How many steps are involved in transmitting information through quantum entanglement

The 2022 Nobel Prize in Physics has been awarded to three scientists for "experiments with entangled photons": Alain Aspect, John Clauser and Anton Zeilinger have made independent breakthroughs over the past few decades; their work has shown not only how quantum entanglement works, but also showed how mysterious properties can be a channel for transmitting quantum information from one photon to another.

In retrospect, when physicists such as Max Planck, Einstein, Bohr and Schrödinger were discussing quantum mechanics at the beginning of the 20th century, it was clear that nature had its own hidden channel of communication at the level of subatomic particles - quantum entanglement. Einstein described this phenomenon scientifically in a paper published in 1935, calling it a "phantom supergiant action".

Maria Spiropulu, professor of physics at Caltech and director of the INQNET Quantum Network Project, equates quantum entanglement to shared memories: "Once you're married, it doesn't matter how many times you may have been divorced," she explains. Because you create memories together, "you're connected forever." At the subatomic level, "shared memories" between particles can instantly transfer information about quantum states - such as atomic spin and photon polarization - between distant particles. This information is called quantum bits, and quantum bits simultaneously have an infinite number of potential capabilities that allow them to process information faster: exactly what physicists are looking for in quantum invisible transfer systems.

But for quantum bits to act as information processors, they need to share information in the same way that classical computer chips share information: into entanglement and transmission. The process of using entanglement for quantum information transfer is called quantum invisible transfer and is roughly divided into five steps as follows.

01Step 1: Entanglement

Using a laser, a stream of photons passes through a special optical crystal that splits the photons into pairs. The pairs of photons are now entangled, which means they share information. When one changes, the other changes as well.

02Step 2: Opening the quantum invisible state transfer channel

Then, one of the two photons is sent to a distant location via a fiber optic cable (or another medium capable of transmitting light, such as air or space). This opens a quantum channel for invisible transmission of states. The photon at the distant location (marked above as photon 1) becomes the receiver, while the photon left behind (marked as photon 2) is the transmitter. The channel does not necessarily indicate the direction of the information flow, since the photons can be distributed in a roundabout way.

03Step 3: Preparing the message for transmission

The third photon is the information carrier, which is added to the mixture and encoded with the information to be transmitted. The type of information to be transmitted can be encoded as so-called photon properties or states, such as its position, polarization and momentum (this is where quantum bits come in handy: encoded information is considered according to zeros, ones and their superposition).

04Step 4: Transmission of the encoded message

One of the strange properties of quantum physics is that the state or properties of a particle, such as its spin or position, cannot be known until it is measured. This can be thought of as dice: a die can hold up to six values, but its value is unknown until it is rolled. Measuring a particle is like rolling a dice, which locks in a specific value.

In the invisible transfer state, once the third photon is encoded, the properties of the second and third photons are measured jointly, which means that their states are measured simultaneously and their values are locked (just like looking at the values of a pair of dice). The measurement behavior changes the state of the second photon to match the state of the third photon, and once the second photon changes, the first photon at the receiver end of the quantum channel quickly enters the matching state.

The information now lies in photon 1 (the receiver). However, even though the information has been transmitted to a distant location, it is still encoded, which means that like an unfolded die, it is uncertain until it is decoded or measured. Therefore, the measurement of photon 1 needs to be matched with the joint measurement of photons 2 and 3; the results of the joint measurement of photons 2 and 3 are recorded and sent to the location of the photon so that it can be repeated to unlock the information.

At this point, photons 2 and 3 disappear because the act of measuring them destroys them: the photons are absorbed by whatever object was used to measure them, such as our eyes.

05Step 5: Complete the transmission

In order to decode the state of Photon 1 and complete the invisible pass, Photon 1 must operate based on the joint measurement, also known as rotating it, which is like rolling a die before. This step decodes the message - similar to the way binary 1s and 0s are converted to text or numeric values. On the surface, the invisible transfer state appears to be instantaneous, but because the decoding instructions for joint measurements can only be sent using light (in this case over a fiber optic cable), photons can only transmit information at the speed of light. This is important because invisible transmission would otherwise violate Einstein's principle of relativity, which states that nothing travels faster than the speed of light; if it did, this would potentially upend physics.

The encoded message in photon 3 (the messenger) is now transmitted from the location of photon 2 (the transmitter) to the location of photon 1 (the receiver) and is decoded.

Voilà! Quantum invisible transmission of states is complete.

Now, researchers have discovered many different ways to entangle, transmit, and measure subatomic information. In addition, they are upgrading from transmitting information about photons to transmitting information about larger particles (such as electrons or even atoms), although their ultimate goal is the transmission of information. Regarding the immediate promise of quantum invisible transfer, Spiropulu put it succinctly, "It's transformative."

Reference link：

https://www.popsci.com/science/quantum-teleportation-history/