1. Development status of superconducting quantum computers
At the annual meeting of the American Physical Society on March 5, 2018, Google demonstrated Bristlecone, a 72-qubit superconducting quantum chip it is testing . Google physicist Julian Kelly said that the research team hopes to use larger quantum chips for the first time to demonstrate supremacy and complete calculations that cannot be performed by traditional computers. The chip is named Bristlecone because its qubits are arranged to resemble the scales of a pine cone.
On January 10, 2019, at the CES exhibition, IBM announced the launch of the world's first commercial integrated quantum computing system: IBM Q System One. The quantum computer has the computing power of 20 qubits . Q System One is wrapped in a heat-resistant glass box two or three meters high, and its core part is hung on the top of the box, like a gorgeous chandelier.
On October 23, 2019, Google completed the development of Sycamore, a 53-qubit superconducting quantum computer. Google claims that the computing speed of Sycamore is 1.5 trillion times that of traditional computers. For example, for an extremely complex calculation, traditional computers need to calculate 10,000 years to get the result, but Sycamore only needs 200 seconds to complete. John Martinis, the chief physicist of the research and development team, said that the speed of Sycamore has far exceeded that of any traditional supercomputer in the world.
On March 24, 2020, at the 2022 APS Annual Meeting, the global physics event, the Quantum Laboratory of Alibaba Dharma Institute announced a series of latest developments. Reaching 99.72%, the quantum laboratory of Bodhidharma Academy successfully designed and manufactured a two-bit quantum chip , achieving a single-bit control accuracy of 99.97%, and a two-bit iSWAP gate control accuracy of up to 99.72%.
On September 12, 2020, Origin Quantum released theThe origin of the 6-bit superconducting quantum processor Kuafu KF C6-130 chip , which is China's first domestically engineered superconducting quantum computer. As a superconducting quantum computer that can operate stably out of the laboratory environment, it can play a role in biomedicine, chemical industry, big data, aerospace, code-breaking and other fields.
On May 7, 2021, a research team composed of Pan Jianwei, Zhu Xiaobo, and Peng Chengzhi from the Institute of Quantum Information and Quantum Science and Technology Innovation of the Chinese Academy of Sciences independently developed a 62-bit superconducting qubit chip Zuchongzhi (Zu Chongzhi), and completed the superconducting qubit chip on this basis. Quantum computing prototype "Zuchongzhi".
2. Classification of Quantum Computer Technology
The current mainstream quantum computer technology routes include: superconducting circuits, ion traps, semiconductor quantum dots, photons, etc., each technology route has its own advantages and disadvantages. These routes are still in the stage of basic theoretical research and prototype product development and verification, and many key technologies have not yet been broken through, which still requires the joint efforts of scientists all over the world!
Superconducting quantum technology uses a refrigerator to cool a circuit using a Josephson element to an extremely low temperature so that it enters a superconducting state, thereby realizing a qubit, and performing quantum gate operations by means of microwave pulses, thereby realizing a qubit. Superconducting quantum technology is considered to be one of the most likely solutions to realize practical quantum computing first . Google, IBM, Intel, Original Quantum (China), Beijing Quantum Institute (China), etc. have all laid out superconducting quantum technology routes.
The ion trap technology uses the interaction force between the charge and the electromagnetic field to restrain the motion of the charged particle body, and uses the two energy levels composed of the ground state and the excited state of the confined ion as a qubit. equipment within.
Semiconductor quantum dot technology is a mechanism that eliminates the influence of other electrons on an electron by isolating it from the outside in three-dimensional space. It is an important low-dimensional semiconductor material, and its size in three dimensions is not larger than twice the exciton Bohr radius of its corresponding semiconductor material. Similar to superconducting circuits that require extremely low temperatures to work, isolated electrons must be cooled to extremely low temperatures to achieve stable qubits.
Optical quantum technology is a method of using photons as qubits, emitting a single photon through a single photon source, using the vibration direction (polarization) of light as a qubit, and performing quantum operations by inputting it into an optical quantum circuit to achieve quantum calculate.
3. Composition of superconducting quantum computer
A superconducting quantum computer usually consists of four parts: a quantum chip system, a quantum computer measurement and control system, a quantum computing environment support system, and a quantum computer operating system . The schematic diagram of a superconducting quantum computer is as follows:
The quantum chip system is a system based on a superconducting quantum chip , which is the computing core of a quantum computer, and it performs quantum calculations.
The quantum computing measurement and control system is the control system of the quantum computer, which is used to control the operation of the quantum chip.
The quantum computing environment support system includes an ultra-low temperature refrigeration system and an active vibration reduction system for the quantum computer host, which provides a working environment guarantee for the stable operation of the quantum computer.
The quantum computer operating system is the fundamental framework for quantum computers to provide quantum computing software systems. It has functions such as multi-quantum computing, automatic calibration of quantum chips, and efficient management of quantum resources.
The block diagram of the control system of the superconducting quantum computer is as follows:
4. Superconducting quantum chips
A superconducting quantum chip is the core of a superconducting quantum computer . The chip will integrate multiple superconducting qubits, encode quantum information on the qubits, and perform quantum logic gate operations through microwave control, thereby realizing quantum computing.
The key circuit components of a superconducting quantum chip include: qubits, readout chambers, control lines, and readout lines . The figure below is a diagram of the internal circuit of a superconducting quantum chip with 5 qubits .
1. The five cross-shaped devices marked Q0−Q4 in the figure are transmon qubits.
2. The curved device marked resonator in the figure is the readout cavity, which is used to read out the state of a single qubit .
3. The devices marked XY and Z in the figure are control lines. The xy control line transmits microwave signals to achieve energy level reversal and other operations. The z control line and the
current change of the Z control line will change the applied magnetic field. At the same time The magnetic flux of the SQUID will also change.
4. readout is a read line for reading qubits.
The schematic diagram of the superconducting quantum chip with 5 qubits in the above picture is as follows:
4.1 Qubits
The qubit of the Josephson junction superconducting quantum chip is realized through the Josephson junction circuit. The Josephson junction is a structure (SC2) composed of a superconductor (SC1)-insulator (insulator)-superconductor. The thickness of the insulating layer is usually in nanoscale. No current will be generated after a voltage is applied across the Josephson junction, because this structure is an open circuit. When the insulating dielectric thin layer (insulator) is thin enough, electrons can tunnel between SC1 and SC2, flowing from one end to the other to generate current. The Josephson knot is shown below.
a is the superconducting Josephson junction, b is the symbol of the Josephson junction in the circuit, and c is the circuit model of the Josephson junction
A widely used technique is to replace a single Josephson junction with a loop interrupted by two identical "junctions", forming a direct current superconducting quantum interference device (DC-SQUID). Due to the interference between the two ends of the SQUID, the effective critical current of the two junctions can be reduced by applying a magnetic flux penetrating the loop, and this effect can be used to adjust the energy of the Severson junction by changing the external magnetic flux.
superconducting qubit
Superconducting qubits are mainly divided into three categories according to different degrees of freedom: charge qubits, flux qubits, and phase qubits . These three kinds of superconducting qubits are all plagued by different noises, resulting in very short decoherence times. The noise sources mainly include charge fluctuations, magnetic flux fluctuations, and quasi-particle noise. The circuit diagrams of the three superconducting qubits are as follows.
a is the charge qubit, b is the flux qubit, and c is the phase qubit
On the basis of the above three types of superconducting qubits, many new superconducting qubits are derived: such as transmon qubits, C-shunt flux qubits, fluxonium qubits, etc. The new superconducting qubit circuit is shown in the figure below.
1 is a symmetric transmon qubit, 2 is an asymmetric transmon qubit, 3 is a C-shunt flux qubit, and 4 is a C-shunt Fluxonium qubit
Symmetrical transmon and asymmetrical transmon will not change the circuit topology in engineering. In the entire tunable frequency range, the flux sensitivity is suppressed and high coherence is maintained. At present, almost all superconducting quantum computers use Transmon technology .
Transmon qubit
The full name of the Tansmon qubit is "plasma oscillation qubit with transmission line bypass", which essentially adds a large bypass capacitance to the charge qubit . This capacitor greatly smoothes the charge dispersion relationship, making the energy of the circuit system very insensitive to the fluctuation of the charge, and naturally suppressing the charge noise. Tansmon qubits include Transmon, Xmon, Gmon, 3D Transmon, etc.
Another major innovation of Transmon is to replace the gate capacitance originally used to control the qubit with a dispersion coupling with a transmission line resonant cavity. This design provides a very useful measurement method for qubits - "quantum non-destructive measurement". The schematic diagram of Transmon qubit and its circuit is as follows.
a is the effective circuit model of the Transmon qubit. CB is a large capacitor connected in parallel with the superconducting quantum interference device (SQUID), Lr and Cr are connected in parallel to form the equivalent circuit of the readout resonator, and the rightmost circuit is the SQUID biased by the magnetic flux of the SQUID.
b is a schematic diagram of the two-dimensional structure of the Transmon qubit
4.2 Readout cavity
The readout cavity is one of the key circuit components of the superconducting quantum chip, and the readout cavity is used to measure the state of the qubit (0 or 1). The reading cavity is coupled with the qubit, and there will be different results in different states of the qubit. For example, after the qubit is connected, the spectral line will undergo a dispersion frequency shift, which can be used to distinguish the state of the qubit. In the process of superconducting quantum computing , the reading of the state of the qubit is indirect, and the qubit will be coupled to the reading cavity to avoid destroying the state of the qubit during measurement .
The reading chamber can be understood as an LC circuit oscillator. In the chip layout, the reading chamber has a snake-like structure, which is actually a coplanar waveguide with a specific resonance frequency.
4.3 Control line
In the superconducting quantum computing chip , the state of the qubit is changed through the control line . Each qubit realizes the control of the qubit through two control lines . The control line is divided into an XY control line and a Z control line . The XY control line is to control the bit state by inputting microwave signals, and the Z control line is to change the frequency of the qubit by generating magnetic flux through the input current.
4.4 Reading lines
The function of the readout line is to output the state information of the qubit , and the readout line is coupled with the readout cavity and then connected to the pin of the chip.
5. Current status of superconducting quantum computers in China
On November 9, 2022, IBM launched the "Osprey" (Osprey) chip. "Osprey" is a superconducting quantum chip with the most qubits in the world so far, with 433 qubits integrated inside it!
Representative superconducting quantum chips developed in China include: Hefei Yuanquan 24-bit superconducting qubit chip KF-C24-100; Zhejiang University independently developed a 26-bit qubit "Tianmu No. 1" superconducting quantum chip; Independently developed 62-bit superconducting qubit chip Zuchongzhi (Zu Chongzhi) with Pan Jianwei's team.
In terms of superconducting quantum chip technology, the United States is the world's leading technology. There is still a certain gap between China's superconducting quantum chip and the United States in the number of qubits and quantum programmability related technologies. Due to the late start of computer and semiconductor technology in China, the overall foundation is not as solid as that of the United States. Chinese people need to give time and patience to relevant technology R&D personnel, and relevant scientists and R&D personnel need to forge ahead to win glory for the country!
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