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Author: Luan Chunyang (Ph.D., Department of Physics, Tsinghua University)
Producer: China Science Expo
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Recently, there has been another big news in the field of trapped-ion quantum computing! A paper published in the top international journal Nature was praised by reviewers as a "great progress" and "noteworthy milestone" in the field of quantum analog computing.
The world's largest quantum simulation calculation based on ion two-dimensional arrays
(Image source: Reference [1])
This is the research team of Academician Duan Luming from the Institute for Interdisciplinary Information Sciences of Tsinghua University, who achieved stable trapping and sideband cooling of up to 512 ion 2D arrays for the first time in the world, and at the same time used 300 ion qubits to realize the quantum simulation calculation of the tunable coupling long-range cross-field Ising model.
This research work broke through the highest number of bits (61 ion qubits) in the original multi-ion quantum simulation calculation, and set a record for the world's largest quantum simulation calculation based on ion two-dimensional arrays.
So, what exactly is a trapped-ion quantum computing system? Why has this research received so much attention?
Ion traps – "magic traps" that trap ions
For the concept of "ion trap", I believe that many people have heard it for the first time, and in the literal sense, it can be vividly understood as "ion + trap". In fact, these are the two core elements of it. In fact, you can simply understand it as "a magical trap capable of capturing and imprisoning ions", which is the core function of trapped-ion quantum systems.
So why do we trap ions? What does ion trapping have to do with the quantum computing we often hear about?
First of all, ions are actually charged atoms, and there is a naturally stable energy level structure inside them. We can pick two specific energy levels from the inside of an ion and encode them into a stable two-level system (i.e., qubits).
Second, for a two-level system in a single trapped ion, we can encode the higher energy state as /1⟩ state and the lower energy state as /0⟩ state. At the same time, since the transitions between the internal energy levels of ions follow the principle of probability in quantum mechanics, the energy states of individual ions can be superimposed in the /1⟩ state and the /0⟩ state, thus participating in the parallel operation of quantum computers as ion qubits.
Detection of ion qubits and internal energy state manipulation
(Image source: Ref. [2])
In other words, if we can stably trap N ions in an ion trap system, we can theoretically encode into N independent ion qubits. Subsequently, driven by a specific laser light field and a microwave field, these ion qubits can perform parallel quantum operations to the Nth power of 2, thus demonstrating super parallel computing capabilities.
When it comes to quantum computing systems, the first thing that may come to your mind is the "superconducting quantum computing system", which was particularly popular some time ago. In fact, as early as 1995, two physicists, Ignatio Cirac and Peter Zoller, proposed for the first time that stable trapped ions could be used to realize the operation of quantum logic gates, so as to build real quantum computing systems.
Nearly 30 years have passed, trapped-ion quantum systems have continued to develop and mature, and have become one of the mainstream quantum computing systems.
In 1995, trapped-ion quantum computing was first proposed
(Image source: Ref. [3])
The dilemma of trapped-ion quantum computing systems – the finite number of ions
Since trapped-ion quantum computing systems started so early and their performance is very good, why is trapped-ion quantum computing not as hot as superconducting quantum computing before this?
This is because trapped-ion quantum computing schemes have always faced a big problem that is difficult to solve, that is, the number of ions that can be stably trapped in a single trap system is too small.
The mainstream Paul type (Paul) ion trap, in which the ions will be subjected to two forces at the same time and be in a state of dynamic equilibrium, one of which is the "binding electric field force", which is to bind the movement of ions by artificially applying a composite electric field of "radio frequency + DC"; The other is the "Coulomb force", which is a mutually exclusive interaction force that exists inside the charged ions and follows the basic Coulomb's law.
Schematic diagram of ions bound by the composite electric field of "RF + DC", the axial yellow arrow represents the DC electric field, and the alternating green arrow represents the alternating current electric field
(Image source: Drawn by the author)
Under the combined action of the external confined electric field force and the internal Coulomb force, multiple ions will spontaneously arrange themselves into one-dimensional ion chains with a spacing of several microns along the axis of the ion trap, which is also known as the "ion one-dimensional configuration".
In the experiment, in order to precisely manipulate the quantum states of the individual ions, it is also necessary to perform individual laser addressing operations for each ion using a focused laser beam. Previously, quantum physicists had used this method to achieve high-precision qubit manipulation in one-dimensional configurations of 53 ions and 61 ions, respectively.
Ion qubits trapped in an ion trap system (25 one-dimensional ion chains of 171Yb+ ions)
(Image source: The physical picture of the ion trap platform built by the author)
However, the number of ions that can be accommodated in this "one-dimensional ion configuration" is extremely limited, and it cannot meet the large-scale needs of trapped-ion quantum computing.
If we just keep more ions in the state of a one-dimensional ion chain, we need to rebalance the external bound electric field force with the internal Coulomb force, so that the intensity of the bound electric field force along the axis of the ion trap has to be reduced. However, this in turn leads to the axial movement of the ion chain, which makes it more susceptible to external electromagnetic noise, which ultimately limits the number of ions that can be stably trapped.
In general, a single ion trap at room temperature can stably trap only a one-dimensional configuration of a few dozen ions. Even if the vacuum of the ion trap is increased and the ambient temperature of the system is further reduced, the one-dimensional ion chain of 100-200 ions can only be stably imprisoned at most. In other words, the number of ions that can be contained in a one-dimensional ion chain is far from the qubit scale required for future general-purpose quantum computers.
So, how can more ions be stably trapped in the same ion trap system?
The key to stabilizing the trapping of more ions is to expand into a two-dimensional structure and then cryostore
In fact, it is theoretically not difficult to further expand the number of ion bits in a single ion trap. This is because the scalability of the ion trap can be greatly improved by simply upgrading the original "one-dimensional configuration" to a "two-dimensional array".
However, it is a challenging task to achieve stable trapping of large-scale two-dimensional ion arrays experimentally. The key to the stable captivation of up to 512 ions by Academician Duan Luming's research team is the "two-dimensional ion array + cryogenic cold trap technology".
First of all, in order to realize the two-dimensional ion array, it is necessary to re-set the appropriate "bound electric field force" strength of the ion trap system, so as to extrude the original one-dimensional ion chain into the ion configuration in the two-dimensional space. To this end, the research team designed and optimized a special electrode structure, and used an integrated processing scheme to fabricate an ion trap system that can stably trap two-dimensional ion arrays.
The special electrode structure prepared by the integrated processing scheme, the red dot matrix is the schematic diagram of the two-dimensional ion array
(Image source: Reference [1])
At the same time, in order to reduce the mutual interference between ions during laser addressing operations, the research team further increased the distance between ions. In this way, the individual ions in the two-dimensional array can be precisely manipulated, which improves the stability of the entire trapped-ion quantum computing system.
Secondly, in order to maintain the long-term stability of the two-dimensional ion array, the cryogenic cold trap technology is also needed to ensure that the two-dimensional ion array is in an ultra-low temperature state (-6.1K). This is because the low temperature environment can effectively reduce the collision probability between the trapped ions and the background gas molecules, and at the same time inhibit the abnormal thermal motion effect of the ions themselves. To achieve this, the research team placed the entire ion trap system in a liquid helium environment, which significantly improved the stability of the two-dimensional ion array.
Stable trapping of up to 512 ions in a 2D array (171Yb+ ions)
(Image source: Reference [1])
It is with the above two unique secrets that the research team of Academician Duan Luming has achieved the stable trapping of up to 512 ions in a two-dimensional array for the first time, and at the same time realized the single-qubit resolved quantum state measurement of 300 ions, setting a record for the world's largest single-qubit resolved multi-ion quantum simulation calculation.
Trapped-ion quantum computing – a powerful "star of the future"
At present, trapped-ion quantum computing systems have set the highest fidelity single-qubit gate (99.9999%), the highest fidelity two-qubit gate (99.94%), and the longest single-qubit coherence time (5500s), respectively, and have been recognized by the international academic community as one of the most promising physical systems for large-scale quantum computing.
Not long ago, on April 16, the trap ion research team from Quantinuum also announced the latest progress - the quantum volume of the trapped-ion quantum computing system (model: H1-1) developed by them has exceeded one million (220=1048576). The larger the quantum volume, the greater the scale and depth of the quantum algorithms that can be executed by the quantum computer.
As a comparison, the quantum volume of MOSS in the movie "The Wandering Earth 2" is only 213, which means that the quantum volume of the trapped-ion quantum computing system is 128 times larger than that of MOSS!
2024 is destined to be an extraordinary year in the field of quantum computing, let us look forward to more shining new advances in trapped-ion quantum computing!
Bibliography:
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