Quantum Computing  

 

 

 

Hardware Structure of Quantum Computer

Quantum computers aren't just the stuff of theory any more - they're real, and they're spectacular. In these notes, we're going on a journey to demystify the sophisticated hardware that powers these incredible machines.

Quamtum computers are real and we have real working hardwares from various companies. In this note, I will try to go through overall pictures of Quamtum computer hardware which are commonly available now.

Before You Read

Recently I found a good introduction video for Quantum Computer Hardware from IBM. I think it would be helpful for you to watch that video first before you start reading the note here. Check this out !!!!

How does it looks like ?

How would a quantum computer look like ? Obviously it would not look like a personal PC that you would have. Just taking exterior view, it would look as below. Not so fun !! A huge cylinder with a buch of racks and cables around the cylinder.

 

Image : re-edited from various sources listed in Reference and YouTube section.

(A),(D) : Cylinders(Cryostat) where Quantum processor and cooling system is placed

(B),(C) : Support systems for what's inside the cylinder, like power suppliers, cooling gas, control box etc

 

In more modern design, the system went through a drastic metamorphosis and become some fancy IT structure like this rather than factory like structures shown above.

Image Source : IBM Debuts Next-Generation Quantum Processor & IBM Quantum System Two, Extends Roadmap to Advance Era of Quantum Utility

What's inside the cylinder ?

If you take look inside of the cylinder, you would find more fun stuffs as shown below. These are what you usually get if would google 'Quantum Computer' in image. The complexity and exact details would vary depending on who build the system and when they were built but overall structure would look similar. First of all a lot of cables (some of them are flexible and some of them are rigid). Multiple layers separated by disk-like separator. In these image, the most important part is missing. It is quantum chip (Quantum Processor) that is missing in this image. Actually all of these complex, sphagetti like (or chandelier like) structure is a supporting system that is required for the operation of a quantum processor.

Image : re-edited from various sources listed in Reference and YouTube section.

Where is the Quantum Chip ?

If I google things about Quantumu computer, the most of image that I get is something like sphagetti(or chandelier - like structures shown in previous section. Where is the quantum processor located in this structure ? Usually the quantum processor (core part of the quamtum computer) is at the bottom of the structure as illustrated below. Also in most cases, the back of chipset has a lot of RF connectors because the most of the qbit operation is done by injecting a specific RF frequency and the read-out of the qbit status is also by analyzing the signal at the specific frequency.

Here you would get some vague idea of how big a quamtum chip is. All of these chandelier like structure, sphagetti like cables and cylnderical refregerator are just to make the small chip to work.

 

Image : re-edited from various sources listed in Reference and YouTube section.

How cold should it be around Quantum Chip ?

Most of the readers would know that extremely low temperature is required for a quantum processor to work regardless of the type of the processor. In other word, it should be extremely cold in the quamtum computer to work. How cold it should be ?

  • Most require cryogenic cooling to millikelvin ranges, very close to absolute zero. For reference, absolute zero is -273.15 C (-459.67 F ).
  • Superconducting quantum computers - ones that use superconducting circuits - typically need to operate below 15 mK, or -273.14 C. This allows the circuits to enter a superconducting state.
  • Semiconductor or silicon-based quantum computers can work at slightly higher temperatures near 1 K, or -272.15 C. But they still require significant cooling.
  • Trapped ion quantum computers need to be cooled to the range of 10-100 mK using cryogenic systems and vacuum chambers.

As mentioned above, there are slight differences between the type of quantum processor in terms of operating temperature but regardless of those types we can say that the required operating temperature should be very close to Kelvin temperature 0.

 

NOTE : Why the quantum chip require such a low temperature ?

There are many reasons for this. Some of the most common reasons are :

  • Enable superconductivity - Many qubits like superconducting circuits need to become superconductors to exhibit desired quantum behaviors. Superconductivity only occurs at extremely cold temperatures, typically below 15 millikelvin.
  • Preserve Energy Gap: Quantum states in a qubit are separated by an energy gap. At higher temperatures, thermal energy can be enough to cause transitions between these states without any external control, leading to errors. Low temperatures ensure that thermal energy is not sufficient to cause such unintended transitions.
  • Increase coherence times - The lower the temperature, the longer qubits can remain in their intended quantum state before decohering or collapsing. Longer coherence times allow more complex quantum circuit operations within the available time. Cooling increases usable coherence lengths.
  • Reduce thermal noise and vibrations - Qubits are delicate and microscopic. Thermal energy and vibrations can disturb their quantum states leading to errors. Cooling to near absolute zero suppresses thermal noise to allow more accurate operations.
  • Improve Accuracy and Precision - Quantum computations rely on the precise control of qubit states. Any fluctuation in temperature can lead to inaccuracies. Thus, maintaining a stable, ultra-cold environment helps in achieving the precision required for quantum algorithms to run correctly.
  • Allow ultrahigh vacuums - Background gas particles can interfere with qubits. Cryogenic cooling enables maintaining vacuums under 10-10 torr needed to practically eliminate interference for reliable computation.
  • Simplify control electronics - Operating at ultra-cold temperatures allows for easier isolation and control of the quantum processors and minimizes unstable behaviors of the electronics. This improves precision in manipulating the qubits.

Why is It segmented ?

If you google the image of the quantum computers, you would notice that most of the structures within the cylinder (cryostat) is segmented (compartmentarized) as shown below. WHY ?

Image : re-edited from various sources listed in Reference and YouTube section.

It is due to the multi-tiered cooling stages and the functional compartmentalization within the system. Each segment represents a different stage or component of the cooling process as well as different parts of the quantum computing hardware. Here's what each segment typically represents:

  • Topmost Segment (closest to room temperature): This segment often contains the warmest components and may house electronics that interface with the quantum processor but do not require the lowest temperatures. These could include microwave electronics used to send signals to the qubits.
  • Intermediate Segments: As you move down the cryostat, each segment represents a progressively colder stage of the cooling process. These intermediate stages might be cooled to temperatures as low as a few kelvins and are designed to progressively remove heat from the system.
  • Lower Segments (close to the quantum chip): These segments contain the coldest parts of the cryostat, which are essential for the operation of the quantum chip itself. The lowest stage typically is the millikelvin stage, where the quantum processor operates. In superconducting quantum computers, this is where the superconducting qubits are located.
  • Bottom Segment: This segment often houses the mixing chamber of the dilution refrigerator, which is the part of the system that reaches the lowest temperatures. It's here that the quantum chip is situated

Another reason for this kind of segmentation is because the segmentation also helps with the organization and management of the various cables and control lines running in and out of the cryostat. Each layer can be optimized for its specific function, whether that's cooling, magnetic shielding, vibration isolation, or signal processing. The complex web of wiring you see typically carries control and readout signals to and from the qubits, and these signals need to be carefully managed to prevent thermal noise from interfering with the quantum processor.

How to make it cold ?

As mentioned above, it is required to make a extremly cold environement around Qunatum chip to make it work. It is not just an ordinary cold... it should be super cold which is even colder than outer space (well below than 1 degree kelvin). Then the question is how we can push down the temperature to such a low level.

As the current technology, it is not possible (or not practical) to push the room temperature down to such a low temperature. So the process of cooling is implemented in multiple stages and multiple different technologies. This is related to the segmentation / compartmentalization of the quantum computer as shown below. Different cooling mechanism is performed at each compartment.

Follwings are general description of what would happen at each sections. (NOTE : this is just general description. The details would vary a little depending on different hardware implementation).

  • A1/B1 (Warm Stage): This is the highest and warmest stage, which is still at a relatively cool temperature but significantly above the operating temperature of the quantum bits (qubits). Here, initial cooling may begin using standard refrigeration techniques. Electronics for initial signal processing may also be housed here.
  • A2/B2 (Intermediate Cooling Stage): The second stage may utilize liquid helium to cool the environment further. At this stage, the system is cooled to just a few degrees above absolute zero.
  • A3/B3 (Lower Cooling Stage): Here, a dilution refrigerator might be employed, which works by mixing two isotopes of helium (Helium-3 and Helium-4) to create a cooling effect through the dilution process. The cold mixture is circulated and helps to draw heat away from the system.
  • A4/B4 (Cold Stage): This is the coldest part of the setup, directly surrounding the qubits. At this stage, temperatures close to absolute zero are achieved. All heat must be carefully managed to prevent decoherence of the qubits. The cables you see would be highly specialized to operate at these temperatures and minimize thermal noise.complicated mess!

How is it connected to external world ?

Quantum computers are like a wild jungle of wires and cables! Imagine a maze of different cables, each with its own special job, all tangled up like spaghetti. There are thick ones, thin ones, some for carrying tiny lightning-fast messages, and others to keep everything running cool as a cucumber. It's a crazy mix where old-school wires meet super-cool space-age tech. It's not just a few cables either – we're talking loads of them, crisscrossing in a dance that only a few brainy folks really get. This tangle of cables is what makes quantum computers do their magic, but boy, it looks like a complicated mess !

Some examples of these messy cabling is captured in following images.

Image : re-edited from various sources listed in Reference and YouTube section.

The type and functionalities of the cables connected to quantum computer are not a single type of cables. There are many different types of cables with single functionality. There are many different types of cables performing different functionalities are connected like multi lane and multi layer highways as listed below.

Function

Type of Cable

Transmitting Radio Frequency (RF) Signals

Coaxial Cables

High-Speed Data Transmission

Fiber Optic Cables

Operation at Extremely Low Temperatures

Cryogenic Cables

Power Supply

DC Cables

Carrying Analog and Digital Signals

Signal Cables

Thermal Management

Thermal Cables

Interface with Control System

Data and Control Cables

Connecting RF Components

RF Interconnects

Transmitting Digital Signals at High Speeds

High-Speed Digital Cables

Ensuring Stability and Preventing Electrical Interference

Grounding Cables

NOTE :Are they going to put the system to the market with all the mess ?

Of course not. There has been a lot of effort not only in terms of evolving the quantum chipset itselft but also in terms of simplifying all those wirings to make it fit into such a nice and tidy looking containers as shown below. In the pictures below, you don't see any of messy wirings outside of the structure and the entire system just looks like a modern arts.

Image : re-edited from various sources listed in Reference and YouTube section.

Will we have a Desktop Quantum Computer ?

I think I have heard this question frequently a few years ago, but rarely hear the same questions recently (as of Dec 2023). As more people (ordinary persons) get deeper understanding on how quantum computer works, it seems that most people has a certain level of understandings on why miniaturizing the quantum computer would not seem possible. But it would be benificial to understand why the miniaturization of quantum computer is difficult (almost impossible). Followings are some of the major obstacles we need to overcome to make the quantum computer small.

  • Extreme Cooling Requirements: Quantum processors often require cryogenic temperatures close to absolute zero to operate. Developing smaller scale cryogenic systems is a significant engineering challenge.
  • Control and Readout Electronics: Operating a quantum computer requires sophisticated control systems to manipulate qubits precisely. Each qubit requires individual control and readout, which means complex and bulky electronics.
  • Shielding from Electromagnetic Interference: Quantum computers require shielding from all forms of electromagnetic interference, which can disrupt quantum operations. Effective shielding often requires bulky materials and construction.
  • Quantum Decoherence: Qubits are extremely sensitive to their environment. Any external noise or unintended observation can cause them to lose their quantum state. Isolating qubits to prevent decoherence while reducing the size of the quantum computer is a major hurdle.
  • Error Correction: Quantum error correction is essential for practical quantum computing. Current error correction techniques require many physical qubits to create a single, reliable logical qubit. This redundancy increases the size and complexity of quantum processors.
  • Interconnects and Wiring: The wiring necessary for control and readout of qubits is both delicate and voluminous. Managing these connections in a smaller space without introducing heat or cross-talk between the lines is difficult.
  • Material and Fabrication Limitations: The materials used for creating qubits, such as superconducting circuits or trapped ions, have specific physical and chemical requirements that currently limit how small and closely they can be packed together.
  • Power Consumption: While individual qubits consume very little power, the infrastructure needed to maintain and read qubits (like cooling and shielding) can be power-intensive. Miniaturizing this infrastructure while keeping power consumption manageable is a challenge.
  • Thermal Management: Removing heat from the control electronics without affecting the qubits' low-temperature environment is a challenge, especially as components are packed closer together in a miniaturized system.
  • Quantum Interconnects: For quantum information to be useful, it must be transported between qubits and systems. Developing quantum interconnects that can operate at a small scale is a complex problem.

But who knows. If scientists and engineers find breakthroughs which seems impossible as of now, we may have much smaller quantum computer just as we experienced in the history of electric computer as we use today (from huge room sized computer to a palm sized computer/mobile phone). What kind of breakthrough we need to find to achieve this challenging goal. Some of those break through can be listed as below:

  • High-Temperature Superconductors: Discovering or engineering materials that can act as superconductors at higher temperatures could allow for simpler and smaller cooling systems.
  • Room-Temperature Qubits: Creating qubits that can operate at room temperature would eliminate the need for bulky dilution refrigerators, which currently take up most of the space in quantum computing setups.
  • Advancements in Qubit Design: Developing new types of qubits that are inherently more resistant to decoherence could reduce the need for complex error correction protocols and allow for a more compact design.
  • Quantum Error Correction: More efficient error correction techniques could reduce the number of physical qubits required to create a single fault-tolerant logical qubit, thus shrinking the overall size of the quantum processor.

NOTE : I think the most critical breakthrough would be development (or finding) of high-temprature superconductors that can operate at room temperature. In this case, we can get rid of such a huge and complicated cooling system.

Reference

 

 

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