Physics: Unite to build a quantum Internet
Nature.com 12 April 2016
Advances in quantum communication
will come from investment in hybrid technologies, explain Stefano Pirandola and
Samuel L. Braunstein.
Almost
25 years ago, physicists discovered a way of 'teleporting' a quantum system
from one place to another without moving it1. There are physical limits to such
teleportation: nothing can be transmitted faster than the speed of light; and
Heisenberg's uncertainty principle restricts what we can know about the state
of a quantum system at a given time. Nevertheless, the transported system is a
replica that perfectly mimics the original, thanks to the weirdest feature of
quantum mechanics — entanglement. Described by Albert Einstein as “spooky
action at a distance”, this property enables distinct quantum systems to become
intimately correlated so that an action performed on one has an effect on the
other, even for systems that are too far apart to physically interact.
Quantum
states are fragile and cannot be sent through conventional lines of
communication; quantum teleportation offers a reliable and efficient way to
transfer quantum information across a network. It provides the most promising
mechanism for a future quantum internet, with secure communications and a
distributed computational power that greatly exceeds that of the classical
Internet.
Quantum
information comes in a variety of forms — the polarization state of a photon,
the spin of an electron or the excitation state of an atom. Many technologies
have been developed for teleporting such states2. But there are practical
restrictions on what can be teleported, and how. Certain technologies will be
better than others for particular tasks, and each has its limitations.
Polarized photons have been used to transfer quantum information over more than
100 kilometres3,
but only probabilistically. Superconducting devices can send information
without losses through a chip, but only for a split second, after which the
information is scrambled by interactions with the environment.
Hybrid
approaches might overcome these limitations. A global, distributed quantum
computer or internet will need to integrate different sorts of quantum
technologies. For example, light-based teleportation, for long-distance quantum
communication, will need to be linked to matter-based quantum memories and
quantum computers for data storage and data processing. Here, we outline the
main challenges and call for researchers to focus on the interfaces between
quantum technologies as well as advancing individual methods.
Two approaches
The
best technique at present for long-distance communication is the teleportation
of quantum information that is embedded in optical light. Quantum information —
measured in units known as quantum bits, or qubits — can be encoded either by
the discrete properties of a pulse of light, such as its polarization state, or
by the continuous aspects of an electromagnetic wave, such as the intensity and
phase of the wave's electric field4. To teleport this information, both
the sender and receiver must own one of a pair of entangled quantum systems
(see 'Quantum teleportation'). When the sender
alters the state of their system, the receiver's system is also affected.
Polarization
qubits perform best in terms of distance — holding a record of 143 kilometres3.
But currently, only 50% of these qubits can be teleported2. Teleportation requires that the
sender can carry out an operation known as a Bell detection, in which the
polarizations of two qubits are correlated perfectly in four possible
configurations. But there is no practical way to measure all four outcomes.
Simple optics and photodetectors can distinguish two, at most. Extra qubits add
technical complications5.
Inconclusive
outcomes such as these are acceptable for quantum cryptography, in which secret
keys are generated at random, and part of the information can be discarded. But
quantum communication demands that information is sent in full.
Teleportation
over long distances2 brings further technical
challenges, such as compensating for atmospheric turbulence and movement of the
ground. It is also likely to require new technologies to synchronize both ends
— using atomic clocks, for example. Modern classical communications rely
heavily on satellite technology. Transferring quantum information to the ground
from a satellite in a low Earth orbit (at an altitude of about 500 kilometres)
is within the reach of current technology, thanks to ground-based telescopes
with metre-sized apertures that can collect most of the light from a beam that
has spread out during its passage from a satellite. But transferring quantum
information from the ground to a satellite, or between satellites, is more
difficult because satellites cannot carry large optics.
By
contrast, it is easy to measure all Bell-detection outcomes for
continuous-variable systems such as electric fields, using only simple linear
optics and standard photodetectors. Such systems can convey simultaneously the
equivalent of many qubits, which makes them appealing for use in high-rate quantum
communications6. But because the range of distances
over which they can teleport is currently limited, continuous-variable systems
are used less frequently than are qubits.
Approaches
are needed that combine the best features of discrete variables (teleportation
over long distances) with those of continuous variables (fast, deterministic
teleportation). Teleportation that uses such combinations has been demonstrated
over table-top distances. One experiment7combined a discrete qubit with a
continuous-variable entangled source to teleport quantum information
deterministically. Further studies should help both to extend the distances
covered by these experiments and to integrate qubits with other types of
quantum technologies, including quantum memories for the storage of teleported
information.
Studies
of hybrid technologies will require greater collaboration and interaction
between teams with different specializations.
Julian
Kelly/Martinis Group
A
superconducting quantum chip with nine qubits.
Quantum internet
One
of the greatest challenges for implementing a globally distributed quantum
computer or a quantum internet is entangling nodes across the network8.
Qubits can then be teleported between any pair and processed by local quantum
computers.
Ideally,
nodes should be entangled either in pairs or by creating a large,
multi-entangled 'cluster state' that is broadcast to all nodes. Cluster states
that link thousands of nodes have already been created in the laboratory9. The challenges are to demonstrate
how they might be deployed over long distances, as well as how to store quantum
states at the nodes and update them constantly using quantum codes.
Quantum
networks require memories to store quantum information, ideally for hours —
shielding it from unwanted interactions with the environment. Such memories are
needed for quantum computing at nodes and also for the faithful, long-distance
distribution of entanglement through quantum repeaters.
Quantum
memories need to convert electromagnetic radiation into physical changes in
matter with near-perfect read–write fidelity and at high capacity. 'Spin
ensembles' represent one type of quantum memory. Ultracold atomic gases
consisting of about one million atoms of rubidium can convert a single photon
into a collective atomic excitation known as a spin wave. Storage times are approaching
the 100 milliseconds required to transmit an optical signal across the world.
Solid-state
quantum memories are even more appealing. Crystalline-solid spin ensembles —
created by inserting lattice defects known as nitrogen-vacancy centres into
diamonds, or by doping rare-earth crystals — can remain coherent for hours at
cryogenic temperatures.
Superconducting
qubits, which are defined by physical quantities such as the charge of a
capacitor or the flux of an inductor, interact within a quantum processor by
releasing and absorbing microwave photons. For the successful integration of
solid-state quantum memory, reversible storage and retrieval of quantum
information must be made possible. This will require an efficient interface
between the microwave photons and the atomic spins of a solid-state quantum
memory that is attached to the processor. If successful, this hybrid technology
would become the most promising architecture to be scaled up into a large,
distributed quantum computer.
“The development of a quantum internet needs investment on a
much larger scale.”
The
incorporation of superconducting processors into a quantum internet also
requires that locally processed and stored microwave photons interface with
optical signals (often carried in fibres), which are the most robust carriers
of quantum information over long distances. A hybrid solution, known as an
optomechanical quantum transducer, is emerging10. These devices exploit
nanomechanical oscillators (such as microscopic vibrating mirrors) to transform
optical photons into microwave photons, and vice versa. But their efficiency
must be improved to ensure that qubits are not lost during the conversion
process and that all of their quantum features are preserved. The conversion
efficiency is currently about 10% (ref. 10).
The
next 15 years could see the construction of a hybrid-technology quantum
internet. In the vision we outline, superconducting quantum processors will be
integrated with solid-state memories for local quantum storage and then
augmented with microwave–optical transducers for long-distance optical
communication. After two remote nodes have been connected in this way,
entanglement can be distributed between distant quantum processors to enable
teleportation.
Next steps
To
make this vision a reality, the following three steps should be priorities for
quantum teleportation science.
First,
more research — theoretical and experimental — is needed at the interface
between discrete and continuous variables; dedicated conferences would help.
This would enable us to blend these currently distinct approaches to exploit
the best of both. Satellite experiments with polarization qubits should be
pursued, and continuous-variable teleportation should be extended beyond the
lab for communication within cities using free space or optical fibres.
Second,
the most successful technologies will be those that integrate data
communication and data storage. We need to invest in the development of a more
efficient interface between superconducting quantum processors and solid-state
quantum memories. This would improve the performance of the storage and
retrieval of microwave photons. A tangible next step could be on-chip
teleportation between a superconducting qubit and a nitrogen-vacancy centre in
a local quantum memory.
Third,
investment should be made in technologies that show promise of scalability. For
example, microwave–optical transducers that can efficiently connect microwave
photons with optical photons on a chip for long-distance quantum communication
should be designed and integrated. Two remote chips could be linked by paired
transducers, paving the way for long-distance quantum teleportation between
superconducting qubits.
These
steps will necessitate a closer interaction between researchers in
superconducting quantum computing and those who are developing long-distance
quantum optical communications. Industry must also be involved, especially
multinational corporations that are leaders in computer hardware and
telecommunications. Quantum technology is attracting private stakeholders, but
the development of a quantum internet needs investment on a much larger scale.
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