurement resonator.
Improving quantum clocks via semidefinite programming
(pp05530574)
Michael Mullan
and Emanuel Knill
The accuracies of modern quantum logic clocks have surpassed those of
standard atomic fountain clocks. These clocks also provide a greater
degree of control, because before and after clock queries, we are able
to apply chosen unitary operations and measurements. Here, we take
advantage of these choices and present a numerical technique designed to
increase the accuracy of these clocks. We use a greedy approach,
minimizing the phase variance of a noisy classical oscillator with
respect to a perfect frequency standard after an interrogation step; we
do not optimize over successive interrogations or the probe times. We
consider arbitrary prior frequency knowledge and compare clocks with
varying numbers of ions and queries interlaced with unitary control. Our
technique is based on the semidefinite programming formulation of
quantum query complexity, a method first developed in the context of
deriving algorithmic lower bounds. The application of semidefinite
programming to an inherently continuous problem like that considered
here requires discretization; we derive bounds on the error introduced
and show that it can be made suitably small.
Quantum predictive learning and communication
complexity with single input
(pp05750588)
Dmitry
Gavinsky
We define a new model of quantum learning that we call \e{Predictive
Quantum (\pq)}. This is a quantum analogue of \pac, where during the
testing phase the student is only required to answer a polynomial number
of testing queries. We demonstrate a relational concept class that is \e{efficiently
learnable} in \pq, while in \e{any} ``reasonable'' classical model
exponential amount of training data would be required. This is the first
unconditional separation between quantum and classical learning. We show
that our separation is the best possible in several ways; in particular,
there is no analogous result for a functional class, as well as for
several weaker versions of quantum learning. In order to demonstrate
tightness of our separation we consider a special case of oneway
communication that we call \e{singleinput mode}, where Bob receives no
input. Somewhat surprisingly, this setting becomes nontrivial when
relational communication tasks are considered. In particular, any
problem with twosided input can be transformed into a singleinput
relational problem of equal \e{classical} oneway cost. We show that the
situation is different in the \e{quantum} case, where the same
transformation can make the communication complexity exponentially
larger. This happens if and only if the original problem has exponential
gap between quantum and classical oneway communication costs. We
believe that these auxiliary results might be of independent interest.
On QMA protocols with two short quantum proofs
(pp05890600)
Francois Le Gall, Shota Nakagawa, and Harumichi Nishimura
This paper gives a QMA (Quantum MerlinArthur) protocol for 3SAT with
two logarithmicsize quantum proofs (that are not entangled with each
other) such that the gap between the completeness and the soundness is
$\Omega(\frac{1}{n\polylog{n}})$. This improves the best
completeness/soundness gaps known for NPcomplete problems in this
setting.
Fast iSWAP gate using two $Q$ switches
(pp06010608)
Jie Song, Yan
Xia, XiuDong Sun, and HeShan Song
An effective approach to the construction of iSWAP gate has been
proposed. Working with three atoms inside three coupled cavities, we
analyze the transport and the confinement of a single photon along two
different directions. It is shown that two $Q$ switches can be built by
tuning the transition energy of each atom. Applying a classical field,
we can implement highspeed gate operation between photon and atom. In
addition, the influence of decoherence on the gate fidelity is also
discussed.
On the existence of losstolerant quantum oblivious
transfer protocols
(pp06090619)
Jamie Sikora
Oblivious transfer is the cryptographic primitive where Alice sends one
of two bits to Bob but is oblivious to the bit received. Using quantum
communication, we can build oblivious transfer protocols with security
provably better than any protocol built using classical communication.
However, with imperfect apparatus, one needs to consider other attacks.
In this paper, we present an oblivious transfer protocol which is
impervious to lost messages.
An optical scheme for quantum multiservice network
(pp06200629)
Fabio Alencar
Mendonca, Daniel Barbosa de Brito, and Rubens Viana Ramos
Several quantum protocols for data security having been proposed and, in
general, they have different optical implementations. However, for the
implementation of quantum protocols in optical networks, it is highly
advantageous if the same optical setup can be used for running different
quantum communication protocols. In this direction, here we show an
optical scheme that can be used for quantum key distribution (QKD),
quantum secure direct communication (QSDC) and quantum secret sharing (QSS).
Additionally, it is naturally resistant to the attack based on
singlephoton detector blinding. At last, we show a proofofprinciple
experiment in 1 km optical fiber link that shows the feasibility of the
proposed scheme.
The security of SARG04 protocol in plug and play QKD system with an
untrusted source
(pp06300647)
Bingjie Xu,
Xiang Peng, and Hong Guo
The SARG04 protocol is one of the most frequently used protocol in
commercial plugandplay quantum key distribution (QKD) system, where an
eavesdropper can completely control or change the photon number
statistics of the QKD source. To ensure the security of SARG04 protocol
in plugandplay QKD system with an unknown and untrusted source, the
bounds of a few statistical parameters of the source need to be
monitored. An active or a passive source monitor schemes are proposed to
verify these parameters. Furthermore, the practical issues due to
statistical fluctuation and detection noise in the source monitoring
process are quantitatively analyzed. Our simulation results show that
the passive scheme can be efficiently applied to plugandplay system
with SARG04 protocol.
How to counteract systematic errors in quantum state
transfer
(pp06480660)
Chiara
Marletto, Alastair Kay, and Artur Ekert
In the absence of errors, the dynamics of a spin chain, with a suitably
engineered local Hamiltonian, allow the perfect, coherent transfer of a
quantum state over large distances. Here, we propose encoding and
decoding procedures to recover perfectly from low rates of systematic
errors. The encoding and decoding regions, located at opposite ends of
the chain, are small compared to the length of the chain, growing
linearly with the size of the error. We also describe how these errors
can be identified, again by only acting on the encoding and decoding
regions.
Finding hidden Borel subgroups of the general linear
group
(pp06610669)
Gabor Ivanyos
We present a quantum algorithm for solving the hidden subgroup problem
in the general linear group over a finite field where the hidden
subgroup is promised to be a conjugate of the group of the invertible
lower triangular matrices. The complexity of the algorithm is polynomial
when size of the base field is not much smaller than the degree.
The quantum query complexity of AC0
(pp06700676)
Paul Beame and
Widad Machmouchi
We show that any quantum algorithm deciding whether an input function
$f$ from $[n]$ to $[n]$ is 2to1 or almost 2to1 requires $\Theta(n)$
queries to $f$. The same lower bound holds for determining whether or
not a function $f$ from $[2n2]$ to $[n]$ is surjective. These results
yield a nearly linear $\Omega(n/\log n)$ lower bound on the quantum
query complexity of $\cl{AC}^0$. The best previous lower bound known for
any $\cl{AC^0}$ function was the $\Omega ((n/\log n)^{2/3})$ bound given
by Aaronson and Shi's $\Omega(n^{2/3})$ lower bound for the element
distinctness problem.
Quantum discord of a threequbit Wclass state in noisy
environments
(pp06770692)
Hui Guo,
JinMing Liu, ChengJie Zhang, and C. H. Oh
We study the dynamics of the pairwise quantum discord (QD), classical
correlation (CC), and entanglement of formation (EOF) for the threequbit
Wclass state W>_{123}=\frac
12(100>_{123}+010>_{123}+\sqrt{2}001>_{123}) under the influence of
various Markovian noises by analytically solving the master equation in
the Lindblad form. Through numerical analysis, we find that EOF
decreases asymptotically to zero with time for the dephasing noise, but
it undergoes sudden death for the bitflip noise, the isotropic noise,
as well as the dissipative and noisy environments. Moreover, QD decays
to zero in an asymptotical way for all the noises we investigated. Thus,
when the Wclass state W>_{123} is subject to the above Markovian
noises, QD is more robust than EOF against decoherence excluding the
phaseflip noise, implying that QD is more useful than entanglement to
characterize the quantum correlation. We also find a remarkable
character for the CC in the presence of the phaseflip noise, i.e.,
CC displays the behavior of sudden transition and then keeps constant
permanently, but the corresponding QD just exhibits a very small sudden
change. Furthermore, we verify the monogamic relation between the
pairwise QD and EOF of the Wclass state.
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