[LINK] NBN also a quantum net

[stephen at melbpc.org.au]

Physicists take steps toward delivering quantum information to the home

July 18, 2011 by Lisa Zyga 
http://www.physorg.com/news/2011-07-physicists-quantum-home.html


(PhysOrg.com) -- Today, fiber optics technology transports information in 
the form of classical data to homes and businesses. 

But researchers are currently working on ways to combine quantum data 
with the classical data in fiber optics networks in order to increase 
security. 

In a new study, scientists have shown how quantum and classical data can 
be interlaced in a real-world fiber optics network, taking a step toward 
distributing quantum information to the home, and with it a quantum 
internet. 

The physicists, Iris Choi, Robert J. Young, and Paul D. Townsend, from 
the Tyndall National Institute at the University College Cork in Cork, 
Ireland, have published their study on combining quantum and classical 
signals in a recent issue of the New Journal of Physics. 

While the feasibility of transferring qubits on modern fiber-to-the-home 
(FTTH) networks has previously been demonstrated, this is the first time 
that researchers have investigated how the operation would work in a real-
world network.

“I believe that our work constitutes the first really hard-nosed, 
pragmatic attempt to address the question of whether quantum key 
distribution (QKD) can work on a real fiber-to-the-home (FTTH) network,” 
Townsend told PhysOrg.com. 

“The new scheme that we have developed and tested demonstrates that the 
answer is ‘yes it can.’ I say pragmatic and hard-nosed because we have 
taken a widely deployed classical FTTH system and have adapted QKD to 
interwork with it, leaving the design of the classical part of the system 
essentially unchanged. The alternative approach, sometimes taken in QKD 
research, is to leave out the classical system completely or to adapt it 
to work with the QKD. In our view this is not very practical for cost 
reasons.”

The biggest challenge in transferring qubits in real-world networks is 
overcoming the crosstalk between the classical and quantum channels. 

Crosstalk is induced by spontaneous Raman scattering of photons in the 
optical fiber. Since the classical channels involve strong laser pulses 
while the quantum information is carried by single photons, the crosstalk 
primarily affects the quantum channel, making the error rate so high that 
the quantum channel is unable to operate.

Previous research has shown that the Raman noise level can be reduced by 
optical filtering, although this technique is too expensive for practical 
use. So Choi, Young, and Townsend have developed and demonstrated a novel 
noise suppression scheme that involves creating gaps in the scattering, 
and sending quantum data in these gaps. 

First, the researchers chose a configuration that used two different 
wavelengths for transmitting the quantum and classical channels. In this 
configuration, only the Raman-scattered light in the “upstream” channel 
(going away from a user’s house) can generate crosstalk for that user. 

Then, the researchers identified quiet periods between the bursts of 
noise generated by Raman scattering in the upstream channel. Using a time 
and wavelength-multiplexing scheme, the researchers demonstrated that 
quantum data generated by a quantum key distribution (QKD) scheme can be 
transmitted during these quiet periods with high fidelity.

While building a purely quantum network could avoid the problem of 
crosstalk altogether, the researchers explain that combining quantum 
channels with classical channels is by far the more practical option.

“I see this as an absolute requirement – a ‘must have,’ Townsend said. 

“That’s because optical fiber network infrastructure is enormously 
expensive to deploy, so it must last for a long time – perhaps 25 years 
or more – and be able to support a wide range of current and future, yet 
to be defined, systems and services. So it is extremely unlikely that an 
operator would ever deploy a network, or even dedicate fibers within an 
existing network, purely for quantum communications – it’s just too 
expensive to do so. Consequently, we have to develop techniques that 
enable classical and quantum channels to work together on the same 
network if we want quantum communication systems to become a practical 
reality.”

By demonstrating that both quantum and classical information can be 
transmitted on a single optical fiber network in a way that satisfies 
real-world requirements, the researchers hope to bring quantum 
information technology one step closer to commercial applications.

“As we have demonstrated, in principle the technology to do this is 
available now,” Townsend. 

“However, in reality further research is likely to be required to reduce 
the cost and improve the performance of certain key parts of the system 
such as the single photon detectors, before widespread applications 
emerge. In general, the ‘value proposition’ for QKD on FTTH and other 
networks is under intensive discussion today, but at the moment no clear 
consensus has emerged concerning if and when it might be adopted to 
replace classical encryption techniques. 

However, as demonstrated by this research, the QKD field is not standing 
still and systems are continuing to evolve to become more practical, 
improving the potential for adoption of the technology in the future.”


More information: Iris Choi, et al. “Quantum information to the home.” 
New Journal of Physics 13 (2011) 063039 DOI:10.1088/1367-2630/13/6/063039
Copyright 2011 PhysOrg.com. 
--

Cheers,
Stephen