422+)*-&#8MTDKADP#*.&#+)*-#
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Quantum Information Processing— Similarities and Differences with Classical Information —9Q@I@#>LHTDPQHRW#

Attacks

Eavesdropping

P2P secure communication

Cypher breaking

Water printing (invisible ink)

Falsification (change contents)

Signature

Spoofing (impostor scam)

Falsification

Account hacking (takeover)Auction (Bid)
Voting (Election)##

Public Key
Cryptography=GPD@R(conventional)(Decipher by)

4VNHP@RHML#C@RD#ME#QDBPDRQ

Quantum
Computing<MJSRHML

Needs<MJSRHML

Post-Quantum

Quantum

Cryptography

Cryptography<GMPR/#4LRP@LBD#DV@KHL@RHML#NPMAJDKQ
7MLF/#<R@RD#QDBPDRQ#$+,#WD@PQ0##,)#WD@PQ0%?DPW#JMLF/#381 &#NPHT@RD#JDRRDPQ

Difference in the ordinary tapping and QND tapping

1974
2010
1980
1990
2000
7576 7778 79 81 828384 858687 8889 919293 9495 969798 99 01 020304 050607 0809 11 12 1314151617 18
Under MC
GraduateNTT

My first paperon “Quantum”
Optical fiber（QND）
communication

Usual LAN: linear extraction of the signal→ obeys fluctuation-dissipation theorem→ additional noise both in the wire-tapped and main signals.

QND LAN: No quantum noise added in any of reflected/transmitted signals.

2010
1980
1974
1990
2000
7576 7778 79 81 828384 858687 8889 919293 9495 969798 99 01 020304 050607 0809 11 12 1314151617 18
Under MC
Graduate

NTT

Essex
My first paperon “Quantum”Dr. Univ.
Optical fiber（QND）
Thesiscommunication
First Review inJapan (Articles&
Presentations)First Symp.
on Quantum
Cryptography
and Quantum
Computation

Quantum Computation & Cryptography Symposium
191993
1993
93
93

1974
2010
1980
1990
2000
7576 7778 79 81 828384 858687 8889 919293 9495 969798 99 01 020304 050607 0809 11 12 1314151617 18
Under MC
Graduate

NTT

Optical fiber
communication

Essex
Univ.
My first paperDr.
My first paperon quantum
Thesison
cryptography“Quantum” Noise
Review
Reduction
Articles &SOKENDAINature

2010
1980
1974
1990
2000
7576 7778 79 81 828384 858687 8889 919293 9495 969798 99 01 020304 050607 0809 11 12 1314151617 18
Under MC
Graduate

NTT

Optical fiber
communication

Essex
My first paperon “Quantum”Dr. Univ.
（QND）
ThesisFirst Review inJapan (Articles&
Presentations)

Osaka Univ.
Sci.Rep.
Nature Nature Photonics
IOP best paper

SOKENDAIMy first paperon quantum
cryptography

Sci.ReNatureCommun.Nature
Photon.
NatureCommun.
NatureCommun.
Japan-EU
Workshop on
Quantum Tech.
ECC 2018

JST-CREST Project

JST-CREST Project

Global Quantum Network

Global Quantum Network(1) Why necessary?

(1) Why necessary?

(2) What are the problems?

(2) What are the problems?

(3) How did we deal with these problems?

(3) How did we deal with these problems?

(4) How are we going to cope with these problems?

(4) How are we going to cope with these problems?

(5) Is their any alternative(s)?

(5) Is their any alternative(s)?

+)*-#9BR(#1#THQHRMP#EPMK#></=GD#?HQHRMP/#:S@LRSK#BPWNRMFP@NGW#HQ#CPHTDL#MSR#AW##
NMQR'OS@LRSK#BPWNRMFP@NGW(##3MLR#WMS#RGHLI#QM0
8MAS#5KMRM/#3M#WMS#RGHLI#RG@R#><#UHJJ#QGSR#CMUL#RGD#PDQD@PBG##
ML:S@LRSK#2PWNRMFP@NGW0=GD#?HQHRMP/#8M(
8MAS#5KMRM/#=G@RQ#RGD#@LQUDP((1) Why necessary?
・long-haul QKD↓2018.1.20 Nikkei p.2↓

!

JST-CREST Project

Global Quantum Network

現状：ラボ内程度から1km程度まで(1) Why necessary?

2012, 21 m @ 780 nm(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?
(4) How are we going to cope with these problems?
(5) Is their any alternative(s)?
2015, 1.3 km @ 630 nm

JST-CREST Project

光ファイバー透過率:
Fibertransmissivity

Global Quantum Network

Loss per unit吸収係数:
length距離:
Length of ﬁberStandard fiber
住友電工(2013)(1) Why necessary?
(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?

Dark counts
可視光Visible／Near Infrared(4) How are we going to cope with these problems?
1550nm帯

Long-haultransmission is impossible
可視光では長距離光ファイバー通信はできない。
for V／NI(5) Is their any alternative(s)?

Entanglement decoherencein an optical fiber

Entanglement preparation
Optical PDC

fidelity

Fidelity F=0.46±0.03 ↑F=1.00±0.03

0
0.5
1.0
anti
no
correlated
correlation correlation

JST-CREST Project

Global Quantum NetworkClose to 0.5

Quantum error correction
Express a logical qubit by n physical qubits.
→ dim(H:whole Hilbert space) = 2n(1) Why necessary?
(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?
・Frequency conversion ・noise cancellation by DFS(4) How are we going to cope with these problems?
(5) Is their any alternative(s)?

Assume at most 1 error occurs among n qubits.
→ We need to specify “which physical qubit sufferedfrom which error?” (3n cases)or “no error?” (1 case) →total 3n + 1 cases.
→ In order to specify all 3n + 1 cases by 2n-1 dimensional space, 2n-1 ≧ 3n1
becomes a necessary condition.
→ The minimum number of n to satisfy this is 5.

Actually, 5 is sufficient!

Now, assume that the bit-flip error occurred on the extreme left qubit. ()

Then the measured value for the syndrome operators become,Define
Defi
De
fine
fi
ne thehe ffolfollowing
ollo
ol
lowi
lo
wing
wi
ng fourffou
ourr syou
synd
syndrome
ndro
nd
rome
ro
me
oobs
observables:
bser
bs
erva
er
vabl
va
bles
bl
es:: error occurred on which qubit):es
Syndrome
observables(forfindingwhich

For any, it is easy to show

keeps 

That is, measurement of any
as it is.
 Also, any two syndromes commutes → all are observable simultaneously.

Considering the 15 error cases, the relation between the
syndrome values and the kind of error is summarized as:This means that for any arbitrary (and unknown) state the state##########isunchanged by the measurement of the four syndromes, resulting in (1,1,1,-1), whichindicates that "bit flip occurred for the extreme left qubit."
Then, once knowing (1,1,1,-1), the error can be corrected by applying the inverse of, whichis  itself. (quantum error correction),

Quantum error correction
Express a logical qubit by n physical qubits.
→ dim(H:whole Hilbert space) = 2nAssume at most 1 error occurs among n qubits.

The 16th case, 1,1,1,1 (= no error), is not listed here.
This idea is cool! However,・It is difficult to realize experimentally.
・We assume that the error correction circuit operates error-free.
・High-frequency operation is needed since “at most 1 error” is assumed.
・When there is a loss of qubit (transmissivity T), the whole system sufferesfrom T-5 (T-7 for another famous code and T-9 for yet another code).

→ We need to specify “which physical qubit sufferedfrom which error?” (3n cases)or “no error?” (1 case) →total 3n + 1 cases.
→ In order to specify all 3n + 1 cases by 2n-1 dimensional space, 2n-1 ≧ 3n
1 becomes a necessary condition.

5→ The minimum number of n to satisfy this iss 5.

Use of two qubits

光ファイバー透過率:
Fibertransmissivity

Loss per unit吸収係数:
lengthIn phase case：

距離:
Length of ﬁberStandard fiber

qubitＡ

住友電工(2013)

Random
phase
noise

still
in phase

Randomphase
noisestill outof phase

qubitＢ

Dark counts

Out of phase case

可視光
Visible／Near Infrared

qubitＡ
1550nm帯

Long-haultransmission is impossible
可視光では長距離光ファイバー通信はできない。
for V／NI

qubitＢ

Let’s use “in phase” and “out of phase” as the
new computational bases “0” and “1.”collective
decoherence
encoding

This is easy if one can prepare two qubits
from the beginning.

decoding

given qubit
your ancilla

The problem is how to encode the given qubit state
into given+ancilla qubits.

the qubit
bi infoencoded in DFS(1) Entanglement preparationcollecti
collective
tive
ti
ve
decoherence
decoherenc
nce
nc
en
encodingng

Optical PDC

decoding
deco
decodingcong

given qubit
your ancillathe qubit
bit info
encoded in DFS

fidelityEntanglement decoherencein an optical fiber

Fidelity F=0.46±0.03 ↑Close to 0.5

F=1.00±0.03

0
0.5
1.0
anti
no
correlated
correlation  correlation

Extraction of the entanglement
protected in the DFS

fidelity

F=0.87±0.07

Progress in suppressing collective noise using DFS (1)
Alice

[1]
Entanglement purification

1 3
Linear optics

rA(for a specific entanglement)
S 1 3 Fibe Noise A

1 3

O
U
Fib
R
er2B 4C 2 4Noise B
E

Classical commun.

2 4
LinearBob optics[2] QI protection using DFS

1JHBD

(for arbitrary isolated state)<HF#;DE!
";DE#<HF[3] QI protection using DFS


<HF
<HF#;DE

Theory PRA(2001)
Experiment Nature(2003)( beam optics
artificial phase noise)<HF#;DE!
";DE#
<HF


<HF
<HF#;DE

Alice
1 3
Linearr opt
optics[1]
Entanglement purificationr A Noise A(for a specific entanglement)
S 1 3 Fibe Nois

1 3

O
U
Fib
R
er2B 4C 2 4Noise B
Nois
E

Classical
ClassicaClass
ical commu
ica
commun.

2 4LinearBob optics[2] QI protection using DFS
Theory PRL(2005)
Experiment NJP(2007)( 10m fiber
natural phase noise)

1JHBD

(for arbitrary isolated state)


<HF
<HF#;DE<HF#;

<HF#;DE!
";DE#<HF[3] QI protection using DFS

1JHBD

(for entangled states)

Progress in suppressing collective noise using DFS (1)Experiment
Nature Photonics(2008)( 500m fiber
natural phase noise)

Theory PRA(2001)
Experiment Nature(2003)( beam optics
artificial phase noise)
Theory PRL(2005)
Experiment NJP(2007)( 10m fiber
natural phase noise)

1JHBD

(for entangled states)


<HF
<HF#;DE
<HF#;
DE


<HF#;DE!
";DE#<HF

Experiment
Nature Photonics(2008)( 500m fiber
natural phase noise)

Overview

Efficiency improvementof faithful entanglement distribution
using decoherence-free subspace

Division of Materials Physics, Department of Materials Engineering Science,
Graduate School of Engineering Science,
Osaka University
Ikuta et al: Phys. Rev. Lett. 106, 110503 (2011).

2. We have demonstrated our DFS scheme experimentally.
We have confirmed that the rate of sharing entanglement is
Violationof CHSH inequality
proportionaltothe transmittance of the quantum channel.
Lower bound of the fidelity

Rikizo Ikuta, Yohei Ono, Toshiyuki Tashima,
Takashi Yamamoto, Masato Koashi, and Nobuyuki Imoto

1. We have proposed a scheme for boosting up the efficiency ofentanglement distribution based on a decoherence-free subspace
(DFS)
Alice
Bob
over lossy quantum channels.
A B
R→ Bob prepares a coherent light pulse
instead of a single photon
as an ancillary system for the DFS.

Error analysis for the multi-photon events

Error analysis for the multi-photon events

Success event

Success event

Main error event

Alice
A

Bob
B

Alice
A

Bob
B

Alice
A

: photon-pair generation rate
: average photon number of the coherent light pulse at: transmittance
Alice’sside of the quantum
channelSuccess probability
Error probability
Requirement :
Alice
B

Bob
B

Alice

Bob
B

A

: photon-pair generation
on rratrate
ate
at: average photon number
berr of thebehe ccoher
coherenteren
er
en light pulse at: transmittanceum
Alice’sside of the quantum
channelSuccess probability
ility
Error prob
Er
probability
Requirement
Requirememen :
emc.f. : T. Yamamoto et al,
Nature Photonics 2, 488 (2008).

A

Main error event

c.f. : T.. Yamamo
Ya
Yamamoto
mamotomamo
to et al,aalhotonics
hotonics 2,2, 48848 (2Nature Photonics(2008).

Bob(This condition is independent of

R

.)

Alice
ice
A

B

Bob

(This condition is independent ofR

Distribution efficiencyof entangled states

Distribution efficiencyof entangled states

Progress in suppressing collective noise using DFS (2)Overview
1. We have proposed a scheme for boosting up the efficiency ofentanglement distribution based on a decoherence-free subspace
(DFS)
Alice
Bob
over lossy quantum channels.
A B
R→ Bob prepares a coherent light pulse
instead of a single photon
as an ancillary system for the DFS.
Lower bound of the fidelity

2. We have demonstrated our DFS scheme experimentally.
We have confirmed that the rate of sharing entanglement is
Violationof CHSH inequality
proportionaltothe transmittance of the quantum channel.

Alice
1 3Linear optics

Entanglement purification[1]
(for a specific entanglement)

rA
Noise AS 1 3 Fibe1 Nois
3
O
U
Fib
R
er2B 4
Noise B
Nois
C 2 4
E

Classical commu
commun.

2 4 Linear optics
Bob
1JHBD

[2] QI protection using DFS
(for arbitrary isolated state)<HF#;DE!
";DE#
<HF


<HF
<HF#;DE
<HF#;
DE

1JHBD

[3] QI protection using DFS (for entangled states)!

";DE#<HF[4] Use of counter

Alicepropagating strongcoherent ancilla

A B


<HF
<HF#;DE<HF#;DEtransmittancetransmnsmittnsm
ittancitt
anc

Theory PRA(2001)
Experiment Nature(2003)( beam optics
artificial phase noise)
Theory PRL(2005)
Experiment NJP(2007)( 10m fiber
natural phase noise)
Experiment
Nature Photonics(2008)( 500m fiber
natural phase noise)
BobR

PRL106, 110503 (2011)η ∝ T attained.)Progress in suppressing collective noise using DFS (3)Entanglement purification[1]
(for a specific entanglement)[2] QI protection using DFS
(for arbitrary isolated state)!
<HF#;DE";DE#<HF[3] QI protection using DFS
(for entangled states)[4] Use of counter

Alicepropagating strongcoherent ancilla

A B

Theory PRA(2001)
Experiment Nature(2003)
beam
optics artificial phase
1JHBD
Theory PRL(2005)

noise
<HF#;DE<HF
Experiment NJP(2007)( 10m fiber
natural phase noise)
Experiment #Nature Photonics(2008)
500m fiber (natural phase noise)transmittanceBob

R

PRL106, 110503 (2011)η ∝ T attained

Progress in suppressing collective noise using DFS (3)Entanglement purification[1]

PRA(2001)
Only the phaseTheory
noise
Experiment
Nature(2003)(for a specific entanglement)[2] QI protection using DFS

beam
optics artificial phase
1JHBD
Theory PRL(2005)
noise
Experiment NJP(2007)

Only the phase noise

(for arbitrary isolated state)!
<HF#;DE";DE#<HF


<HF#;DE<HF( 10m fiber
natural phase noise)
Experiment #Nature Photonics(2008)
500m fiber (natural phase noise)

Only the phase noise[4]
R
Only theA Bphase noise[3] QI protection using DFS
(for entangled states)

Use of counterpropagating strongcoherent ancilla

AlicetransmittanceBob

PRL106, 110503 (2011)η ∝ T attained[5] Scientific Reports 7, 4819 (2017 July)“Experimental demonstration of robust entanglement distribution over reciprocal noisy channels
assisted by a counter-propagating classical reference light”
Rikizo Ikuta, Shota Nozaki, Takashi Yamamoto, Masato Koashi & Nobuyuki Imoto

Progress in suppressing collective noise using DFS (3)Entanglement purification[1]

PRA(2001)
Only the phaseTheory
noise
Experiment Nature(2003)(for a specific entanglement)[2] QI protection using DFS

JST-CREST Project

Global Quantum Network

beam
optics artificial phase
1JHBD
Theory PRL(2005)

noise
<HF#;DE<HF
Experiment NJP(2007)

Only the phase noise

(for arbitrary isolated state)!
<HF#;DE";DE#<HF( 10m fiber
natural phase noise)
Experiment #Nature Photonics(2008)
500m fiber (natural phase noise)

Only the phase noise[4]
R
Only theA Bphase noise[3] QI protection using DFS
(for entangled states)

Use of counterpropagating strongcoherent ancilla

AlicetransmittanceBob

PRL106, 110503 (2011)η ∝ T attained

Our present CREST project[5] Scientific Reports 7, 4819 (2017 July)phase & polarization noise“Experimental demonstration of robust entanglement distribution over reciprocal noisy channels
assisted by a counter-propagating classical reference light”
Rikizo Ikuta, Shota Nozaki, Takashi Yamamoto, Masato Koashi & Nobuyuki Imoto(1) Why necessary?
(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?
・Frequency conversion ・noise cancellation by DFS(4) How are we going to cope with these problems?
(5) Is their any alternative(s)?

Output Power

Quasi-Phase matched (QPM) crystal

riod
odic
od
ically poled structure compensates pha
hase
ha
se
Periodicallyphasemismatch
Periodically poled
structure

Observation of wavelength conversion
ECDL : external cavity diode laser
EDFA : Erbium-doped fibre amplifier

Δλ〜0.2 nm

λ=780 nmf = 82 MHz
τ=1.2 ps

QPM

Δf =150kHz

No
QPM

Max : 2 W

L
Phase mismatch
QPM
Properly adjusting poling period
Λ,phase mismatch will becompensated.
In our experiment:
Wave guided PPLN( periodically poled lithium
niobate)T. Nishikawa et al. Opt. Express"17, 17792(2009) "Wavelenth conversion of single photon

Conversion efficency ： 0.62
（0.70@ cw laser
light）••••

Crystal length
L=20mmPolling periodtempBand widthnm

Λ〜19μm50℃

Conversion
efficiency：pump powerα|0〉 ＋ β|1〉650mW@1600nmIntensity
ity correlation

Single photon
Laser lightClassical
light

Observation of Nonclassicality：contstantObservation of entangled photon conversion

200mW @390nm

Helarded single
photon：crystal length

0.3

DFG module

Observation of entangled photon conversion

DFG moduleα|0〉 ＋ β|1〉

Observation of entangled photon conversion

DFG moduleα|0〉 ＋ β|1〉

DFG module

Observation of entangled photon conversion

DFG module

DFG moduleα|0〉 ＋ β|1〉

Observation of entangled photon conversion

DFG module

DFG moduleα|0〉 ＋ β|1〉

Reasons of the degradation of the
fidelity

Observation of entangled photon conversion

Reconstructed density
matrixParametric down converion

Pump power VS noise counts

Beforeconversion
fidelity：0.95±0.01

Unexpected photon counts
75%:Raman scattering from Pump
25%:Dark counts of the detector

UV(390 nm, 200 mW)

Timing jitter of the detectors

After conversion
Polarization entangled photon pair300 ps @Si-APD, InGaAs-APD

fidelity ：0.75±0.06

R. Ikuta, Y. Kusaka, T. Kitano, H. Kato, T. Yamamoto,
M. Koashi, N. Imoto, Nat. Commun. 2 537 (2011).

Faster detectors will decrease the noise counts

Superconducting single photon
detectors (SSPD)

Experimental Ikuta
resultset.al.,PRA

Quantum st
stat
state
ate
at
e
tomography
1. APD +
SSPD

F

EoF

S（Bell parameter）

Before conv.

0.97±0.01

0.97±0.03

2.73±0.02

After conv. (APD + SSPD)

0.87±0.06

0.68±0.15> 2

2.35±0.10

Almost noise-free conversion
has been observed
0.93±0.04
0.88±0.10byusing SSPDs

2.62±0.09

S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang
Optics Letters, Vol. 35, Issue 13, pp. 2133-2135 (2010)

Si-APD
efficiency〜50% @800 nm

InGaAs-APD〜25% @1550 nm

Dark counts

100 Hz1.6×105 Hz

Timing jitter

300 ps300 ps(2013)
2. SSPD + SSPD

(APD+APD):
F=0.75±0.06
After conv. (SSPD + SSPD)

JST-CREST Project
超長距離量子ネットワーク（量子中継システム）
Quantum repeaters
Bell measurement
Bell測定（Opticalcircuit／
Quantum
量子メモリ(光回路/光子検出器)
memoryphotodetectors）
光子
PhotonGlobal Quantum Network

Entanglement
量子相関（エンタングルメント）(1) Why necessary?
(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?
・Frequency conversion ・noise cancellation by DFS(4) How are we going to cope with these problems?
(5) Is their any alternative(s)?

Long-haul entanglement

長距離エンタングルメント→ Quantum cryptography, Quantum teleportation, Quantum dense coding, Quantum cloud→量子暗号、量子テレポーテーション、量子稠密通信、量子クラウドコンピューティン
computing
グ。。。

Elementarydevices／technologies
要素技術
1.長寿命量子メモリ

1.
Long lifetime quantum memories
2.量子メモリ・光エンタングルメント
本研究課題で推進
2.
Memory-photon entanglement
3.高効率ファイバー光伝送（通信波長帯）
4.高効率・低雑音光子検出器
3.
High-efﬁciency ﬁber transmission of photons
4. High-efﬁciency & low noise photodetectors

Single excitation of Rb atom

Initialization

with single photon emission

Initialization: Polarization of atomic
stateInteraction Hamiltonian:
Raman scattering
Pump(anti-) Stokes Photon
excitation

Λ type three energy level systemwith atomic ensemble

StokesPhotonPump

Rbensemble

Read out process

Cold Rb atomic ensemble

"
"
"

量子相関
Entanglement（エンタングルメント）"
Clean laser

Cold 87Rb Atomic Ensemble

Λ-type three level system in Rb
Resonance
atomwavelength between upper level and two lower levels:
780 nmResonance frequency between two lower levels:

6.8Ghz""Vacuum stateSingle excitation in atomic cloud

Read out of single excitation

Preparation of single excitation in
atomic cloudWrite(V pol.)Write(V pol.)Anti-Stokes(H pol.)Rb

ensemble(H pol.)"
To Single photon detection(Notice of single excitation)

APD

Read out of single excitation fromatomiccloud
Anti-Stokes【 】Stokes

Rb

ensemble(V pol.)Read(H pol.)Read out of Single excitation"

AntiAnti

Write"

Single excitation

Anti-

Write

Stokes""

Stokes

Stokes

"

Read

"
"

"

Atom-Photon Entanglement

Polarization Insensitive QFC

Momentum of collective spin excitation in atomic

QFC installed in Sagnac interferometer

ensemble1522 nm

780 nm

1600 nmPump

QFC
CW direction

ACW direction

Atom-Telecom Photon

Experimental results

Entanglement

Atom statemeasurement

Density Matrix without
QFC

Density Matrix with QFC

780 nmState Tomography

780 nm1522 nmR. Ikuta, T. Kobayashi, T. Kawakami, S. Miki, M. Yabuno, T. Yamashita, H. Terai, M. Koashi, T. Mukai, T.

Yamamoto, N. Imoto, “Polarization insensitive frequency conversion for an atom-photon entanglement
distribution via a telecom network”
NATURE COMMUNICATIONS 9, 1997 (2018).

Ion-Telecom Photon Interface
Ca+ ion single photon source with our QFC

F=0.69 0.07
EOF=0.25 0.
13

F=0.78 0.05
EOF=0.37 0.
11

R. Ikuta, T. Kobayashi, T. Kawakami, S. Miki, M. Yabuno, T. Yamashita, H. Terai, M. Koashi, T. Mukai, T.
Yamamoto, N. Imoto, Polarization insensitive frequency conversion for an atom-photon entanglement
distribution via a telecom network. arXiv:1710.09150

Summary

Global Quantum Network(1) Why necessary?

Conversion Efficiency
866 nm -> 1530 nm

g(2)=0.67 0.07 over 10 km

Joint work with Hayasaka@NICT in Tanaka Team and M. Keller@Sussex Univ.
T. Walker, K. Miyanishi, R. Ikuta, H. Takahashi, S. V. Kashanian, Y. Tsujimoto, K. Hayasaka, T. Yamamoto, N.
Imoto, M. Keller, “Long-distance single photon transmission from a trapped ion via quantum frequency
conversion”
Phys. Rev. Lett. 120, 203601 (2018)(2) What are the problems?
・transmission loss ・noise（decoherence）(3) How did we deal with these problems?
・Frequency conversion ・noise cancellation by DFS(4) How are we going to cope with these problems?
・Incorporating quantum memories such +as Rb, Ca(5) Is their any alternative(s)?