February 9, 2021. Near the city of Oldsmar (Florida, USA). At eight in the morning, the automated systems of a water treatment plant that supplies 15,000 households multiplied the levels of caustic soda used to treat water by a hundred. In small amounts, this corrosive substance serves to remove metals and reduce acidity. But, in larger doses, it is poisonous.
A hacker had remotely accessed the computer system that controls the chemical processes of the treatment plant. Apparently, an operative noticed that same afternoon, just in time to prevent the disaster. Today, the case continues to be investigated by the FBI, which has not yet managed to clarify the identity of the attacker, or whether he acted from US soil or from abroad.
Luckily, it was not a scare. But it served to make clear that there is a risk that something similar will be repeated in the future. “ The attacks –terrorists, between nations… – will be increasingly technological , for this reason, there are many countries and companies interested in supporting the protection of critical infrastructures, such as the drinking water network or electricity supply, communication between administrations, or the medical records of the health systems ”, points out to VERY the physicist Hugues de Riedmatten, director of the Group of Quantum Photonics with Solids and Atoms of the Institute of Photonic Sciences (ICFO), in Barcelona.
This is what he answers when we ask him why the European Union, China and the United States are so interested in subsidizing research on the quantum internet . Because it turns out that security will be precisely one of its star applications. “In the conventional internet, we have very good encryption methods that we use on a daily basis, for example, in banking transactions. However, we know that if we had enough computing power, a hacker could gain access to the system. Something that cannot happen with the quantum internet ”, adds Tracy Northup, a scientist at the Institute for Experimental Physics at the University of Innsbruck (Austria).
As a result of this interest, the Quantum Internet Alliance (QIA) was created in 2018, of which more than twenty research centers from all over Europe are part. Among them are the Northup team and the Riedmatten team. Stephanie Wehner, a physics and computer whiz working at Delft University of Technology (QuTech) in the Netherlands, is the QIA coordinator. Its ambitious goal is to develop a European network in which information is transmitted in a quantum state from end to end . In an article published in Science in 2018, Wehner established the phases to achieve this: they are still in the first, connecting Delft with three other Dutch cities. “We think it’s doable, but we won’t know for sure if we don’t try! European scientists are at the forefront of the most significant achievements in this field in recent decades, and it is time to put that experience to good use. Now we have to develop the hardware through which the quantum information will circulate, plus the software that will control how it is distributed, and the applications that will work on that network ”, Northup tells MUY.
The basis for this idea is in quantum physics, which, according to Riedmatten, “allows us to encode information in a different way than has been done so far , taking advantage of its superposition and entanglement properties.” That? How? Lets start by the beginning.
If we want to transfer information from one computer to another in a quantum way, the bargaining chip is the photon – the elementary particle that makes up light. “Since they are quantum particles, you can put them in a property superposition. And one of the most interesting properties to manipulate is polarization – to circulate the photon to the left or to the right. Quantum information occurs in the superposition of polarizations in a photon. It is what is called a qubit –qbit or quantum bit, in English–, formed by two quantum states ”, indicates to VERY Eden Figueroa, director of the Quantum Information Technology Group in the Department of Physics and Astronomy at Stony Brook University (New York). While the traditional bits that surf the classic internet are 0 or 1, the qubits have the possibility of representing the values 0 and 1 at the same time . Therefore, they have a much greater capacity to save information. Furthermore, this curious property of quantum mechanics – called superposition – is what makes information so elusive and impossible for third parties to manipulate. It turns out that, when a qubit is observed or intercepted, its delicate mixed state collapses and becomes 0 or 1. With this, the quantum information it carried is destroyed, which leaves the trace of the intrusion.
And how do we get the qubits of information that we want to send into the photons? “You can encode your photon, charging it whatever polarization state you choose. For that, you create the photon and pass it through a device called an electro-optical modulator, to which you put a certain voltage so that the phase of the electric field changes. That change is what determines the superposition of polarizations for that photon. Thus, you get the qubit that you want ”. It seems easy, at least, because of the familiarity with which Figueroa explains it.
Now that we have the raw material, the qubit, the number one requirement to talk about the quantum internet is entanglement . “If you have a pair of entangled photons and you send each one in different directions, to two separate points, for example, per hundred kilometers, every time you make a measurement on one photon, the other reacts immediately, altering the information at the other end ”, Explains Figueroa. It is not that a particle is in two places at the same time, but that there are two particles that have the same state at the same time. Although they are in different places, their state is shared and a change in one implies a change in the other .
Across the pond, Figueroa and his six postdoctoral students just demonstrated in January of this year that it is possible to connect the campus of the State University of New York with the Brookhaven National Laboratory, separated by 158 kilometers, using quantum memories to room temperature, so that the information encoded at the source was maintained when it was received at the destination. “We are developing different technologies that are needed to build the first prototype of the quantum internet,” he tells us in a telephone interview.
The first thing is to create simple photon sources, that is, with each pulse you apply to the source, a single photon comes out. The next thing is to have entangled photon sources: every time you press the button, the source emits two photons whose state is correlated ”. But his darling is quantum memories , “a very beautiful technology that is based on atoms,” he enthusiastically tells us, which he has been working on for ten years and which is requirement number two to start building quantum networks. Imagine that you have two entangled photons that are traveling in the fiber and you want to do the measurement in one. For that, you need a system that can store the information of that photon, while you wait for the other photon to arrive at the measurement station at its destination point ”, he says.
Another specialist in quantum memories is Riedmatten. In his words, “they serve to make a synchronization between the sections of a network. They save the result of an operation, until the other sections of the network can also carry out this operation or task successfully ”. An example of a typical operation would be to establish entanglement between two points. It is a process that does not always work, nor is it achieved the first time: “If you have a 100-kilometer stretch of optical fiber, only 1% of the transmitted photons arrive, so the probability of generating entanglement is very small. It takes multiple attempts, until the quantum memory emits a signal warning that it has been achieved ”, he points out. When we want to establish a multinode network , in which two points are connected through at least a third, a quantum memory allows us to store the entanglement between points 1 and 2, while it is possible to link with 3. “Then, we can put together all the sections so that the distance is longer ”, says the physicist.
It is what we call a quantum repeater . According to Riedmatten, “if all nodes have to work at the same time, the total probability decreases exponentially to the number of nodes. But if you can save it in memory every time it works, the total time will not depend on the number of sections ”.
Another advantage of quantum repeaters is that they allow photons to be remotely entangled , that is, they do not need to be entangled from the same source. Imagine that you have a pair of interlaced qubits in section A and B, and another pair in section C and D. “When making a joint measurement between B and C, memories A and D will be correlated, although there has never been interaction between they. When doing the measurement, the photons in B and C disappear, but the entanglement remains between A and D. It is a key to getting the internet over long distances ”, Riedmatten emphasizes.
Although there are other methods, the memories most used in experiments use cold atoms to store quantum states . This technology combines efficiency – the probability that a photon enters memory without losing entanglement with its partner is 90% – with an acceptable coherence time – the time that information can be stored so that overlaps are not lost quantum, which is currently a few hundred milliseconds.
Unsurprisingly, these memories look little like their digital counterparts. Those with cold atoms are rubidium clouds cooled by lasers in a vacuum chamber. Lasers are pointed in all directions to slow atoms to the point where they barely move, lowering their temperature to tens of microkelvin. Then, the light is mapped or imprinted on them: “The photon enters these cold atoms, and it stops completely. It’s as if the light could be turned off, ”observes Riedmatten.
Although the atoms do not occupy anything, the devices based on this technology are large and bulky . They occupy a table measuring two by two meters, because of the laser equipment around it. To overcome this drawback, his ICFO group is investigating another smaller and more manageable system: “solid state devices, where a crystal directly captures the photon and prevents it from moving. We don’t need lasers, ”explains Riedmatten. Specifically, the crystals they use in their laboratory are made up of rare earth atoms, “because they have a configuration of electrons that isolates them from their environment, makes them less malleable and provides very long coherence times,” he adds. A different proposal is that of Figueroa, which uses quantum memories at room temperature composed of rubidium vapor in a glass tube two centimeters long, which would fit in a shoe box. His intention is to make them more affordable and manageable, with less technical requirements than cold atom or solid state memories.
Thus, the ultimate goal of the ongoing research is “to demonstrate for the first time a working quantum repeater. It has not yet been possible to preserve entanglement to be able to measure it with quantum memories ”, says Figueroa. Quite a scientific and engineering challenge. “In recent years, we have seen very interesting demonstrations of point-to-point connections over short distances using different systems. The next step will involve more complex connections –multinode–, over long distances and with more qubits, to be able to carry out more complex quantum communication tasks ”, according to Northup. For the moment, his team has managed to send a photon entangled with a calcium ion – one of the hardware systems that could put together the quantum internet – 50 kilometers by fiber optics, without losing the entanglement. And Riedmatten’s most recent milestone was to demonstrate an entanglement between solid-state quantum memories using light at telecommunication frequencies, last January. The idea is to be able to adjust its technology to the current fiber optic networks that we already have.
Meanwhile, the goal in the global race towards the quantum internet points to increasingly distant targets. According to Figueroa, the ideal to achieve a system that covers the entire planet is a combination of fiber and free space. “If the entanglement is to be done between 100 or 150 kilometers, it can be achieved by fiber. If you want to do it between continents, you must use satellites. What is not clear is how you can get the memory to receive the photon through free space, by satellite ”, he tells us. His Stony Brook University team is collaborating with the University of Padova and the Matera-Laser-Ranging-Observatory (both in Italy) to answer this question.
In parallel, Riedmatten’s dream is to bring together all the qualities we need to have a good quantum memory in the same system. “We have not yet succeeded. One technology is better for long-term storage, another is better for efficiency, another is better for storing more than one qubit … ”, he acknowledges. His second big bet is to create entanglement between long-distance quantum memories, “but outside the laboratory.” Because, for now, all the demonstrations in which it has been achieved have taken place over kilometers of fiber optic cable, yes, but rolled up in a room and protected from outside interference.
For now, it looks like “ the quantum internet is not for tomorrow, but it will be for the very long term . We are going to make a small network for now and then we are going to extend it. To expand it on a continental level, it takes dozens of years. Doing it locally, in a city or a country, maybe it can be done in the next five or seven years. In fact, there are plans to try it between Barcelona and Madrid ”, Riedmatten tells us.
Nor will it be a substitute for the classic internet, but both will coexist in the future, according to experts. Sending a non-top secret email or streaming a movie are things the mainstream WWW does very well, while its quantum brother specializes in secure communications .
For his part, Figueroa predicts that, within ten years, another utility of this type of network will be operational: quantum astrometry . “It consists of a system of sensors connected through entanglement that cover a fairly long distance, hundreds of kilometers, and that allows you to detect with great precision astronomical particles that come from outer space – such as dark matter, neutrinos … -“. And, twenty years from now, “once we have quantum repeaters in various places, we can do distributed quantum computing. In other words, joining different quantum computers to make simulations much larger and more powerful than those made by a single quantum processor ”, adds the scientist. A little earlier, perhaps, blind quantum computing – or in the cloud – will become a reality.
As Northup explains, “ in the future, there will be scientific problems that only quantum processors can solve , but not all researchers will have access to their own. So they will have to submit the work to be solved by a quantum computer elsewhere, remotely. This will be able to do the operations that are requested, without knowing what the question was. Thus, no one can access the content of the work or its data, not even the company that owns the technology in question. Quite the opposite that happens with the cloud services that we use today, such as gmail, for example ”, points out this researcher.
When we want to know, when push comes to shove, what use to the ordinary citizen all this quantum shed, Northup trusts that “it will benefit people because it will offer secure communications in everyday life, for example, protecting the privacy of our medical or financial data ” . But, like everything else, it also has its risks: it can increase the security of communications, yes, although it could also make traditional networks more fragile. “If quantum computing technology fell into the wrong hands, it would be a problem, just like nuclear bombs. Who is going to use it first and who is going to use it later is something that the governments are still trying to define ”, admits Figueroa.
And is it not going to serve to increase the differences in power between countries? We asked him. “That is an issue that worries me a lot. Imagine, you are talking to a Mexican who works in the United States! What we do in my team focuses on a type of technology that uses simple systems; that is, they are not too complicated, but they work. One of my dreams is to make them as affordable as possible so that other nations can acquire them. They will probably begin to be marketed in the next five years. The desirable thing is that, eventually, all countries are endowed with quantum internet networks, as has happened with the classic internet ”.
