2010 International Workshop

on Quantum Information Science

Organized by
JST ERATO-SORST QCI Project,
in cooperation with the University of Tokyo.

Our thanks go to the speakers and the attendees.

The pictures taken during the workshop are uploaded. We apologize that there is no picture for the first session.

Date: March 8th, 2010.

Time: 9:00-19:10

Annex of Ichijo Hall, in the Faculty of Agriculture. The University of Tokyo, Hongo Campus.

Yayoi Auditorium, Annex (Seihoku
Gallery)
Link to the
map of the Yayoi Auditorium.

Please be aware that the annex is located on the other side of the gate
(left side on the map).

Title: Sharpening Occam's Razor with Quantum Mechanics

Abstract: Much of science involves the construction of mathematical models that
make predictions about the future, based on relevant information collected from
the past. In the spirit of Occam's razor, simpler is often better; should two
predictive models equally simulate the future, the one that requires less
information from the past is preferred. For a large class of stochastic
processes, we show that by encoding possible pasts into non-orthogonal quantum
states, we can construct predictive models that require less information about
the past than any possible classical approach. This indicates that to construct
a device of minimal entropy that perfectly replicates the behavior of such
systems, quantum dynamics is a necessity. These results imply that certain
phenomena could be significantly simpler than classically possible, should
quantum effects be involved.

Title: Adaptive phase measurements of phase-squeezed states

Abstract: Optical phase measurement is of great
importance in various fields, e.g., optical communication and gravitational wave
detection. Precise phase measurement, however, is not an easy task due to the
difficulty of direct phase measurement. Conventional methods of phase
measurement, that is, heterodyne or dual homodyne measurements, are simultaneous
measurements of conjugate variables (quadrature-phase amplitudes of the field)
in which extra noises from the measurement back-action are inevitably
added. Adaptive phase measurement is an alternative way to measure phase in
which single homodyne measurement and feedback loop are used. Adaptive homodyne
measurement enables better phase estimate than the conventional methods. We
experimentally demonstrate adaptive phase measurement for a stochastically
varying phase on continuous-wave phase-squeezed beam. Here we use phase-squeezed
beam to achieve further improvement of the phase estimate. In this presentation
we will present these experimental results.

Title: Experimental demonstrations of Gaussian operations via teleportation-like
schemes

Abstract: In optical realization of continuous-variable (CV) quantum
teleportation, classical communications of Homodyne measurement outcomes enable
transportation of unknown quantum states. Nonclassicality of this procedure
comes from the entanglement in ancillas which are shared by the sender and the
receiver. Seen from the standpoint of CV quantum information processing, a
quantum teleportation is an identity operation. Using the same technique, we
demonstrate several nontrivial Gaussian operations, i.e. squeezing operation,
quantum nondemolition interaction, etc. Nonclassicality of squeezed vacuum
states used as ancillas is converted to the nonclassicality of the processing,
via feedforward of Homodyne measurements. The input states that are assumed to
be unknown are transformed unitarily up to small excess noise, which is
analogous to CV teleportation.

Title: Quantum Teleportation of non-Gaussian states of light

Abstract: We report the experimental realization of continuous variable quantum
teleportation of a non-Gaussian state of light. The input state similar to a
"squeezed photon" is generated by photon-subtraction using the degenerate
central modes of an OPO. The OPO cavity bandwidth (7.5 MHz) defines the temporal
characteristics of our non-Gaussian state which is a 160 ns long wavepacket of
light. Quantum state tomography reveals strong non-classical features for this
state and its experimental Wigner function in the origin (negativity) is -0.152,
for a density matrix purity of 0.59.

Our experimental broadband teleporter follows the canonical cv teleportation scheme and operates on a 10 MHz bandwidth to teleport all frequency components of the input wavepacket, with -4.5dB equivalent squeezing in the relevant temporal mode. We benchmark our setup using the fidelity F and measure F=0.74 for vacuum input, above the no-cloning limit 2/3. Although the output state for non-Gaussian input does not show negativity yet, we are currently approaching this goal by 1) working on input state negativity 2) increasing the effective squeezing parameter.

Our experimental broadband teleporter follows the canonical cv teleportation scheme and operates on a 10 MHz bandwidth to teleport all frequency components of the input wavepacket, with -4.5dB equivalent squeezing in the relevant temporal mode. We benchmark our setup using the fidelity F and measure F=0.74 for vacuum input, above the no-cloning limit 2/3. Although the output state for non-Gaussian input does not show negativity yet, we are currently approaching this goal by 1) working on input state negativity 2) increasing the effective squeezing parameter.

Title: Sub-shot-noise-limit discrimination of on-off keyed coherent signals via
a quantum receiver with a superconducting transition edge sensor

Abstract: We demonstrate a sub-shot-noise-limit discrimination of on-off keyed
coherent signals by an optimal displacement quantum receiver in which a
superconducting transition edge sensor is installed. Use of a transition edge
sensor and a fiber beam splitter realizes high total detection efficiency and
high interference visibility of the receiver and the observed average error
surpasses the shot-noise-limit in a wider range of the signal power. Our
technique opens up a new technology for the sub-shot-noise limit
detection of coherent signals in optical communication channels.

Title: Entanglement detection for interference fringes in atom-photon systems

Abstract: A measurement scheme of atomic qubits pinned at given positions is
studied by analyzing the interference pattern obtained when they emit photons
spontaneously. In the case of two qubits, a well-known relation is revisited, in
which the interference visibility is equal to the concurrence of the state in
the infinite spatial separation limit of the qubits. By taking into account the
super-radiant and sub-radiant effects, it is shown that a state tomography is
possible when the qubit spatial separation is comparable to the wavelength of
the atomic transition. In the case of three qubits, the relations between
various entanglement measures and the interference visibility are studied, where
the visibility is defined from the two-qubit case. A qualitative correspondence
among these entanglement relations is discussed. In particular, it is shown that
the interference visibility is directly related to the maximal bipartite
negativity.

Title: A Class of Quantum LDPC Codes Constructed From Cyclic Difference Set

Abstract: Low-density parity check (LDPC) codes are a significant class of
classical codes with many applications. Several good construction methods have
been proposed for their quantum counterparts, normally they are based on CSS
codes which need to construct self-containing classical block codes. Considering
the cyclic difference set codes can be decoded with message passing algorithm,
we propose a novel method, based on Stabilizer code, to construct quantum low
density parity check code by extending the concept of cyclic difference set in
this paper. We give the detail construction steps and prove the codes
constructed by this method have commutativity property. This method has high
code rate and less constraint. It is also surprising we can construct the famous
[5,1,3] stabilizer code by this method. At last, we present some examples of
quantum stabilizer codes by our construction method.

Title: Entanglement-assisted quantum error correction

Abstract: Entanglement-assisted quantum error-correcting codes (EAQECCs) make
use of pre-existing en-tanglement between the sender and receiver to boost the
rate of transmission. It is possible to construct an EAQECC from any classical
linear code, unlike standard QECCs which can only be constructed from
dual-containing codes. Operator quantum error-correcting codes (OQECCs) allow
certain errors to be corrected (or prevented) passively, reducing the complexity
of the correction procedure. We combine these two extensions of standard quantum
error correction into unified entanglement-assisted quantum error correction
formalism. This new scheme, which we call entanglement-assisted operator quantum
error correction (EAOQEC), is the most general and powerful quantum
error-correcting technique known, retaining the advantages of both
entanglement-assistance and passive correction. We present the formalism, show
the considerable freedom in constructing EAOQECCs from classical codes, and
demonstrate the construction with examples.

Title: Private Quantum Decoupling and Secure Disposal of Information

Abstract: Given a bipartite system, correlations between its subsystems can be
understood as the information that each one carries about the other. In order to
give a model-independent description of secure information disposal, I propose
here the paradigm of private quantum decoupling, corresponding to locally
reducing correlations in a given bipartite quantum state without transferring
them to the environment. In this framework, the concept of private local
randomness naturally arises as a resource, and total correlations are divided
into eliminable and ineliminable ones. I will prove upper and lower bounds on
the quantity of ineliminable correlations present in an arbitrary bipartite
state, and show that, in tripartite pure states, ineliminable correlations
satisfy a monogamy constraint, making apparent their quantum nature. A relation
with entanglement theory is provided by showing that ineliminable correlations
constitute an entanglement parameter. In the limit of infinitely many copies of
the initial state provided, I compute the regularized ineliminable correlations
to be measured by the coherent information, which is thus equipped with a new
operational interpretation. Results, in particular, imply that two subsystems
can be privately decoupled if their joint state is separable.

Title: Additivity in quantum Shannon theory

Abstract: A noisy quantum channel has many different capacities that
characterize its ability to transmit information. These capacities correspond to
different operational tasks, such as the transmission of classical information
or quantum information with or without the assistance of shared
entanglement. Additivity of a channel's capacity is equivalent to a complete
understanding of the channel because it implies that its calculation is a
tractable optimization problem. The only capacity that we can really claim to
understand at this point is the mutual information of a quantum channel because
the formula for the capacity is additive for all quantum channels. All suggested
formulas for other capacities at this point are additive only for a handful of
channels or there exist known counterexamples that violate additivity of the
given formulas. In this tutorial overview, I survey the landscape of additivity,
covering the different capacities of a quantum channel, reviewing classes of
channels for which additivity of a particular capacity formula holds, and
mentioning the counterexamples for which additivity does not hold.

Title: Architecture and system design for quantum computer

Abstract: Qubus computation is a type of quantum information processing (QIP)
where computational qubits couple through a quantum bus (qubus). Such
computation is flexible in terms of physical systems, properties and it scales
exceptionally well. In this talk we introduce a number of new designs for
qubus-type quantum devices and discuss system designs and architecture for
scalable large-scale quantum computer.

Title: On the computational power of BB84 states

Abstract: BB84 states play an essential role in BB84 quantum key distribution
protocol. We show that BB84 states are a powerful tool even to construct quantum
cryptography other than QKD. In classical cryptography, interactive hashing (and
interactive hashing theorem) is one of powerful tools to construct classical
cryptographic protocols. We will see that BB84 states act as the interactive
hashing. BB84 states are usually used in the non-interactive fashion. This is
beneficial to construct round-efficient quantum cryptographic protocols.

Title: On quantum based oblivious transfer

Abstract: Due to the famous impossibility results, unconditional security is
seen as impossible for oblivious transfer in the quantum world. In this paper,
we try to overcome the known impossibility results by proposing a protocol which
is not perfectly secure, but is unconditionally secure in practical sense. The
relation to the impossibility results is discussed. Other advantages of our
protocol include the fact that the honest players do not need quantum memory and
entanglement.

Title: Quantum Counterfeit Coin Problems

Abstract: The counterfeit coin problem requires us to find all false coins from
a given bunch of coins using a balance scale. We assume that the balance scale
gives us only ``balanced'' or ``tilted'' information and that we know the number
$k$ of false coins in advance. The balance scale can be modeled by a certain
type of oracle and its query complexity is a measure for the cost of weighing
algorithms (the number of weightings). In this paper, we study the quantum
query complexity for this problem. Let $Q(k,N)$ be the quantum query complexity
of finding all $k$ false coins from the $N$ given coins.

We show that for any $k$ and $N$ such that $k < N/2$, $Q(k,N)=O(k^{1/4})$, contrasting with the classical query complexity, $\Omega(k\log(N/k))$, that depends on $N$. So our quantum algorithm achieves a {\it quartic} speed-up for a natural problem.

We show that for any $k$ and $N$ such that $k < N/2$, $Q(k,N)=O(k^{1/4})$, contrasting with the classical query complexity, $\Omega(k\log(N/k))$, that depends on $N$. So our quantum algorithm achieves a {\it quartic} speed-up for a natural problem.

Title: Untraceable quantum ballots?

Abstract: We start by presenting the cryptographic task of secret ballot
elections in its most general way. We concentrate on four of its security
properties and after their brief analysis, we show how they impose restrictions
on honest party's behavior. These restrictions turned out to be so strong that a
no-go theorem can be derived from them. This no-go theorem practically forbids
the usage of quantum states for implementing ballots (and taking advantage of
quantum no-cloning theorem) as a way of preventing ballot copying.

- Workshop Chair:
- Hiroshi Imai (University of Tokyo)

- Workshop Program Organizer Chair:
- Min-Hsiu Hsieh (chair) (JST ERATO-SORST QCI Project)

- Workshop Program Organizers:
- Francesco Buscemi (Nagoya University)
- François Le Gall (University of Tokyo)

- Local Arrangements:
- Kenji Tsujino (JST ERATO-SORST QCI Project)
- Hiroko Takeshima (JST ERATO-SORST QCI Project)

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