Just as classical computers are unthinkable without memories, quantum memories will be essential elements for future quantum information processors. Quantum memories are devices that can store quantum information for a long time with very high fidelity and efficiency. Quantum memories are devices that can store the quantum state of a photon, without destroying the volatile quantum information carried by the photon. The quantum memory should be able to release a photon with the same quantum state as the stored photon, after a duration set by the user.
Quantum memories open manifold opportunities like high-speed quantum cryptography networks, large-scale quantum computers, and quantum simulators. Quantum memories will be key components in future quantum networks, such as quantum repeaters which can provide a solution for long-distance quantum communication beyond the limit of 500 km because of losses in fibers. Sending quantum information further afield requires a quantum Internet—a network of quantum routers linked by fibers. These routers must receive quantum information, store it in quantum memory, and then send it on through the network.
The field of quantum memories has recently seen a lot of progress, with landmark experiments such as the storage of entangled photons and the demonstration of memories with a storage efficiency approaching unity. Unfortunately, developing quantum memory is a very hard problem, since any interactions with the environment can erase any information held in an object’s quantum state. The main problem of quantum memories is the low retrieval efficiency of the systems from the registers of the quantum memory.
Quantum memories require coherent matter systems, otherwise, the quantum information stored inside the medium will be lost due to decoherence. Many different systems are being studied, including individual atoms and ions, cold and hot atomic vapors, various kinds of defects in solids, and quantum dots. The things that are easy to manipulate, like electrons, tend to undergo lots of interactions. Things that are a bit insulated against the environment, like the spins of an atom’s nucleus, tend to be hard to address—you often have to go through the electrons to get to them.
Rare-earth-ion (RE) doped crystals are highly interesting matter systems for quantum memories, owing to their unique optical and spin coherence properties at low temperatures (around 4 K). To store photons in RE-doped crystals Researchers used the Atomic Frequency Comb technique.
Physicists from the University of Basel have developed a memory that can store photons. The researchers were able to store them in an atomic vapor and read them out again later without altering their quantum mechanical properties too much.
Researchers have built a quantum memory-enabled source of spatially structured nonclassical light, based on the principle of wavevector multiplexing. The high-capacity memory using laser-cooled atoms can store up to 665 quantum states of light simultaneously. According to researchers, their system has a larger capacity than any other existing quantum memory.
A team of researchers from the U.S. and Italy has built a quantum memory device that is approximately 1000 times smaller than similar devices—small enough to install on a chip.
Chinese advances on quantum memory
Chinese scientists have developed an integrated quantum memory with on-demand retrieval capability, a significant step toward building quantum networks. A research team with the University of Science and Technology of China said they realized the on-demand storage of time-bin qubits in an on-chip waveguide memory, utilizing the Stark-modulated atomic frequency comb protocol. The study was published in the journal Physical Review Letters In 2021.
A qubit storage fidelity reaches 99.3 percent ± 0.2 percent, indicating its high reliability, according to the study. For scalable and convenient applications, great efforts have gone into the integrated quantum memory based on various waveguides fabricated in solids. However, on-demand storage of qubits, which is essential for quantum information processing, is still challenging in its implementation, using such integrated quantum memory.
A team of researchers affiliated with several institutions in China has succeeded in sending entangled quantum memories over a 50-kilometer coiled fiber cable. In their paper published in the journal Nature, the group describes several experiments they conducted involving entangling quantum memory over long distances, the challenges they overcame, and problems still to be addressed.
In this new effort, the researchers in China succeeded in entangling quantum memory between buildings 20 kilometers apart and across 50 kilometers of coiled cable in their lab. The first experiment was based on the use of a small cloud of atoms placed in a desired quantum state—it represented a memory state—reading and writing operations were done using photons. To engage the memory state, the researchers forced them into an optical cavity, allowing photons to interact with the atoms in the cloud. Once the memory state was set, the cloud emitted a photon to announce its readiness.
That photon was then polarized, allowing it to carry information regarding the state of the memory collective, which meant it could be used to entangle the memory. But preventing it from being lost during transmission required shifting its wavelength to one that is commonly used in fiber cable communications. It was at this point that the memory was ready to travel across the cable. The process proved to be approximately 30 percent efficient. The second experiment involved creating just two quantum bits of memory from photons and sending them through 50 kilometers of coiled fiber. Neither experiment is likely to lead to the creation of a quantum internet, but both demonstrate that scientists are edging ever close to the ultimate goal.
UChicago scientists control single subatomic quantum memories in semiconductors in Sep 2020
In a new study, scientists at the University of Chicago managed to do exactly that. The team demonstrated control of atomic quantum memories in silicon carbide, a common material found in electric cars and LED light bulbs. Then, they used this control to create an “entangled state,” representing a connection between the quantum memories and electrons trapped in the semiconductor material.
Published Sept. 21 in Nature Materials, the study effectively shows how one could encode and write quantum information onto the core of a single atom, unlocking the potential for building qubits that can remain operational—or “coherent”—for extremely long times. The study results hold major implications for quantum computing, according to the authors.
“Just like a desktop computer has different types of memory for various purposes, we envision quantum technologies will have similar needs,” said co-first author Alexandre Bourassa, a graduate student at the Pritzker School of Molecular Engineering at the University of Chicago. “Our trapped electron is like a CPU, where different nuclear spins can effectively be used as a quantum RAM and hard-drive to provide both medium- and long-term storage of quantum information.”
Semiconductor materials are arrangements of atomic nuclei held together by electronic bonds. Some but not all of these nuclei possess a property called “spin,” which enables them to behave as tiny quantum magnets. Nuclei that do have spin can be used to encode quantum information. “The spins of atomic nuclei are one of the most robust quantum systems we know of,” said co-first author Chris Anderson, a UChicago postdoctoral scholar. “Their quantum state can last for hours or even days. This makes them ideal for building quantum memories. In a world where most quantum technologies can only retain their information for a fraction of a second, this is an eternity.”
To interact with these nuclei, the scientists used techniques similar to those used in magnetic resonance imaging (MRI), but replaced the bulky magnetic chamber with just a single electron. Using this “atomic scale MRI,” the scientists were able to address and control the nuclei that form the core of individual atoms. “The trick is to precisely control the number of nuclei carrying the desired spin. If there are too few, one will not have enough available memories in the device, but if there are too many, it won’t be possible to isolate and control them independently,” said graduate student Nikita Onizhuk, a co-author who developed a theoretical model to interpret and guide the experimental breakthroughs.
“Integrating theory, computation and experiments has been critical to optimize these quantum memories,” said Giulia Galli, the Liew Family Professor of Electronic Structure and Simulations at the University of Chicago and a senior scientist at Argonne National Laboratory. Working with theory and materials growth collaborators, the team showed that it’s possible to optimize these quantum memories.
“We believe that we can develop materials that have tens of high-quality quantum memories within a smaller footprint than a single state-of-the-art transistor you’d find in today’s integrated circuits,” said David Awschalom, the Liew Family Professor in Spintronics and Quantum Information in the Pritzker School of Molecular Engineering. Awschalom is also a senior scientist at Argonne, the director of the Chicago Quantum Exchange, and the director of Q-NEXT, a Department of Energy National Quantum Information Science Research Center led by Argonne National Laboratory. Awschalom added that this work establishes the key components necessary for creating quantum technologies in semiconductor devices and will be an important platform for a future quantum internet.
New record of quantum memory efficiency created in April 2019 by Hong Kong University of Science and Technology
Like memories in computers, quantum memories are essential components for quantum computers — a new generation of data processors that obey quantum mechanics laws and can overcome the limitations of classical computers.
Quantum memories are generally a light-matter interface since the quantum information to be stored is usually encoded on a beam of light. Optical quantum memory is a device that takes a photon and encodes it with information. Unfortunately, the devices developed to date have been too large and inefficient to operate in a chip-scale quantum device.
Photonic quantum memories allow for the storage and retrieval of flying single-photon quantum states. However, production of such highly-efficient quantum memories remains a major challenge as it requires perfectly matched photon-matter quantum interface. Meanwhile, the energy of a single photon is too weak and can be easily lost into the noisy sea of stray light background. For a long time, these problems suppressed quantum memory efficiencies to below 50% — a threshold value crucial for practical applications.
Now for the first time in history, a joint research team led by Prof. DU Shengwang from the Department of Physics and William Mong Institute of Nano Science and Technology at HKUST; Prof. ZHANG Shanchao from SCNU who graduated his PhD study at HKUST; Prof. YAN Hui from SCNU and a former postdoctoral fellow at HKUST; as well as Prof. ZHU Shi-Liang from SCNU and Nanjing University, has found a way to boost the efficiency of photonic quantum memories to over 85% with a fidelity of over 99%.
The team created such a quantum memory by trapping billions of rubidium atoms into a hair-like tiny space — those atoms are cooled down to nearly absolute zero temperature (about 0.00001 K) using lasers and magnetic field. The team also found a smart way to distinguish the single photon from the noisy background light sea. The finding brought the dream of an ‘universal’ quantum computer a step closer to reality. Such quantum memories can also be used as repeaters in a quantum network, laying the foundation for a new generation of quantum-based internet.
“In this work, we code a flying qubit onto the polarization of a single photon and store it into the laser-cooled atoms,” said Prof Du. “Although the quantum memory demonstrated in this work is only for one qubit operation, it opens the possibility for emerging quantum technology and engineering in the future.”
Researchers Improve Quantum Memory Capacity Using Multiplexing reported in Dec 2017
Researchers at the University of Warsaw have developed high-capacity memory using laser-cooled atoms that can store up to 665 quantum states of light simultaneously. According to researchers, their system has a larger capacity than any other existing quantum memory. The results of the research could lead to ways to further increase the capacity of quantum memory. Simultaneous processing of many qubits, key to efficient quantum parallel computation, could open up new possibilities in imaging and in communications.
Researchers built the quantum memory using spatial multiplexing aided by a single-photon resolved camera. Their setup contained a magneto-optical trap (MOT), where a group of rubidium atoms inside a glass vacuum chamber was trapped and cooled by lasers in the presence of a magnetic field to about 20 microkelvins.
In the write-in process, the cloud of atoms was illuminated by a laser beam and driving a two-photon transition, which results in spontaneous emission of visible blue light. Each scattered photon was emitted in a random direction and registered on the single-photon sensitive camera. The information about the scattered photons was stored inside the atomic ensemble in the form of collective excitations — spin-waves that could be retrieved on demand as another group of photons.
The quantum information about all the stored photons resides in a single cloud of cold atoms. Each atom is engaged in the storage of each photon, making the memory resilient to quantum decoherence. Researchers confirmed this resilient state by observing quantum interference of two distinct excitations, differing by just one quantum number.
The current prototype quantum memory from the University of Warsaw team requires two optical tables, and uses nine lasers and three control computers.
New quantum memory device small enough to fit on a chip in 2017
A team of researchers from the U.S. and Italy has built a quantum memory device that is approximately 1000 times smaller than similar devices—small enough to install on a chip. In research described in the journal Science, researchers from Caltech, the National Institute of Standards and Technology (NIST), and the University of Verona, Italy have collaborated on the development of a nano-sized cavity containing neodymium.
The device is very small, approximately 10 by 0.7 micrometers and has an odd shape, long and thin with a notched triangular shape, with mirrors on either end. It is made of yttrium orthovanadate with small amounts of neodymium, which form a cavity. That cavity in turn creates a crystal cavity that enhances the interaction between light and the cavity’s neodymium at the single photon level. These cavities traps single photons encoding data information (zero, one or both).
To operate the device, the researchers fired laser pulses at it, causing photons to assemble in the comb, which forced them to be absorbed—the configuration also caused the photons to emerge from the comb after 75 nanoseconds. During the time period when the photons were absorbed, the researchers fired dual laser pulses at the comb to delay the reemergence of the photons for 10 nanoseconds, which allowed for on-demand retrieval of data. During the time period when the photons were held, they existed as dual pulses—early and late.
To show that the device was actually storing data information, the team compared the wavefunction of the photons both before and after storage and found them to be virtually unchanged, meaning they still held their zero, one or both state—it had not been destroyed, which meant the device was truly a quantum memory device.
The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the network nodes
The properties of quantum memories ensure greater fidelity of information transmission compared to traditional memories, with a clear advantage in terms of efficiency. Optical quantum memories are essential elements in quantum networks for long distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of its readout time.
Quantum memories for a quantum internet network
A team of physicists led by the professors Philipp Treutlein and Richard Warburton from the University of Basel has now developed a particularly simple and fast quantum memory that stores photons in a gas of rubidium atoms. A laser controls the storage and retrieval processes. The technology used does not require cooling devices or complicated vacuum equipment and can be implemented in a highly compact setup. The researchers were also able to verify that the memory has a very low noise level and is suitable for single photons.
“The combination of a simple setup, high bandwidth and low noise level is very promising for future application in quantum networks,” says Janik Wolters, first author of the study. The development of such quantum networks is one of the goals of the National Center of Competence in Quantum Science and Technology (NCCR QSIT) and of the EU Framework Programme for Research and Innovation that have funded this study.
In the future, quantum networks could lead to unconditionally secure communication, the networking of different quantum computers and the simulation of complex physical, chemical and biological systems.
Researchers create quantum memory that’s stable for six hours
Most of the work on quantum storage in nuclear spins involves the use of doped materials. These have a regular crystal structure, but a small number of atoms within the structure are replaced by a foreign atom. In this case, the main crystal is a yttrium silicate; the atom it’s doped with is an isotope of europium. The spin of the europium nucleus can be manipulated directly using photons, which simplifies the setup. And preliminary work had indicated that its nuclear spin is extremely stable—some experiments have observed lifetimes of up to 23 days.
The primary thing that causes the spin states to flip is the influence of nearby yttrium atoms, which may change orientations of their spin and thereby change the local environment. But the europium’s own electrons also have spins, and these change much more rapidly than any of the nuclear ones. The authors found that by applying a strong magnetic field (on the order of a Tesla), they could create what they called a “frozen core” in which all the spins of the europium atom’s electrons stayed put. This stabilizes the magnetic environment around the nucleus, stretching out the lifetime of the spin.
This process reduces the chance of that europium’s spin will flip down to 9 x 10-5 each second. When the researchers stored a quantum state in the spin, it stayed there for about six hours (give or take an hour). This development could potentially serve as the basis for a quantum relay—the europium atoms receive the state of a photon and then emit a new photon that’s sent further down the communication chain, limiting the loss of photons that naturally occur over longer fiber-optic connections. But the authors are positively giddy, suggesting that it might make sense just to throw the material into the back of a truck and drive it to the destination.
At a very moderate speed of 100 km/hour, the authors calculate that by the time you got the crystals 600 km, 13 percent of them would still hold the intended information. Sending photons the same distance would result in the loss of all but one in 10-12 due to things like scattering.
What doesn’t show up in their calculations is that everything operates in a very intense magnetic field at a temperature of two Kelvin. So, it’s not quite as simple as throwing a crystal in the glove compartment and hitting the gas; chances are good that photons will still do the long-distance transmission. But it’s certainly possible that this material, or an improved variant of it, will be on the receiving end of some of those photons when we develop the first quantum networks.
Memory devices on satellites to enable the quantum internet reported in Sep 2021
The installation of memory and ‘repeater’ devices in space, to enable use of the quantum internet, have been proposed in research by the University of Strathclyde and an international collaboration. The study suggests that quantum memories (QM), which store information in quantum form, and repeaters, which are used in the transmission of the information, can be deployed to facilitate use of advanced internet technology. This is done through distribution of quantum entanglement, a phenomenon in which two particles are interlinked, potentially at vast distances from each other.
The research showed that satellites equipped with QMs provided entanglement distribution rates which were three orders of magnitude faster than those from fibre-based repeaters or space systems without QMs. The study has been published in the journal npj Quantum Information. It was led by Humboldt University in Berlin and also involved the Institute of Optical Sensor Systems of the German Aerospace Center (DLR) and JPL (Jet Propulsion Laboratory NASA).
Dr Daniel Oi, Senior Lecturer in Strathclyde’s Department of Physics, a partner in the research, said: “We show in this paper that this method would have much higher performance than previously proposed schemes and we identify promising physical systems with which to implement it.
The proposal in the research uses satellites equipped with QMs in low-earth orbit. It is focused on the use of quantum key distribution (QKD) for encryption and distribution, and of QMs to synchronise detection events which could otherwise have been happening by chance. The researchers describe their study as “a roadmap to realise unconditionally secure quantum communications over global distances with near-term technologies.” The paper states: “With the majority of optical links now in space, a major strength of our scheme is its increased robustness against atmospheric losses. We further demonstrate that QMs can enhance secret key rates in general line-of-sight QKD protocols.”
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