Quantum Computing and the Need for Cryogenic RF Components

Quantum computing is a rapidly emerging technology that harnesses the laws of quantum mechanics to solve complex problems. Complex problems present challenges with complicated variable interaction. Quantum computing is an essential tool in situations where multiple interdependent elements are at work. Quantum computing solutions can be applied to a wide array of complex problems like modeling the behavior of atoms in an individual molecule or optimizing route navigation for a fleet of tankers in a global shipping network. Two fundamental goals of the quantum community revolve around solving problems that were previously unsolvable and streamlining traditional problem-solving techniques. Quantum computing can potentially aid in chemical discovery, traffic simulation, banking encryption, physics modeling, weather forecasting, and much more.

Figure 1: Quantum Computer

The Difference Between Quantum and Traditional Computing

A key distinction between traditional and quantum computing are the physical fundamentals of bits and qubits (quantum bits). Individual bits in our computers, phones, or tablets are binary and hold two options. A common analogy that explains binary is flipping a coin. The heads and tails of a coin are like that of the “0” and “1” outcomes of binary bits. Contrarily, quantum qubits have “superpositions” that apply to the holding states between binaries – points where the coin is still flipping through the air, determining “heads” and “tails” at the same time. Qubits provide inherent plurality in their ability to hold “neither” and “both” states. For example, N number of bits can hold N pieces of information, while N number of qubits can hold 2N pieces of information. If a traditional computer holds 8 bits of information, a quantum computer holds 256 qubits of data. With 30 bits of data stored and manipulated in a classic computer, there are over a billion pieces of information stored in a quantum computer.

Different Forms of Qubits

Pursued by large companies and research groups, qubits take multiple physical forms including qubits, trapped ion qubits, superconducting qubits, topological qubits, and more. Companies like Google and Intel use superconducting qubits, whereas IBM uses transmon qubits, and IonQ and Honeywell use trapped ion qubits. Qubit types are selected for reasons surrounding error rates, coherence times, and employee expertise.

Creating Quantum Behavior

Interestingly enough, not all qubits exhibit quantum behavior. In fact, forcing a qubit to hold superposition is difficult. The time in which a qubit holds superposition is called “coherence time,” and coherence times are generally very short. Think of the time a coin spends resting versus the time it spends spinning through the air. Unless influenced to spin, a coin spends most of its time in one resting state. Quantum computing is similar in that qubits require the right environment and energy to hold a superposition. One method of increasing coherence times is to place qubits in a low-energy setting like a cryogenic environment. Cryogenic environments increase coherence times by eliminating high-energy particles that could knock qubits out of superposition, almost as if someone was to catch your spinning coin in mid-air.

Absolute Zero

Creating an ideal setting where coherence times can be increased requires a temperature that is close to absolute zero Kelvin when atoms are completely still. To achieve an environment that is close to absolute zero, qubits that rest on a quantum chip must be placed in the coldest level of a cryostat. Cryostats, also called dilution fridges, are cylindrical fridges with different vacuum sections that are separated by cold plates. The plates between each level prevent heat from transferring from one section to the next. The top level of a cryostat generally sits around 70K (-334 F) while the bottom level must maintain Millikelvin temperatures (-458 F). Reaching and maintaining an equilibrium temperature in the Millkelvin range requires a lot of power. Additionally, all of the energy entering the system that controls qubit behavior creates heat through friction and attenuation. Energy cannot be created or destroyed. With each RF line attenuation, displaced electrical power is redistributed as heat.

With heat constantly entering the cryostat system, maintaining Millikelvin temperatures is challenging. The larger the space is, the harder it is to cool. Fridge volume is a critical consideration in how many qubits can be processed at once. This limitation paired with consistent heat entering the system provides cryogenic RF passive product manufacturers two main goals when developing cryogenic products:

• Superb thermal grounding that enables heat transferal into cold plates and out of the system.
• Increased RF component density. (Any lines coming into sections of the Cryostat must be extremely cold and isolated from heat.)

Cryogenic RF Components

For RF transmission lines, there are companies successfully creating terminations and attenuators that are operational at these extremely low temperatures. An attenuator in an RF line lessens signal power without inhibiting signal integrity. Housed in a metal/Teflon tube that ensures proper specification performance, attenuators are small resistive chips composed of substrate bases and sputtered metals that attach to a wire, plate, or other connectors. XMA Corporation has a popular line of cryogenic RF attenuators. XMA Corporation designs and manufactures cryogenic RF components with superb thermal grounding and increased density.

It’s important to note that heat seeks an equilibrium state through the path of least resistance. In an effort to create a consistent temperature in its environment, heat will try to warm the cold around it through the easiest route it can find. Unfortunately, the path of least resistance for heat produced by attenuation is down the RF line, where it contaminates vacuum areas below it. Cryogenic attenuator manufacturers face a challenge in decreasing resistance in the path from the chip to the wall of the attenuator, through the body of the attenuator, and into the cold plate. XMA Corporation works with scientific partners to offer comprehensive, high-performance products that distribute heat into cold plates and out of RF lines.

Other than efficiently dispersing heat, quantum engineers are battling scale-up issues. More qubits lead to more RF lines, but producing larger fridges has many obstacles. Instead, the focus has shifted to increasing product density. XMA Corporation designed the bulkhead attenuator that eliminates a connection point at each port and requires less space in a fridge. XMA has also released cryogenic SMP and SMPM attenuators that boast smaller connection radii. To compensate for radii size discrepancies, XMA manufactured a series of cryogenic block attenuators with multiple ports that allow scientists to install 8, 16, or even 24 ports at one time. XMA’s cryogenic block attenuators are an effective solution for crowded fridges as they increase density and decrease installation time. XMA continues to improve and compact their high-density blocks. Ultimately, XMA’s goal is to provide high-performance attenuator products that are superior in density and thermal anchoring.

References:

• XMA Corporation
• IBM Quantum Systems