High-quality quantum process tomography of time-bin
qubit’s transmission over a metropolitan fiber
network and its application
Peiyu Zhang (张佩瑜)
†
, Liangliang Lu (陆亮亮)
†
, Fangchao Qu (渠方超),
Xinhe Jiang (蒋新贺), Xiaodong Zheng (郑晓冬), Yanqing Lu (陆延青),
Shining Zhu (祝世宁), and Xiao-Song Ma (马小松)*
National Laboratory of Solid-state Microstructures, School of Physics, College of Engineering and Applied Sciences,
Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
*Corresponding author: xiaosong.ma@nju.edu.cn
Received April 6, 2020; accepted June 10, 2020; posted online July 16, 2020
We employ quantum state and process tomography with time-bin qubits to benchmark a city-wide metropolitan
quantum communication system. Over this network, we implement real-time feedback control systems for
stabilizing the phase of the time-bin qubits and obtain a 99.3% quantum process fidelity to the ideal channel,
indicating the high quality of the whole quantum communication system. This allows us to implement a field
trial of high-performance quantum key distribution using coherent one way protocol with an average quantum
bit error rate and visibility of 0.25% and 99.2% during 12 h over 61 km. Our results pave the way for the
high-performance quantum network with metropolitan fibers.
Keywords: quantum process tomography; quantum networks; quantum communication; quantum key
distribution.
doi: 10.3788/COL202018.082701.
Quantum internet connects quantum computers with
quantum communication channels
[1,2]
, facilitating the
transmission of information carried by qubits. Recently,
free-space quantum communication has had tremendous
advancement
[3]
. On the other hand, fiber-based quantum
communication is a natural candidate for the realization of
transmitting quantum information in the metropolitan
scale. This is because of its compatibility with an estab-
lished fiber network for classical communication
[4–12]
.
To obtain the full knowledge of the transmission process
over the fiber channel is quintessential for the security and
reliability of quantum communication systems. A method
for reconstructing the quantum process is known as quan-
tum process tomography (QPT)
[13]
. Based on the method,
we can fully describe the channels and understand the pos-
sible errors during transmission
[14–16]
. A time-bin qubit is a
promising quantum information carrier over fiber net-
works [e.g., intercity quantum teleportation
[17,18]
and quan-
tum key distribution (QKD)
[19–21]
] because it is easy to
prepare, is polarization independent, and stable in the fi-
ber. However, to the best of our knowledge, there are no
tests of QPT in a fiber network based on time-bin qubits
encoded in weakly coherent states, let alone in an installed
metropolitan telecommunication fiber network
[14,15,22–24]
.
Here, we carry out tomographic protocols based on
time-bin encoding to characterize an installed commercial
fiber network between the two campuses of Nanjing Uni-
versity. The physical distance between the two campuses
is about 30.5 km. We use a fiber loop (about 61 km in total
with a loss of 28.02 dB) to guide the photon back to the
Gulou Campus at Nanjing University. By doing so, we
double the attenuations of the signal, which enables us
to characterize our QKD system under various operating
conditions and provides important metrics of our system
with high-transmission loss. Full reconstruction of the
channel helps us better understand the channel condi-
tions. To verify the reliability of the QPT experiment, we
then implement a field trial of coherent one way (COW)
QKD
[20]
with continuous and autonomous feedback con-
trol over 12 h. We obtain the averaged quantum bit error
rates (QBERs) of 0.25% and visibilities of 99.2%, respec-
tively, matching well with the QPT results. Such a tech-
nique can be used as a standardized method for the
calibration of quantum fiber networks in the future. The
COW protocol can be naturally extended to a three-state
protocol for considering the coherent attacks, which has
been studied both theoretically and experimentally
[25–27]
.
An aerial map of the Nanjing University quantum net-
work, identifying the locations of Alice and Bob, is shown
in Fig.
1 with the schematics of the experimental setup as
the insets. There are three nodes in the network with two
nodes (node A and node B) at the Gulou Campus and one
node (node C) at the Xianlin Campus. These nodes are
separated by distances of 0.2 km and 30.5 km, respec-
tively; the superconducting nanowire single-photon detec-
tor (SSPD) is situated at node B. The sender (Alice, at
node C for remote experiment and at node A for looped
back field trials) prepares time-bin-encoded weak laser
pulses and sends them to the receiver (Bob) at node A over
the commercial fibers, considered as a quantum channel
here. At Alice’s side, we use a continuous-wave laser at
1536.61 nm (ITU-T channel 51) with an intensity modu-
lator (IM) to generate the time-encoded pulses. The pulse
width is about 1.5 ns, and the pulse separation is about
COL 18(8), 082701(2020) CHINESE OPTICS LETTERS August 2020
1671-7694/2020/082701(5) 082701-1 © 2020 Chinese Optics Letters