◎Reporter Wu Changfeng
Quantum memory is used to store the entangled state of photons. As a synchronization device for entanglement establishment and entanglement exchange in different links, it is the key to the acceleration of entanglement distribution in quantum repeaters.
my country has used the Micius satellite to achieve long-distance entanglement distribution up to 1,200 kilometers, but has not yet introduced quantum memory.
A few days ago, some media reported that foreign scholars stored the information of a qubit in a crystal and saved the message for up to 20 milliseconds, laying an important foundation for the development of long-distance quantum networks.
Just like traditional electronic computers, the future development of quantum information technology is also inseparable from the storage and reading of information.
So how exactly does this crucial quantum memory store quantum information?
Where is the difficulty in storing quantum information?
more important than classic memory
The function of memory is to store information until it is needed to be read out.
The storage of information is an important means of transmission of human civilization, and it is also a core link of modern information technology.
With the development of human history, the medium of information storage is also changing.
The human brain is the earliest medium for information storage, which enables human beings to survive and evolve continuously.
The transition from language to writing is a turning point in the progress of human civilization. This change enables information to be separated from people and stored and passed on in the form of words.
People have successively used memory in the form of stone carving, rope knotting, books, magnetic disks, and CD-ROMs.
Modern digital information processing is based on binary computers, so classical memory stores bits, that is, one of two classical states: 0 or 1.
The combination of a large number of bits constitutes all kinds of information we need.
Classic memory includes computer, mobile phone memory, hard disk and portable U disk.
From classical information to the era of quantum information, quantum memory is an essential basic device.
In contrast to classical memory, quantum memory can store quantum states.
Classical memory is generally measured in bits, and today's classical memory can reach the order of terabytes.
A memory cell of a classical memory only stores one bit, and the capacity of the memory is actually the number of classical memory cells.
Due to the characteristics of quantum coherence, a storage unit of a quantum memory can store N qubits at a time, that is, N patterns.
Recent studies have shown that solid-state quantum memory can store up to 100 qubits.
This capacity is already far greater than the sum of all classical memories on earth.
However, since quantum information cannot be reproduced and cannot be amplified, the place of quantum memory in quantum information is more important than that of classical memory in classical information.
Up to now, there are many research groups in the world engaged in the research of quantum memory. The more mainstream physical systems are cold atoms, hot atoms and rare earth ion doped crystals.
At present, the independent indicators of quantum memory have relatively good results, but the comprehensive indicators are still far from the requirements of quantum relay.
The key to achieving entanglement in quantum repeaters
Quantum networks are carriers of long-range quantum communication and distributed quantum computing, which can be built based on quantum entanglement.
The remote quantum entangled state can support many quantum information applications including quantum key distribution, quantum computer interconnection, distributed quantum precision measurement and so on.
A single photon is an ideal carrier of quantum entanglement and quantum information. However, the transmission of a single photon in an optical fiber network faces exponential losses. down to one part in 10 billion.
Due to this inevitable channel loss, current fiber-based entanglement distribution distances are limited to the order of hundreds of kilometers.
In classical communication, this problem can be solved by continuously amplifying the classical signal by a relay amplifier.
Unfortunately, conventional relay amplifiers are not suitable for quantum communication due to the limitation of the quantum no-cloning theorem, that an unknown quantum state cannot be reproduced exactly.
Remote quantum entanglement distribution has also become one of the core challenges in the field of quantum information.
A possible solution to this conundrum is quantum relay, the basic idea of which is to divide a large-scale network into multiple segments of a small-scale network.
For example, a 500-kilometer quantum entanglement transmission can be decomposed into five short-range entanglements of 100 kilometers. Under the condition that the short-range entanglement is successfully established in turn, entanglement exchange can be used to establish long-range entanglement.
The problem with this method is that the time of each 100-kilometer entanglement establishment is generally not synchronized. For example, the first section may be established in 0.05 seconds, the second section may be established in 0.02 seconds, and the third section may be established in 0.1 seconds. .
This requires quantum memory to synchronize this process. Once the entanglement of each node is successfully established, it will be stored. When all nodes are successfully established, entanglement exchange between memories will finally establish long-distance entanglement.
Therefore, the core problem to be solved by large-scale quantum networks is the physical realization of high-performance quantum memory.
Specifically, quantum memory is used to store the entangled state of photons. As a synchronization device for entanglement establishment and entanglement exchange in different links, it is the key for quantum repeaters to accelerate entanglement distribution.
The channels used within the basic link include fiber optics as well as free space channels.
The overall structure of optical fiber quantum relay is similar to that of classical optical fiber communication, and it is the most promising technical route to achieve the goal of quantum network.
Quantum relays do not eliminate photon losses, but they can convert the exponential loss of direct transmission through the fiber into a tolerable loss of polynomial magnitude, which will show significant advantages in long-distance communications.
The channel loss in free space is lower than that of optical fibers. my country has used the Micius satellite to achieve long-distance entanglement distribution of up to 1,200 kilometers, but quantum memory has not yet been introduced.
With the rapid development of quantum information technology, future quantum communication satellites can be combined with quantum memory to achieve high-speed quantum communication covering the world.
Chinese scientists perform well
We already know that the challenge in developing long-range quantum communication systems lies in finding a way to repeat the signal without changing it, in particular creating quantum memory-based quantum repeaters.
The quantum repeater includes the entanglement establishment of the basic link and the subsequent entanglement exchange process.
Since the success probability of the entanglement exchange process is determined by the basic principles of quantum optics, and it is generally difficult to improve, in order to achieve high-speed quantum relay communication, the success probability of basic link entanglement establishment becomes crucial.
Two main factors affect the improvement of this success probability. One is the emission probability of the quantum entanglement source, that is, the probability of an entangled photon emission, which actually successfully emits a photon.
The second is the channel transmission loss and the loss of the detection device. After the photon is emitted, the short-range channel transmission and detection process will inevitably introduce loss.
In June 2021, the team of Academician Guo Guangcan of the University of Science and Technology of China (hereinafter referred to as USTC) gave the "USTC" solution.
They realized the basic link of quantum relay based on absorption memory for the first time, and demonstrated the communication acceleration effect of multi-mode quantum relay.
This achievement was featured on the cover of Nature.
The Chinese University of Science and Technology team divides quantum light sources into deterministic quantum light sources and probabilistic quantum light sources.
In principle, the emission probability of the former can reach 1, while the emission probability of the latter is generally controlled below 0.1 in order to avoid multi-photon noise and ensure entanglement fidelity in actual use.
Among the two constraints mentioned above, the first problem can be solved by using deterministic light sources. In order to avoid multi-photon emission events, deterministic light sources are generally implemented based on single quantum systems, including single atoms, quantum dots, and single crystals. Grid defects, etc.
Solving the second problem requires the introduction of multiplexing techniques similar to those used in classical communication, that is, storing multiple photons at once, which requires quantum memory based on atomic systems.
During the entanglement establishment process of the basic link, if N modes are used at the same time, the photons in the N modes can establish entanglement between nodes as long as one mode succeeds, which can greatly improve the success probability of entanglement establishment and improve the final entanglement. The rate of distribution.
Previously, research on quantum memory mainly focused on applications in the field of quantum communication, such as establishing quantum relays based on multi-mode quantum storage to build a long-distance quantum Internet, or realizing removable quantum U-disks based on ultra-long-life quantum storage.
Chinese scientists have made a series of breakthroughs in the field of quantum memory.
In April 2021, the team of Academician Guo Guangcan of the University of Science and Technology of China increased the coherent optical storage time record of 1 minute set by German researchers to 1 hour, setting a new world record, which means that quantum U disks are possible.
In July 2021, the research group of Duan Luming of Tsinghua University realized the phase-preserving storage and reading of single-photon level microwave pulses for the first time by means of the dynamic regulation of the multi-resonator system in the experiment, and used this method to demonstrate the time-division coding qubit. on-demand access.