The first quantum revolution brought about semiconductor electronics, the laser and finally the internet. The coming, second quantum revolution promises spy-proof communication, extremely precise quantum sensors and quantum computers for previously unsolvable computing tasks.
Quantum secure communication relies on sharing of entangled states between parties. Over short distances (less than 100 km), these states can be distributed by sending photons over optical fibers, but losses in those fibers limit long-distance sharing. Currently Most Quantum Communication links are direct point-to-point links through telecom optical fibers and, ultimately limited to about 300-500 km due to losses in the fiber. Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration.
The next important milestone, is development of large scale QKD network to extend QKD from point-to-point configuration to multi-user and large-scale scenario. China has also operationalised the 2,000-km quantum communication main network between Beijing and Shanghai using quantum repeaters.
One solution to extend the range of quantum communication between sender and receiver is to use a sequence of “quantum repeaters” along the optical fiber connection. Quantum Repeaters can be thought of as being analogous to the optical amplifiers that provide an economic and compact solution for long distance classical communication. However, whereas the idea of amplifiers is to regenerate the classical optical signal, what these Quantum Repeater links do is to create sections of lossless transmission line over which the quantum state is teleported.
These devices can store the quantum information in an excited state of matter. The states of two relatively close repeaters can be entangled through photon emission. This process can conceivably be repeated over and over to entangle more distant repeaters together, until the entanglement extends from one continent to another.
“In the future, a quantum internet could be used to connect quantum computers located in different places,” Andreas Reiserer says, “which would considerably increase their computing power!” The physicist heads the independent Otto-Hahn research group “Quantum Networks” at the Max-Planck-Institute of Quantum Optics in Garching.
A quantum internet is thus essentially about the global networking of new technologies that make a much more consequent use of quantum physics than ever before. However, this requires suitable interfaces for the extremely sensitive quantum information. This is an enormous technical challenge, which is why such interfaces are a central focus of fundamental research. They must ensure that stationary quantum bits—qubits for short—interact efficiently with “flying” qubits for long-distance communication without destroying the quantum information. Stationary qubits will be located in local devices, for example as the memory or processor of a quantum computer. Flying qubits are typically light quanta, photons, that transport the quantum information through the air, a vacuum of space or through fiber optic networks.
For this system to work, the photons sent or received by the modem as quantum information carriers must be matched precisely to the infrared wavelength of the laser light used for telecommunications. This means that the modem must have qubits at rest that can react precisely to these infrared photons with a quantum leap. Only in this way the sensitive quantum information can be transmitted directly between the qubits at rest and the flying qubits.
Extensive research by the Garching-based group showed that the element erbium is best suited for this purpose. Its Electrons can perform a perfectly matching quantum leap. Unfortunately, the erbium atoms are very reluctant to make this quantum leap. Therefore, they must be fixated in anenvironment that forces them to react more quickly. To solve this problem, the erbium atoms and the infrared photons are locked up in a suitable space for as long as possible. “You can think of it as a party, which should stimulate the best possible communication between, let’s say, ten guests,” Reiserer explains. The size of the space is crucial here. “In a football stadium the guests would get lost, a telephone box in turn would be too small,” the physicist continues, “but a living room would do just fine.”
The party, however, would quickly be over because the photons travel at the speed of light and are therefore highly volatile and always tempted to leave. This is why the Garching quantum modem uses a tiny mirror cabinet as a “living room” Thereto,the team packed the atoms into a transparent crystal made of an yttrium silicate compound, which is five times thinner than a human hair. This crystal, in turn, is placed like a sandwich spread between two almost perfect mirrors. To eliminate the heat wobbling of the atoms, which is destructive to quantum information, the entire ensemble is cooled to minus 271 °C.
The photons trapped between the mirrors are reflected back and forth through the crystal like ping-pong balls. They pass the erbium atoms so often so that the atoms have enough time to react with a quantum leap. Compared to a situation without a mirror cabinet, this happens much more efficiently and almost sixty times faster. Since the mirrors, despite their perfection, are also slightly permeable to the photons, the modem can connect to the network.
“We are very happy about this success,” Reiserer says. As a next step, he wants to improve the experiment such that individual erbium atoms can be addressed as qubits via laser light. This is not only an important step towards a usable quantum modem. Erbium atoms as qubits in a crystal may even serve directly as a quantum processor, which is the central part of a quantum computer. This would make the modem easily compatible with such quantum terminals.
With such an elegant solution, comparatively simply constructed “quantum repeaters” would also become possible. Every hundred kilometers, the devices would have to compensate the increasing losses of quantum information transported by photons in the fiber-optic network. Such “quantum repeaters” are also the focus of international research. “Although such a device based on our technology would cost about a hundred thousand euros, widespread use would not be unrealistic,” Reiserer says. The Garching quantum modem is still purely fundamental research. But it has the potential to advance the technical realization of a quantum internet.
In 2001, researchers first proposed the so-called DLCZ protocol (named after the authors Lu-Ming Duan, Mikhail Lukin, Ignacio Cirac and Peter Zoller) that involves the creation of entanglement between quantum repeaters situated every hundred kilometers or so along an optical fiber link. These repeaters are first excited by a strong laser pulse. By interfering single photons emitted by two nearby repeaters, it’s possible to entangle their excited states. However, this interference is not always successful and can require several tries. For this reason, repeaters must be able to store the quantum information from the initial pulse for an extended period of time
The key technology for implementing quantum repeaters is quantum memories that allow the storage of quantum states. It is difficult to trap and store a photon, which by definition moves at the speed of light. The key challenge in creating quantum repeaters has been finding a material that could both store and transmit qubits. The DLCZ protocol has mostly been demonstrated in laser-cooled clouds of trapped alkali atoms, such as sodium or rubidium, which have very strong optical transitions (as is evident from the bright orange glow of sodium street lights).
Cold atom systems have already reached the prototype stage, and some groups are developing warm vapor DLCZ memories that do not require atom trapping. In addition, researchers are now developing stoichiometric rare-earth crystals, where the ions form part of the chemical structure of the host, which eliminates inhomogeneous broadening and boosts the rare-earth density, providing a solution to the problem of weak transitions and broadened spectra.
Physicists are exploring new materials to build quantum repeaters. A Princeton-led research team has created diamonds that contain defects capable of storing and transmitting quantum information for use in a future ‘quantum internet.’ The defects can take and store quantum information in the form of electrons for relatively long periods of time and link it efficiently to photons
Crystals with rare-earth ions could lead to quantum repeaters that enable secure quantum communications over long distances.
Researchers proposed a way to store light using the weak but narrow optical transitions of rare-earth ions, such as erbium and praseodymium, doped into solids. Rare-earth ions have the unusual property of their unfilled electronic orbitals lying inside their filled shells, which provide shielding against the strong electric fields of the host atoms of the crystal where they are embedded. The ions’ optical transitions are thus narrow, like those of free atoms in a gas. However, the residual interaction with the solid host shifts the transition frequencies differently for each ion, so that the combined spectrum appears broadened.
In addition, the optical transitions are normally forbidden, but they become allowed due to the host’s perturbation. This means the transitions will be weak, and as a consequence, the incoming light must be tuned close to the resonance frequency to increase the probability of absorption (and therefore storage). However, on-resonance excitation combined with the strong inhomogeneous broadening will cause the resulting atomic excitations to rapidly get out of sync. This dephasing would mean the stored information in the putative repeater would be lost.
The trick for overcoming the dephasing, is to shape the spectrum of the ions into an atomic frequency comb (AFC). In this method, a laser “switches off” those dopant ions whose transition frequencies correspond to the spaces between the teeth in the comb. If the remaining ions, with transitions spaced apart in frequency by δ, are resonantly excited with another light pulse, they will rapidly dephase. But thanks to beating between the different frequencies, the ions will return in sync after a time delay of τ=2π∕δ.
For their system, the Geneva group used europium as a dopant in an yttrium orthosilicate ( Y2SiO5) crystalline host. The ICFO team used the same host, but their dopant was praseodymium. In both cases, after optical pumping to produce an AFC spectrum, the teams excited their ions with a “write” pulse. The excited ion states emitted photons, which in a quantum repeater network would be used to entangle the emitting crystal with another crystal. However, in these simple demonstrations, it was enough to show that the emitted photons were quantum correlated with the excited ions in the crystal. For that, the teams used a “read” pulse to re-excite the ions. The researchers then measured the conditional probability that a single photon was detected following the write pulse and following the read pulse. They found that this two-photon coincidence was much higher than could be explained by any classical process, which implied that the emission was correlated to the quantum state of the ions.
The ICFO team initially generated an entangled pair of photons—one at visible wavelengths and the other in an infrared band used in optical fiber telecommunications. The researchers utilized the visible photon to excite their doped crystal, which meant the infrared photon became correlated with the ion excitation. This demonstrates the feasibility of using the scheme at wavelengths compatible with existing optical fiber networks.
Along the way, the researchers have contributed improvements to optical pumping, polarization encoding, coherent control, and enhanced optical efficiency.
Future work is to do in reducing the noise and optical losses, increasing the storage lifetime, and then demonstrating a prototype quantum repeater node. Still, the groundwork laid by the Geneva and ICFO teams shows crystals doped with rare-earth ions remain a promising architecture for tomorrow’s quantum secure World Wide Web.
Quantum repeaters using Synthetic diamond
Nathalie de Leon, an assistant professor of electrical engineering at Princeton University and the lead researcher, said the diamonds could serve as quantum repeaters for networks based on qubits.In an article published in the journal Science, the researchers describe how they were able to store and transmit bits of quantum information, known as qubits, using a diamond in which they had replaced two carbon atoms with one silicon atom.
Researchers have looked to solids such as crystals to provide the storage. In a crystal, such as a diamond, qubits could theoretically be transferred from photons to electrons, which are easier to store. The key place to carry out such a transfer would be flaws within the diamond, locations where elements other than carbon are trapped in the diamond’s carbon lattice. Jewelers have known for centuries that impurities in diamonds produce different colors. To de Leon’s team, these color centers, as the impurities are called, represent an opportunity to manipulate light and create a quantum repeater.
Previous researchers first tried using defects called nitrogen vacancies — where a nitrogen atom takes the place of one of the carbon atoms — but found that although these defects store information, they don’t have the correct optical properties. Others then decided to look at silicon vacancies — the substitution of a carbon atom with a silicon atom. But silicon vacancies, while they could transfer the information to photons, lacked long coherence times.
“We asked, ‘What do we know about what causes the limitations of these two color centers?’,” de Leon said. “Can we just design something else from scratch, something that addresses all these problems?”
The Princeton-led team and their collaborators decided to experiment with the electrical charge of the defect. Silicon vacancies in theory should be electrically neutral, but it turns out other nearby impurities can contribute electrical charges to the defect. The team thought there might be a connection between the charge state and the ability to keep electron spins in the proper orientation to store qubits.
The researchers partnered with Element Six, an industrial diamond manufacturing company, to construct electrically neutral silicon vacancies. Element Six started by laying down layers of carbon atoms to form the crystal. During the process, they added boron atoms, which have the effect of crowding out other impurities that could spoil the neutral charge.
“We have to do this delicate dance of charge compensation between things that can add charges or take away charges,” de Leon said. “We control the distribution of charge from the background defects in the diamonds, and that allows us to control the charge state of the defects that we care about.”
Next, the researchers implanted silicon ions into the diamond, and then heated the diamonds to high temperatures to remove other impurities that could also donate charges. Through several iterations of materials engineering, plus analyses performed in collaboration with scientists at the Gemological Institute of America, the team produced neutral silicon vacancies in diamonds.
The neutral silicon vacancy is good at both transmitting quantum information using photons and storing quantum information using electrons, which are key ingredients in creating the essential quantum property known as entanglement, which describes how pairs of particles stay correlated even if they become separated. Entanglement is the key to quantum information’s security: recipients can compare measurements of their entangled pair to see if an eavesdropper has corrupted one of the messages.
The next step in the research is to build an interface between the neutral silicon vacancy and the photonic circuits to bring the photons from the network into and out of the color center.
Ania Bleszynski Jayich, a physics professor at the University of California, Santa Barbara, said the researchers had successfully met a longstanding challenge of finding a diamond flaw with characteristics favorable to working with quantum properties of both photons and electrons.
“The success of the authors’ materials-engineering approach to identifying promising solid-state defect-based quantum platforms highlights the versatility of solid-state defects and is likely to inspire a more comprehensive and extensive search across a larger cross-section of material and defect candidates,” said Jayich, who was not involved in the research.
The Princeton team included Brendon Rose, a postdoctoral research associate, and graduate students Ding Huang and Zi-Huai Zhang, who are members of de Leon’s laboratory.