THE ERROR CORRECTING CAT- THE CAT QUBIT


Going back into the quantum world again.   A world of weirdness. If you’re a frequent reader of my blog, you’ve just read a crazy experiment done on time travel through quantum mechanics this is going to be a surprise for you. You can read about it here.

As said, the world of weirdness filled the scientist with curiosity to crack down the mysterious occurrences in the quantum realm. Very recently, scientists have brought up a clever way to overcome few errors using a beautiful yet another interesting concept-The Schrodinger’s cat. This is the most famous cat in all of the science community. It neither lives nor dies. (Such a beauty isn’t it? (No it’s not a zombie)).

Before hopping onto the cat (I meant the topic of that cat), I’m guessing you’ve heard about quantum hardware, quantum processors, quantum circuits, quantum computers, and many more (no, they’re not just adding “quantum” to everything).

Traditional processors and computers run on what we call bits (well, it doesn’t matter what we call, they called “bits”),’1’ and ‘0’. Ah yes!  The binary buddies. Quantum ‘thingies’ run on qubits (quantum bits). They use a property from quantum mechanics, quantum superposition where two states (here 1 and 0) exist at once.

Schrodinger’s Cat is a thought experiment that is illustrated to show the superposition theory of the quantum world.  A cat is placed in a box with a radioactive source and a poison that will trigger if an atom of the radioactive substance decays.  The quantum superposition states that the cat is both dead and alive until you observe what’s inside the box. The moment you observe, it would fall into one category (either dead or alive). Qubits follow the superposition theory and the bit is both 0 and 1 at the same time.

Yale physicists have developed an error-correcting cat to fix some of the trickiest errors in quantum computation.  Many errors occur during quantum computation, they just keep on popping up. While performing a particular task using quantum computer, it is necessary to correct the stream of errors that occur.

Quantum computers have tremendous usage in coming future and are being used by a range of industries currently. It can perform computational tasks in various orders of magnitude faster than today’s supercomputers.

At Yale, they continue to build upon two decades of ground-breaking quantum research which is led by Devoret, Robert Schoelkopf, and Steven Girvin. Their approach to building a quantum computer is called “circuit QED” and employs particles of microwave light (photons) in a superconducting microwave resonator. Circuit QED provides a means of studying the fundamental interaction between light and matter. In Cavity Quantum Electrodynamics (QED), a single photon within a single-mode cavity coherently couples to a quantum object (like an atom). In contrast to this, the photon is stored in a 1-D on-chip resonator and the quantum object is an atom but artificial one. The quantum resonant devices used for circuit QED are superconducting coplanar waveguide microwave resonators (yeah, it’s a big name), which are two-dimensional microwave analogues of the Fabry-Perot interferometer. (These complex terms can be easily visualized by the images below, don’t worry).

Fabry-Perot interferometer
Cavity QED
The set-up of superconducting coplanar waveguide
Superconducting coplanar microwave resonator

In traditional computers, the information is encoded as 0 or 1, the errors can be found by “bit-flips” (the name says it all). They are corrected by building in redundancy: using three “physical” bits of information to ensure one “effective”-accurate bit.  For instance, in the recent launch of the SpaceX rocket, the SpaceX Crew Dragon, they use 3 x86 processors. One was enough for the whole operation but the other 2 were also important as they do the same calculations as the 1st one. During lift-off or the journey to ISS, there is heavy radiation. Although they use radiation-hardened processors, because of the radiation and other factors, bit-flip occurs. Since 3 processors are doing the same calculations, if a bit flip occurs in the main processor, it checks with the remaining 2 processors for error checking and correction. What if bit-flip occurs in all the 3 processors, there is a backup for that case too but the possibility of 3 processors having bit-flips at the same time is extremely rare. (Yes, a single bit can crash the whole project or make it lose the trajectory or blow up or emergency landing failure and many more).

Quantum information bits-qubits-are subject to both bit-flips and phase- flips, in which a qubit randomly flips between super-positions.

Now here, the Schrodinger’s cat bit –the cat Qubit comes into play. It’s difficult to do error correction in quantum computations, so they made this cat Qubit (It’s a stepping stone).  “Our work flows from a new idea. Why not use a clever way to encode information in a single physical system so that one type of error is directly suppressed?” Devoret asked.

Unlike the multiple physical qubits needed to maintain one effective qubit, a single cat qubit can prevent phase flips all by itself. The cat qubit encodes an effective qubit into a superposition of two states within a single electronic circuit- in this case, a superconducting microwave resonator whose oscillations correspond to the two states of the cat qubit. “We achieve all of this by applying microwave frequency signals to a device that is not significantly more complicated than a traditional superconducting qubit,” Grimm said.

Qbit encoding , stabilization and implementation(PDF available in the references)

The researchers said they can change their cat qubit from any one of its superposition states to any other superposition state, on command. Also, the researchers developed a new way of reading out — or identifying — the information encoded into the qubit. “This makes the system we have developed a versatile new element that will hopefully find its use in many aspects of quantum computation with superconducting circuits,” Devoret said.

Brace yourselves; we got a new era of electrons ahead. Quantum computing, nanotechnology, interplanetary travel, and many more.

REFERENCES:
1) https://news.yale.edu/2020/08/12/yale-quantum-researchers-create-error-correcting-cat
2) https://www.sciencedaily.com/releases/2020/08/200812144017.htm
3) https://www.nature.com/articles/s41586-020-2587-z
4) https://cacm.acm.org/news/246782-yale-quantum-researchers-create-error-correcting-cat/fulltext
5) https://phys.org/news/2020-08-quantum-error-correcting-cat.html
6) https://arxiv.org/pdf/1907.12131.pdf

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