I would like to take a look at the Nature paper on Amazon's new architecture called the cat qubit. The cat qubit is a new type of superconducting qubit developed by Amazon. French company Alice & Bob is also working on this kind of qubit.
I briefly mentioned this in an article a long time ago, but I did not go into detail about its internal structure at that time.
"Optimized for Error Correction. Cat Qubits that Prevent (or Reduce) Bit-Flips."
April 10, 2022
Amazon has now released a new machine, but since I had not properly studied cat qubits before, I thought this would be a good opportunity to fully understand them.
Hardware-efficient quantum error correction via concatenated bosonic qubits
The paper discusses the components that make up a cat qubit and how its properties can be utilized for error correction. Since my goal this time is to understand what a cat qubit actually is, I will lightly cover the error correction part and focus mainly on understanding the fundamental concept of cat qubits.
Bosonic Qubits and Cat Qubits
The key feature of this new machine is that, unlike conventional superconducting qubits that start with binary states (0 and 1), it uses bosonic qubits. The term "bosonic" comes from bosons, and in this case, it refers to photons, the quantum particles of light.
The bosonic qubit in this system uses a coplanar waveguide resonator (CPW resonator), which can confine specific electromagnetic waves. In this system, rather than using just two values, 0 and 1, it employs a continuous set of values such as 0, 1, 2, 3, .... This kind of continuous-variable encoding is also used in optical quantum computing.
There are multiple types of bosonic qubits, but the ones that use a special quantum state of light called the coherent state are referred to as cat qubits. Since a superposition of two opposing coherent states is called a cat state, qubits that utilize this concept are called cat qubits.
This is based on Schrödinger’s cat thought experiment, where a cat inside a closed box exists in a superposition of "alive" and "dead" states until observed. Similarly, cat qubits leverage this superposition principle for quantum computation.
In this particular cat qubit system, introducing a large number of photons enhances its robustness against loss, and because it integrates well with superconducting qubits, it enables more efficient quantum error correction—one of the primary goals in quantum computing development.
Stabilization with Buffer Modes
Additionally, cat qubits are further stabilized using a device called a "buffer mode." The buffer mode is based on a component called a non-symmetrically threaded SQUID element, which helps maintain a process known as two-photon dissipation inside the cat qubit.
A major challenge for cat qubits is single-photon loss—when a single photon leaks out and is lost, it causes a phase-flip error. To mitigate this, the system ensures that photons are lost in pairs (two-photon dissipation), rather than one at a time.
The non-symmetrically threaded SQUID element is responsible for enabling this two-photon dissipation mechanism.
Although the buffer mode itself is not a direct component of the cat qubit, it functions as a crucial auxiliary device that maintains the quality of the cat qubit.
Cat Qubits and Bit-Flip Errors
The biggest advantage of using cat qubits is that they drastically reduce bit-flip errors.
- Bit-flip errors occur when a quantum state erroneously transitions from ( |0\rangle ) to ( |1\rangle ).
- In conventional superconducting qubits, bit-flip errors are caused by environmental noise.
- In contrast, cat qubits utilize photons, which are inherently less affected by external noise, and their design makes bit-flip errors fundamentally unlikely.
In cat qubits, the coherent states that correspond to 0 and 1 become increasingly resistant to bit-flip errors as the average number of photons increases, due to an exponential suppression effect.
This means that as the number of photons increases, the probability of mistakenly flipping between 0 and 1 becomes negligibly small.
This allows quantum error correction to focus almost entirely on correcting phase-flip errors rather than having to deal with both types of errors.
Traditional quantum computing required handling both bit-flip and phase-flip errors, making error correction highly complex. However, with cat qubits, physical device properties naturally suppress bit-flip errors, allowing error correction efforts to focus on phase-flip errors alone.
Concatenated Bosonic Qubits
This new machine does not rely solely on individual cat qubits but rather links multiple qubits together to experimentally demonstrate error correction.
One important component of this is the CX gate (controlled-X gate), which consists of a control qubit (C) and a target qubit (X).
- When the control qubit is ( |0\rangle ), the target qubit remains unchanged.
- When the control qubit is ( |1\rangle ), the target qubit undergoes an X operation (bit-flip).
In this system, the control qubit is not a cat qubit but a conventional transmon qubit, which is a commonly used superconducting qubit.
Thus, two different types of qubits—transmons and cat qubits—are connected and used together.
Conclusion
The name cat qubit sounds cute, but it comes from its reliance on Schrödinger’s cat state for quantum computation.
- The key property of this Schrödinger’s cat state is that the probability of bit-flip errors decreases exponentially as the average number of photons increases.
- In conventional quantum computers, both bit-flip and phase-flip errors needed to be addressed simultaneously, making error correction difficult.
- Cat qubits, however, make bit-flip errors almost negligible, allowing error correction to focus solely on phase-flip errors.
While cat qubits are not yet perfect, and many challenges remain, I was interested in this new type of superconducting qubit and happy to have gained a solid understanding of its fundamental mechanisms.
That’s all for now.