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It makes for a hefty hardware overhead. And it only works if the fidelity of the physical qubits is good enough, which puts a huge burden on fidelity. So, this is the obstacle: an acute physical problem that error correction alone can at best turn into a massive engineering problem. What to do?

Many physical platforms are currently competing to make quantum computing a reality. Indeed, many different quantum systems can play the role of the qubit, each with its own strengths and weaknesses. Think of trapped ions, neutral atoms, superconducting circuits, and even photons. Yes, light itself can be used to encode quantum information.

Optical photons bring together many advantages for quantum computing. They are easily produced and can be routed in optical fibers, propagating over long distances and remaining coherent for long times at room temperature, which means they dont require expensive cryostats. Companies like Xanadu in Canada or Quandela in France have developed promising approaches to photonic quantum computing. All in all, its a great platform for scaling, but its much harder to run operations between qubits and program the quantum computer. This makes it more difficult to build all the necessary gate operations to achieve universality.

But it isnt the only way optics can provide a key tool in the operation of a quantum computer. Other platforms rely heavily on optics to control and measure quantum systems. Lasers are used to read out the states of trapped ions, optical tweezers to manipulate the states of neutral atoms, and microwave photons to control superconducting circuits.

There are even state-of-the-art approaches to quantum computing where ideas from quantum optics provide more than just a tooland provide a method that directly addresses the biggest problem in quantum information.

The idea here is to attack the error problem head on: Schrdinger cat states are quantum superpositions of two coherent states of light that are effectively mirror images of one another.

The quantum logical 0 is a collective state of photons in which they all share the same amplitude and phase. It corresponds to the state of light created by a laser. The logical 1 is the same state except that the phase of each photon is the opposite. We take the same laser light as we did before, but delay it just as much as needed so that all photons have the opposite phase of the ones in our first beam.

Such states are often referred to as classical because they correspond to the usual excitations of resonators: using mirrors to trap the light of a laser in an optical cavity, the corresponding coherent state inside is described mathematically in the same way as a mass oscillating at the end of a spring.

The laws of quantum mechanics allow us to prepare not only these two distinct coherent states, but also superpositions of them. In the laser analogy, this would correspond to the laser emitting the same photons with two different phases at the same time. These states are called Schrdinger cat states, named after the famous thought experiment in which a cat could be both dead and alive due to quantum effects. Schrdingers aim was to show how absurd it would be if the principle of quantum superposition could be transposed to our classical world.

In the present case, no cats are harmed, but the idea is the same: we can generate and observe coherent quantum superpositions of classical states, not of cats, but of light. And the idea and first realization of these states originated in optics. Cat states of photons at microwave frequencies were then realized and French Physicist Serge Haroche was awarded a Nobel Prize in Physics in 2012 for this groundbreaking work in quantum optics.

Whats the connection with the error problem? At Alice & Bob, we use superconducting circuits to generate, stabilize, and control qubits based on Schrdinger cat states (see Fig. 2). Cat states are interesting quantum objects that can teach us a lot about the fundamentals of quantum mechanics, but our goal is to create practical quantum computers. And it turns out cat qubits have one particular property that makes them eminently suitable for fault-tolerant quantum computing: a built-in ability to resist bit-flip errors.

See the article here:
Encoding quantum information in states of light - Laser Focus World