Beam me up, Scotty.” Do you remember this phrase? It was the command that Captain Kirk gave to his chief engineer, Montgomery “Scotty” Scott, when he needed to be transported back to the Starship Enterprise. Till now, we have seen humans and objects getting transported in science fiction only. But is teleportation possible in the real world also? Is there any way to make transportation a reality? Well, the answer is “yes.” Teleportation is possible, but only in the quantum world as of now. Moreover, quantum teleportation is way different from the teleportation you see in movies. How? Let’s find out!

So what is quantum teleportation?

While teleportation, as portrayed in movies, can transport even humans from one place to another, quantum teleportation can only transfer quantum information instead of physical matter. As we know, bits are the smallest unit of information in our classical world. They refer only to the binary values such as 0s and 1s, and all the classical information in our computers can be represented in the form of 0s and 1s. Sometimes, they are also classified as yes/no or true/false and are often used in groups called bytes.

However, when it comes to quantum information, qubits take over. In simple terms, qubits are just a quantum version of the classical bits. But, there is a lot more to them. A qubit is technically a two-level quantum system where the two basis qubit states are usually written as |0> and |1>. While a classical bit can either represent a 0 or 1, a qubit can represent state |0>, |1>, or even a linear combination of both, like a|0> + b|1>.

This helps the qubits to store comparatively more information than classical bits. Typically, n qubits can store 2n values of information. As the information stored and the data transfer via qubits is huge, this helps to transfer the information faster, thereby aiding quantum teleportation.

In real-world experiments, distinguished states of atoms, ions, electrons, photons, etc. can act as qubits carrying information. And eventually, quantum teleportation becomes a process where a qubit is transmitted from one location to another, without the qubit being actually transmitted through space. Moreover, the sender may neither know the recipient’s location nor the qubit that will get teleported.

Basics of quantum teleportation

Before moving further with the mechanism of quantum teleportation, we need to become familiar with some fundamental terms to the circuit of quantum teleportation. To be precise, quantum teleportation runs on two main things: quantum entanglement and quantum logic gates.

Quantum Entanglement is a quantum phenomenon in which the quantum states of two or more objects can be described concerning each other, even though the individual objects may be spatially separated. For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin up, the other one will always be spin down. In this way, if we perform measurements on one of the partners of the entangled pair, the results about the other partner will be known automatically.

As explained above, qubits carry all the information in the quantum world. However, qubits are just the raw material for quantum teleportation. They further need to be acted upon by something to make quantum teleportation a reality. And this is where gates find their role.

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Most of us are familiar with classical logic gates that operate on binary bits 0 and 1. However, to work with qubits, we need quantum logic gates. This means that quantum logic gates are the quantum versions of classical logic gates, but with a twist. Several quantum logic gates have varying operations, and each of these gates is reversible and can be represented by a unitary matrix. However, the circuit of quantum teleportation makes use of four quantum gates only: the Hadamard gate (H), the X gate, the Z gate and the controlled not gate. Please don’t get confused; I am going to explain these gates one by one.

Hadamard gate (H): Hadamard gate is one of the most important quantum logic gates used in the circuit of quantum teleportation. The Hadamard gate acts on a single qubit to change its outcome. It maps the basis state |0> to (|0)>+ |1>)/ √2 and |1> to (|0>-|1>)/√2. This means that a measurement after applying the Hadamard gate will have equal probabilities to result in states |0> and |1>.

X-gate: Also known as the quantum Not gate or the Pauli-X gate, X-gate flips the |0> state to |1> state, and vice versa.

Z-gate: Coming to the Z-gate, it doesn’t affect |0> state at all but multiplies the sign of the |1> state by -1. 

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Operations of Hadamard gate, X gate, and Z gate

CNOT gate: An abbreviation for “controlled-not gate,” a CNOT gate has two qubits as input where one is the control qubit, and the other is the target qubit. The operation of the CNOT gate is that it flips the target qubit if the control qubit happens to be |1>. On the other hand, if the control qubit is |0>, then the gate does not affect the target qubit at all.

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CNOT Gate

With this information in hand, we are now in a position to move forward towards the mechanism of how quantum teleportation works.

Mechanism of quantum teleportation

Suppose Alice wants to send some quantum information, say qubit |ψ⟩=a|0⟩+b|1⟩, to Bob. Now, if it had been some classical information, Alice would have easily made a copy of this information and handed it over to Bob. But in quantum mechanics, things are not as simple. There exists a theorem known as the no-cloning theorem, according to which you cannot simply make an exact copy of an unknown quantum state. As a result of this, we can see that Alice can’t simply generate a copy of |ψ⟩ and give it to Bob, and this is where quantum teleportation will come to the rescue.

To begin with, Alice and Bob will generate an entangled pair together, say the state |Boo> = (|00> +|11>)/√2. Then, they will move to different places, each taking one qubit of the entangled pair with him/her. Then, Alice will interact with the qubit |ψ⟩ to be teleported with half of her pair. Following this, she will measure the two qubits which she has.

Circuit of Quantum Teleportation
Circuit of Quantum Teleportation

Alice will make measurements, thereby getting |ψo⟩, |ψ1⟩ and |ψ2⟩. Alice’s measurements will reveal what outcome would be there on Bob’s pair of entangled states. Following this, Alice would contact Bob via a classical channel and ask him to perform operations on his entangled pair, depending upon her measurement, so that Bob can recover the original |ψ> state from his pair of the entangled state. Confused? Don’t worry, the following example (using |ψ⟩=a|0⟩+b|1⟩ and |Boo> = (|00> +|11>)/√2) will make things clear.

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Scheme of performing measurements in quantum teleportation

In this way, |ψ> will reach from Alice to Bob, without even traveling through space. Here, Bob and Alice teleported information in a one-qubit system. However, advanced teleportation in an n-qubit system can also be realized, provided the entangled pair is modified.

Quantum teleportation in Lab

In 2019, scientists confirmed that information could be passed between photons on computer chips even when the photons were not in any physical contact. Moreover, according to National Science Foundation-funded research by the University of Rochester and Purdue University scientists, teleportation may also be possible between electrons. In the coming years, more advanced research is expected to take place in this regime.

Researchers believe that quantum teleportation can revolutionize technology, medicine, and science by providing faster and more efficient processors and sensors. Although at present, quantum teleportation seems to be a long shot, it is expected to become a common reality in the future.

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