Classic storage including computers, memory of cellphones, hard disks, and portable USB flash drive can be seen everywhere.
A storage works to store information and read it out if needed. Information storage serves as an essential way to hand down human civilization and a core part of modern information technology. Media that store information keeps changing due to human development. Language is an original way to communicate, and brain the earliest media to store information, which enables mankind to survive and evolve. Human civilizations develop by transiting from languages to characters, while information can separate itself from people and be saved as well as handed down by means of characters. Mankind used storage including stone carving, knotting, book, disk and CD successively.
The hard disk of computers (image from the network)
Modern digital information is processed based on binary computers, that is why classic storage in bits, namely, 0 or 1, one of the classic states of storage. A large number of bits bring information we need. Classic storage including computers, memory of cellphones, hard disks, and portable USB flash drive can be seen everywhere.
Portable USB flash drive (image from the internet)
A quantum storage is a fundamental component of the essence when classic information evolves into quantum information. Compared with functions of the classic storage, a quantum storage is supposed to store quantum states, that is, arbitrary superposition state of |0⟩ and |1⟩ which are the two basic states. |0⟩ and |1⟩ can overlay, for example, there are various states including |0⟩+|1⟩,|0⟩−|1⟩,|0⟩+i|1⟩, which makes them different from classic 0 and 1.Quantum storage function differently in quantum science:
1, to set up a large-scale network
The quantum network serves as the carrier for long-distance quantum communication and distributed computing. It can be set up based on quantum entanglement. A single photon is the ideal carrier for quantum entanglement and information, while it suffers exponential loss in the transmission of the fiber network. It travels through the 10 thousand-meter optical fiber at the rate of 1%, while at the rate of one billionth when through the 50 thousand- meter one. Here is a classic solution, quantum relaying. Basically, it aims at partitioning the large-scale network into a small one. For example, the 50 thousand-meter quantum entanglement transmission can be divided into five short-distance entanglement of 100 kilometers, and set up long-distance entanglement through exchange when the short-distance one is set up successfully in turn. The question is, every 10 thousand-meter entanglement is not set up simultaneously. For example, the first phase may be set up in 0.05 second, while the second on in 0.02 second, and the third in 0.1 second, etc. Therefore, a quantum storage is needed to synchronize them. The entanglement of every joint will be stored once it is set up, then there will be the exchange among storage and the long-distance entanglement will be set up. Therefore, the large-scale quantum network needs to give priority to making the high-performance quantum storage possible in a physical way.
Quantum network (image from the network)
2, to set up quantum computers
Like classic computers, a common quantum computer also needs a storage to play its complex role in computing. The storage should store corresponding quantum information carrier according to concrete computing chips. Let’s take the quantum computing of linear optics as an example, it is hard to generate a multiphoton directly which is a basic source. A photon can be generated at the rate of P, then N photons at the rate of the Nth power. The entanglement state of dozens of photons is not possible since the current value of P is about 10%. A quantum storage can make this inefficient photon source a certain one whose value of P approaches to 100%. For example, we can try to launch photons for 100 times within the service life of storage, and make the photon source with the rate of 1% a certain one and then get a multiple one.
To make quantum USB Flash drive possible
The service life of storage mentioned above is not longer than seconds. Fixed in a location, they work with photon to function differently. Scientists found that the spinning quantum of the rare earth ion-doped crystal can live to six hours in 2015, which is the highest level among the service life of the quantum system, and expected to live to several days. This result makes a great difference to developing quantum information science.
We can store a single photon in a storage which can be transported to any designated location by means cars, high-speed train and planes within its service life. That is how a quantum USB Flash drive functions. It is a revolutionary communication proposal since it can make quantum entanglement objects accessible to the classic handling. Quantum USB flash drive with entanglement can get to places where communication can get to without the optical fiber network or any classic vehicles. It will be a point-to-point communication mode with a high flexibility and low cost, and is expected to used in various fields including ID authentication, signature, quantum code and communication sharing.
Besides, the capacity of classic storage is usually in bits. Now it can reach TB (the 40th power of 2). The capacity is usually subject to the storage cell since only one bit is stored in a cell. Each cell can store N bits, namely, N modes, because the quantum storage features in coherence. Recent researches showed that the capacity of a solid storage can reach 100 bits, which is much higher than the sum of all storage in the world.
Quantum storage (image from the network)
Generally speaking, a quantum storage plays a more important role in the quantum information than that the classic quantum plays in the classic information because the quantum information can neither be copied not enlarged. There are many groups embarking on relative researches, mainly, the physical system is a cold, a hot atom, and a rare earth ion-doped crystal. Currently, good results are made in every independent indicator of quantum storage, while that in comprehensive indicators fail to meet requirements of the quantum relaying. Low comprehensive indicators are needed by the quantum computing, while this storage is used with breakthroughs made in computing researches.
The quantum USB flash drive mainly challenges to store a single photon in a long-life spinning state efficiently and enable the real system transportation to work under noise better. It is expected to take the lead to be applied with the development of researches mentioned above.
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