The Quantum Computing System Operates at 1.5 Kelvin (-271.6°C) For The First Time

Figure 丨 Google's Sycamore quantum chip (Source: Google)

If you follow reports in the field of quantum computers, you may be familiar with the elegant retro steampunk object below.

 Figure 丨 Dilution refrigerator used by IBM (Source: IBM)

Figure 丨 Dilution refrigerator used by Google (Source: Google)

It will be mistaken for the ontology of quantum computers, but it is not. This is a dilution refrigerator designed to "dissipate heat" to the quantum computer. When this machine costing millions of dollars is in operation, the task it needs to complete is to maintain a temperature near absolute zero to ensure the stability of the qubits. In recent years, although quantum computers are being developed around the world, most quantum computers can only work near absolute zero. Once the quantum computer is connected to the traditional electronic circuit, it will immediately overheat, so this requires special refrigeration equipment.

But now, a team led by Professor Andrew Dzurak of the University of New South Wales in Sydney has taken an important step in solving this problem. In a paper published today in Nature, Professor Dzurak's team, together with collaborators in Canada, Finland, and Japan, reported a proof-of-concept quantum processor unit that differs from most designs being explored worldwide, It does not need to work at a temperature of 0.1 Kelvin. Its operating temperature is 1.5 Kelvin. This temperature is 15 times higher than the operating temperature of Google, IBM and other companies using superconducting qubit chips: Google and IBM mainly focus on the research of superconducting qubits, and the quantum computing systems are driven thereby need to operate in the milli-Kelvin range, only A little higher than absolute zero.

In engineering, this control of the state of microscopic particles is the core difficulty. To control the state of the particles, a feasible solution is to approach the ambient temperature to absolute zero, which is equivalent to "freezing" the particles. This is a key step for the quantum computer to run smoothly. This also allows dilution refrigeration, one of the most cutting-edge refrigeration technologies, to be of use in this field, the dilution refrigerator is the main component of the quantum computer based on superconducting, spin and topological qubit technology. Its principle is similar to the heat absorption of liquid evaporation. The isotope mixture based on helium realizes the flow of heat.

Figure 丨 Dilution refrigeration principle (Source: Internet)

For example, Google, which announced the realization of "quantum superiority" last year. The 53 qubits of Google's quantum computer are composed of a superconducting metal microcircuit, which has two different energy settings or states. But the system must be placed in a bulky dilution refrigeration device (the size of a phone booth), operating near absolute zero. Google researchers have stated that they can hold about 1,000 qubits in a dilution refrigerator. With this development, a mature quantum computer with millions of qubits may require tens of thousands of interconnected dilution refrigeration equipment. Using enough qubits to maintain a quantum computer requires ultra-low temperatures. This approach is not only costly but also requires pushing refrigeration technology to the limit, which is daunting. In the long run, this solution is not conducive to the birth of practical quantum computers. "Each additional pair of qubits in the system will increase the total heat, and the increased heat will cause calculation errors. This is also the main reason why the current design needs to stay close to absolute zero.

Figure 丨 Qubits produced by spin electrons in silicon (Source: UNSW)

Generally speaking, the reading through the electronic spin associated with the spin tunneling, which relies on large type Reservoir Electron. However, at higher temperatures, the energy reservoir will become blurred and lose self Spin dependence. In this work,  reading the spin information by tunneling the electrons between two quantum dots, not the reservoir, and the process is very elastic to heat. "First of all, this means that a simpler and cheaper cooling system can be used: a pump plus a vacuum bottle system with liquid helium (4He) can cool down to 1.5 Kelvin. On the other hand, to reduce the temperature to 0.1 Calvin with a dilution refrigerator, the interface and liquid ratio between the two different isotopes of helium (3He and 4He) must be carefully designed, and multi-stage temperature control and multiple pumps are required. 3He is also a very rare and expensive helium isotope. "