A Deeper look inside Entanglement & Quantum Computers

How to Entangle Particles | Create Entangled Qubits & Upgrade QCs

Hey,

Recently we were discussing about how quantum computers are being upgraded from 27 qubits to 53 or even higher qubits!

Basically a quantum computer works on qubits, just like a classical computer works on bytes.

A bit is either 1 or 0 whereas a qubit can be both 1, 0 or anywhere inbetween, its basically an entangled state of photons, electrons or any fundamental particle. 

But what's behind that is phenomena of quantum entanglement and superposition. Heard these words quite a lot. Right?

But let's take a simple question, how particles are entangled in practical? Tough to answer, right! 

We had an exciting conversation during which I figured it, my friends where confused about it, and quite a bunch of people are too. Also recently I have been taking the quantum computing course by QubitxQubit and iBM and noticed that the most common question during lab sessions was how two particles or qubits are entangled?

So here I list few of the most exciting ways in which particles can be entangled.

Before that lets just see what entanglement is. 

Entanglement is the primary feature or the phenomenon that's not present in classical physics and its the reason scientists defined a different area of study & that's what we call Quantum Physics.

The idea is simple, we have two particles each in one of two states. And put them in a state where their states are indeterminate but correlated. Measuring them individually, will return a random distribution of 0 and 1, thats up spin or down spin (they are not spinning though - anyway  its physics). And the state of one of the two particle depends on the other and this correlation will hold even when they are separated far apart in space. Einstein termed this 'Spooky action at a distance'. 

Lets take a scene, we have a pair of gloves, both separated & packed in different boxes. Now without knowing which glove is inside, one of the boxes is taken to Saturn and the other remain on Earth. Now the task is to know where is the right hand glove. For now the gloves are in superposition. The person on Saturn don't need to bring that box back and look for the glove, instead that box on the Saturn can be opened there & if its a left hand glove, we know the right one is on Earth. That's even being separated by 1.3865 billion km we can know state of one if we know the state of other. This is just to explain how entanglement looks like. The phenomena is only applicable on quantum scale.

Entanglement of particles is really a fascinating process; lets look at that.

1. Entanglement of Photons:

Now this is the most common way of putting two particles in quantum entanglement. Photons are particles with zero charge and zero spin. Once a photon is put in a strong electric field, it separates into  two particles that are on quantum scale. Now these two particles needs to have a net charge and spin of zero, i.e., if one is spin up the other must be spin down. When the system of electron-positron is not observed, the system can exist in both possible states at the same time. In this case these particles act as if they are one and exist in multiple superposition states at the same time such that the total charge and spin is zero. Such particles are in entanglement.

Entangled pair of particles from photon

2. Entanglement : 2nd Generation

Photons are great for demonstrating entanglement and transmitting information on quantum scale. But photons are hard to keep around, since they are always moving somewhere at the speed of light. So it becomes easy to entangle material particles instead. A pair of photons that are produced in an entangled state are taken and directed at a pair of atoms. The photons are absorbed and the end state of absorption depends on polarization of these photons. Since the photons are indeterminate but correlated, you end up with two atoms whose states are indeterminate but correlated, or simply entangled.

3. Entanglement through Cascade Transition:

This is the historical way, in this experiment scientists take a bunch of highly excited calcium atoms where the electron is forbidden to return to the ground state by emitting a single particle; Instead, they decay by emitting two particles from a single photon, one followed by the other. So if we notice one coming out we know the other should be around somewhere. And there total charge and spin be zero, basically an entangled pair of particles.

This shows separated entangled particles

4. Entanglement by interaction:

Done by Rydberg blockade, so you have two ground state atoms separated by tiny space, in a way they don't affect each other. Once you excite those atoms to a very high energy state (called the Rydberg state), they interact over longer ranges. If arranged properly, exciting one atom to the Rydberg state will shift the energy levels of the other. A laser pulse is used to put part of first atom in ground state paired with the part of the second atom that's in Rydberg level. Now the two behave as entangled atoms and shows quantum superposition.

5. Entanglement through prism:

Done using a prism that divides light into two equal parts. So instead of light, take a single photon and strike it on the prism. Now it will be divided into two particles, having a total net charge and spin equals to zero and being in superposition. i.e., these particles are entangled. 

These are few methods in practice for putting two particles in entanglement physically. 

Whoo! super interesting. 

We had also heard this term, entanglement but were never sure how its done. Hearing these terms in Marvel movies, looking at the phenomenon of multiverse has been always fascinating. Now while watching those you can say, you know concepts working behind :)

Travelling through Multiverse

The question still remains, how two qubits are entangled?

But, first have a deeper look at qubits. 

When we assign digital values to switch, in a classical computer or a chip we call it binary digit or a 'bit'; but when we assign values or information to subatomic or fundamental particles we call it 'quantum bit' or a qubit.

A qubit is always an entangled state of particles, as mentioned earlier; therefor its 1, 0 or somewhere in between simultaneously.

So lets say you need to guess a number on a eight byte (eight-bit) data scale, the classical computer will approach in a way, checking one number at a time, say the first guess be one (00000001) and then the second be two (00000010), it will run for 107 time if the output needs to be for example, 107 (01101011). While the case is different in quantum computers, as the qubit can be both 0 and 1, so the first guess is the only guess you need, means it takes just one step to reach any desired output. 

Quantum computer

So this happens, because qubits are entangled and here is how its done.

Basically qubits are being created using photons, electrons or even nucleus of atoms by putting them in entanglement. So putting qubits in entanglement is actually putting entangled particles in entanglement.

For years scientists have been using electron spin of the outer most electron of phosphorus atom for creating qubit. A single atom of phosphorus is taken and is embedded in a silicon crystal with a tiny transistor. 

Phosphorus

The electron has a dipole or a spin, spin up and down; at different energy state. And to know the state (spin) we have to apply a strong magnetic field. This is done using a super conducting magnet or a large solenoid placed in super chilled liquid helium, now why its done in this way has its own reasons.

Particles can be in entanglement until they are indeterminate but correlated, even the tiniest disturbance (noise) can break that coherence and once there is no coherence, particles will be no more in entanglement.  

This animation shows how even photons can end entanglement.

A photon breaking the coherence and ending entanglement

Liquid helium makes the place super chilled, there will be no thermal disturbance, plus this also makes the environment super conducting. While any thermal energy can change the spin of electron, or makes it wiggle between up and down spin, super chilled surrounding helps in lining up electrons in single spin state or basically in a down spin, as there will be no enough thermal energy to flip it.

Once this is done, and we have the electron stable in one spin, we apply magnetic field, to write information on it we use pulse of micro waves, similar to what we have in ovens. These waves at a particular frequency has particular energy that turns the spin of electron in a desired position, up, down or anywhere in between depending on the magnetic filed present in the surrounding, this means you are putting information in the electron, on quantum scale. Now these electrons are in entanglement because of the strong magnetic field applied by solenoid, plus they have information on quantum scale, And Congrats, that means you have successfully generated qubits.  

Qubits in entanglement

Ok so thats half the things done, to read the information, we use a transistor that the phosphorus atom is embedded next to. What actually happens is, the transistor has a bunch of electrons, and we know spin up means more energy than spin down. The electorn will move to a stable spin up state from the electron shell into the transistor, creating +vely charged phosphorus ion, and we notice a pulse of current or simply more current flowing through the circuit when this happens and indicates that the electron was in spin up state. And thats it, although there are things to be done with the nucleus and even turning its spin up and down creating a more stable qubit for a longer time, or using phosphorous 28 isotope, with a net zero spin and charged nucleus.

So this is how we store information at quantum level by creating qubits.

And to our surprise, this quantum information travels at a speed which is more than speed of light, yes you read it right. It's even more than what we can imagine, information between two entangled particles can travel with a speed of 3 trillion m/s thats more than 4 order of magnitude faster than speed of light ( basically 2.99792458 x 10^8 x 10^4 m/s ). 

On a quantum computer you can take two qubits, apply H gate on the controlled qubit followed by a CNOT gate. This will generate a maximally entangled two qubits state as shown below, this is called Bell state. 

This is actually how I entangled two qubits on an iBM quantum computer using code

Another way to entangle qubits

Recently researchers at University at Sussex estimated that a quantum computer with 1.9 billion qubits can essentially crack encryption safeguarding Bitcoins within mere 10 mins. That means the person with that information can access every bitcoin present on the global network, can know who is holding them, and literarily access anything related to bitcoins throughout the internet and that's probably the biggest crypto jackpot anybody can ever imagine.

But then why aren't we building a QC with 1.9 billion qubits, when we are having micro chips with billions of transistors each containing billions of bytes or binary digits. So in simple terms a micro chip inside your cellphone is holding billions and billions of bytes at the same time. 

To make QCs with billions of qubits are not possible, atleast with all the knowledge we have currently. Why? Because QC works on the principle of entanglement and to hold billions of qubits in entanglement simultaneously is not possible currently. As even a tiniest disturbance in any state (whether thermal energy, any EM wave or even a particle like photon) will break that entanglement. 

A Quantum Computer Chip

We have QCs with 127 qubits ( iBM's Eagle processor ) which requires a large room just to cool down the environment and remove every disturbance rather noises. While the QC chip itself being a very tiny one. 

To understand how mush information a 127 qubits computer can hold. 

Say the number of classical bytes required to hold the same information will be more than the total number of atoms present in all 7.5 billion people on Earth. 

Quantum computers have been the best part of my research areas so far, its as fascinating as Space Science. 

This year's Nobel Prizes in physics were also awarded for research done in the field of quantum computing specifically on the Bell inequalities & entangled photons.

And thats a wrap for the day, 

Stay tune for a very interesting blog on Nobel Prizes :)

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