Using Quantum Optics to Enhance Quantum Communication

Data breaching - currently one of our societies biggest concerns. At a time like ours where we are run as a data driven society, we are constantly worried about our security being at risk. Rather we need a safe and secure system to transmit information.  

However, the saviour to society’s security problems might have come just in time through quantum communication networks – a medium for secure data exchange.  

Quantum communication networks harness the laws of quantum physics and quantum information science to develop safe communication networks. This allows us to transmit information without the fear of having a third party intercept the piece of information.  

How do quantum communication networks work? 

In these networks, qubits also known as quantum bits are used to store information in the form of 1s and 0s using the laws of quantum physics. Qubits are produced and distributed in order to share quantum signals through optical fibres. Quantum networks specifically use optical networks and photon-based qubits and allow for information to be transmitted between physically separated quantum processors.  

Yet, quantum communication networks are a lot more complex than any other form of communication through optical fibres. We need to consider the following when considering these types of networks: 

• Quantum Mechanics: In order to enable faster communication, we use quantum key distribution theoretically to secure communication and quantum teleportation to send data at a high rate. You could read more about it in my previous article. 
• Quantum Entanglement: This is used to create a quantum channel to transmit information.  

When we are looking at quantum entanglement, we could also focus on entanglement swapping, where entanglement is transferred onto two particles that originated from different sources and were completely independent beforehand.  

The process for quantum entanglement goes as follows:  

• 2 independent pairs of entangled photons (lets say A-B and 1-2) are emitted from an independent source 
• Take a joint measurement, known as a Bell State Measurement (BSM) on one photon in each pair (A and 2), and the photons fall into an entangled state of one of four Bell states. 
• The remaining photons 1 and B are then in an entangled state even if they are unaware of the other’s presence since they haven’t interacted before 
• Therefore, the entanglement of the initial pairs is swapped

While making a joint measurement on the photons, we would want to create a successful bell state measurement. A bell state is the maximally entangled quantum state of two qubits. A successful BSM has to do with the precise timing of the 2 photon pairs. We can find the timing through pulsed sources, which send photons in separate bunches/groups. To send the photons at the same time, the pulsed sources need to be synchronized. This is a difficult task. 

Possible alternatives include having a continuous photon sources because they do not require synchronization and it becomes easier to incorporate in the real-world quantum communication systems. 

Let’s have a closer look at Bell states: 

The first Bell state ∣Φ + ?can be represented as the following equation: 

Let’s say we wanted to transmit information between Alice and Bob. Alice’s qubit is denoted with A, while Bob’s is denoted with a B.  

The qubit Alice has can be either 0 or 1. If Alice were to measure her qubit, the probability of either outcome would be ? making it completely random. Then if Bob were to measure the result of his qubit it would be the same as Alice’s. If Bob measured, he would also get a random outcome but if Alice and Bob were to later communicate they would find out that the results are interrelated even though they seem to be random.  

Yet the particles might have previously agreed on the outcome that would be displayed if a measurement were to occur when the pair was made. In 1935, Einstein, Podolsky and Rosen stated in the EPR Paradox that there is something missing in that explanation. We could denote this as a hidden variable.  

Quantum mechanics on the other hand lets qubits be in a quantum superposition of 1 and 0 at the same time. This means that the qubits could be in both states.  

In 1964 John Bell showed through probability theories in his paper that the correlations between the qubits were not because of an earlier agreement that’s kept in a hidden variable, rather quantum mechanics allows perfect correlations. This is formally known as the Bell-CHSH inequality: it states that the results of entanglement in quantum mechanics cannot be recreated by local hidden variable theories. This means a measurement on a correlation cannot be over 2 but according to quantum mechanics the maximum is 2√2 

There are 3 other states that lead to the maximum value of 2√2 of the two qubits. These four states are known as the Bell states. These bell states explain the simplest and largest examples of quantum entanglement.  

Bell states are an important factor to investigate how qubits interact. This is especially when we look at quantum machines. In the case of quantum machines, we have qubits interacting giving us quantum gates. Quantum machines also use quantum memory which is similar to RAM on a computer.  

The quantum memory in quantum machines is an important part in creating secure communication networks. We want to use optical channels to connect qubits and quantum memory because light is the best medium for communication. The goal is to interconnect matter and light to develop quantum light-matter interfaces.  

This is where we start to deal with quantum optics: applications of quantum mechanics to explain concepts about light and how it interacts with matter. 

Quantum Light – Matter Interfaces  

• A single atom can take up certain quantum states and when you couple atoms to an optical cavity, the atom could transfer its quantum state from one atom to another 
• Coupling atoms: This could work by having two mirrors sitting in front of each other which enhances the interaction of atoms with light 

Quantum memory is an example of quantum light - matter interfaces.  

It allows the information transporter in a quantum network (photon) to interact with a physical storage medium. We need it to store quantum information carried by light.  

It’s an ensemble of atoms and light stored in the atoms with different ways to recapture the light while preserving the quantum effects. Quantum memory is an important part of making a quantum communication network and quantum information processors.  

At the moment, superconducting qubits are being investigated as a possibility to make quantum computing machines. Now we want to know how can we combine an ensemble of atoms to superconducting devices and make it interface with light? 

A possibility to do this is by making solid state material optical resonators.  

The Kavli Nanoscience Institute at Caltech have done this through photonics crystal resonators where they use lanthanide atoms.  

Lanthanide atoms are used as they are best to preserve the atom’s “quantumness”. The Lanthanide atoms are highlighted in yellow in the periodic table below.  

The atoms put in a crystal will cause optical transitions between the 4f orbitals where it interacts with light. This is good for quantum optics because the 5s and 5p orbitals shield the 4f orbital, allowing it to preserve the atoms quantum aspects. In this case, we can notice that the 5s and 5p orbitals are filled while the 4f orbital isn’t filled.  

These materials were used for quantum memory applications and were also able to store entanglement.  

Looking forward, we want to make on chip micro nanoscale devices that can be integrated in on chip quantum networks.  

These on chip optical quantum memories allow: 

• Single error times to be reduced  
• transduction between microwave and optical fields  

Wait a second... why the nanoscale? 

The benefit of having them on the nanoscale is that it enhances the interaction between light and matter. When the atom is in a free space nothing happens when it interacts with the photon. Then when you place an optical resonator, for example putting the atom between two mirrors, the photon is trapped between the mirrors and the photon can interact with the atom.  

However, there are negative aspects to all possible solutions, including quantum memories.  

Problem with quantum memory

It’s hard to store information in a physical quantum system for long periods of time because the quantum information disappears a short while after because it interacts with the environment.  

Therefore, the goal is to have light in an optical resonator for as long as possible. When we have small resonators, it is more advantageous with nano-structures because it’s operating at a level where single photons interact with single atoms. Here the photon is confined in a smaller area and the electric field correlating to the single photon is higher. This increases the interaction between the atom and the single photon.  

With quantum memories we can enhance our current levels of communication with the use of light and with the help of quantum physics we can create a secure channel to transmit information. If this is the what the future entails, I’m definitely excited! This takes us one step closer to becoming a more secure data driven society.  

Key Takeaways:  

1. Quantum communication networks allow for a safe medium to transmit information 
2. This is achieved using entanglement specifically with the use of the Bell State Measurements 
3. Quantum machines execute quantum communication networks through quantum memories  
4. Quantum memories contain quantum information stored in light and are an example of quantum light matter interfaces  
5. A limitation with quantum memories is that it’s difficult to store information in a quantum machine for certain time periods because the quantum information disappears 
6. If we use the nanoscale we can strengthen the light and matter interactions, improving the potential of quantum memory  
7. This can lead to a future of more secure communication channels! 

Stay tuned for more articles I write on quantum computing and quantum communication networks!