Qubits : Heart of a Quantum Computer

Quantum Computing has been creating a lot of buzz for the past few years. Since it was first conceived it has captivated intelligent minds all over the world. Due to its ability to solve problems in minutes, which the current supercomputers will take millennia, these computers promise to solve problems that have haunted researchers and scientists for years.

The last decade has been exceptional for Quantum Computing. Years of research started showing its results and a big breakthrough came last October when Google claimed to achieve Quantum Supremacy.    

But what makes it stand apart from classical computers? It’s the Quantum Bits or Qubits that gives them this incredible processing power. Unlike a classical bit, which can be either 0 or 1 at a time, qubits can also be any combination of 1 and 0 simultaneously. These qubits exploit quantum phenomena like Superposition and Entanglement to provide you the results as fast as possible while you anxiously stare at the computer.       

Qubits are fascinating but their implementation is an arduous task. The information in the qubits is easily destroyed by thermal heats and other disturbances from the environment. This is known as Decoherence.

The colder and more isolated the qubit is, the less likely it is to flip to a different quantum state when it’s not supposed to. But it’s really difficult to keep the qubits cold and isolated.

So with all these challenges, how scientists and researchers are making these bizarre bits possible? Let’s find out.

Superconducting Circuits

This is the most widely used method for making qubits. Companies like IBM and Google rely on this method for their quantum computers. Superconductor materials that have zero resistance when cooled below a certain temperature, like Aluminium and Niobium, are used.

When the temperature drops below a critical value, two electrons form a weak bond and become a Cooper pair that experiences no resistance when traveling through the metal. The pairing opens a gap in the energy state, which any excitation requires some minimum energy. This gap leads to superconductivity since not any random increase in energy is allowed.  

Each qubit is actually an LC circuit, an inductor, and a capacitor. We manipulate its energy state to represent a superposition of |0⟩ and|1⟩.

Now the challenge is to make the energy levels uneven such that the superposition is confined to |0⟩ and |1⟩. To overcome that, the superconducting circuit includes a Josephson Junction.

The junction behaves as a non-linear non-dissipating inductor. It contains two Aluminum superconducting electrodes that are weakly coupled and are separated by a thin insulator about a thousandth of a hair thick. It is non-linear such that the energy level is unevenly separated so we can use two lower states as the bases for our superpositions.

This inductor is combined with a linear capacitor using a Niobium superconductor to create an LC resonator. With correct tuning, the circuit behaves like an atom with two quantum energy levels, i.e. our qubit.

Quantum operations are performed by sending electromagnetic impulses at microwave frequencies (around 4–6 kHz) to the resonator coupled to the qubit. This frequency resonates with the energy separation between the energy levels for |0⟩ and |1⟩. And the duration of the pulse controls the angle of rotation of the qubit state around a particular axis of the Bloch sphere.

To make a measurement, it sends a microwave tone to the resonator and analyzes the signal it reflects back. The amplitude and phase of the reflected signal depend on the qubit state. Once it is amplified, we know the energy level and therefore we can determine the state of the qubit.

To isolate the qubits, the computer contains a dilution refrigerator to cool down the quantum processor to as near as 0 Kelvin.

Silicon Spin Qubits aka “Hot Qubits”

This is a method that many think is the future of quantum computing. Two separate teams of researchers from Australia and Netherlands published a paper in April this year stating that they had performed the 2-qubit operation at 1.5 Kelvin, which is 15 times hotter than rival technologies can withstand.

Each silicon spin qubit consists of a few electrons held within a quantum dot. These quantum dots are tiny wells or divots in silicon that lay just beneath the gate electrode of a conventional transistor. As charge flows through the transistor, electrons drop into the well, and electrostatic forces hold them in place.

To compute with them, the Australian team applied an AC electric field, while the Netherlands team used an AC magnetic field to manipulate the electron spins, causing the spins to point up (1), down (0), or in both directions at once.

With this technique, electrons can be forced to occupy the same quantum dot only if their spins are opposite. If their spins match, the electrons stay put in their respective wells.

After this successful demonstration, Intel has also shifted its focus to hot qubits as these qubits can be made using transistors and Intel ships 400 quadrillion transistors a year.

Trapped Ion Pair

 Another approach to make qubits is through trapping ions and then precisely controlling them using lasers. To trap ions, scientists start with a steel vacuum chamber, housing electrodes on a chip that is chilled to nearly 450 degrees below zero Fahrenheit. 

Ca and Sr atoms stream into the chamber. Multiple lasers knock electrons from the atoms, turning the Ca and Sr atoms into ions. The electrodes generate electric fields that catch the ions and hold them 50 micrometers above the surface of the chip. Other lasers cool the ions, maintaining them in the trap. 

Then, the ions are brought together to form a Ca+/Sr+ crystal. Each type of ion plays a unique role in this partnership. The Sr ion houses the qubit for computation. The Ca ion, which has a similar mass to the Sr ion, takes away extra energy from the Sr ion to keep it cool and help it maintain its quantum properties. Laser pulses then nudge the two ions into entanglement, forming a gate through which the Sr ion can transfer its quantum information to the Ca ion.

To read out this state, the scientists interrogate the Ca ion with a laser at a wavelength that only the Ca ion's electron will interact with, leaving the Sr ion unaffected. 

What's nice about using this helper ion for reading out is that we can use wavelengths that don't impact the computational ions around it; the quantum information stays healthy. So, the helper ion does dual-duty; it removes thermal energy from the Sr ion and has low crosstalk when we want to read out just that one qubit.

Now answer this: Can we manipulate the Nucleus of an atom as a Qubit?? If yes then how?

The race to make a commercial quantum computer continues as researchers and tech giants keep finding ideas to make qubits that can work at an optimal temperature. Until then we will keep tabs on all the quantum computing breakthroughs until one of them finally establishes the quantum age.

References-

1. https://www.qutisgroup.com/wp-content/uploads/2014/10/Amaia_TFG.pdf

2. https://analyticsindiamag.com/how-this-breakthrough-makes-silicon-based-qubit-chips-future-quantum-computing/

3. https://news.mit.edu/2020/trapped-ion-pair-may-help-scale-quantum-computers-0128