Superconductivity and Quantum Computation

Quantum computation is the use of quantum mechanical properties to perform computation. Analogous to the bits in classical computing, quantum bit or qubit is the most basic unit of information in a quantum computer. A qubit has two basis states, namely |0> and |1>. The general state of a qubit can be represented as a superposition of the basis states.

For a classical computer, the bits can be stored as electric charges on nanometre-scale capacitors (little reservoirs of charge). If so, how can a qubit be physically implemented?

Physical Implementation of Qubits

To understand how a qubit can be implemented in a physical system, one must be aware of DiVincenzo’s Criteria which specifies that in order to construct a quantum computer, the system must meet seven conditions. While the first five are associated with quantum computation, the other two deal with quantum communication. Here, we’ll look into the first five.

DiVincenzo’s Criteria

1. The quantum physical system on which the qubit is realized, must have two orthogonal basis quantum states.
2. Quantum systems are quite fragile when it comes to coherence (interaction with noise). This means that the system needs to be isolated from this environment. Hence, the decoherence time of the system must be longer than the operating time of the system.
3. One must be able to create a set of (universal) quantum logic gates for the qubit.
4. There must be a “qubit-specific” procedure for measuring the qubit.
5. Scalability (the ability to create a set of qubits and not just one) must be possible.

All these criteria are in fact difficult to satisfy. However, there are systems which can be implemented as qubits.

1. Polarization of a photon: The vertical and horizontal polarizations of a photon can be considered as the two basis states. However, it is difficult to produce a light pulse that is guaranteed to contain only one photon, that too at a specific point of time.
2. Two level atom: The ground and excited states of the atoms serve as the basis states.
3. Quantum dots: artificial atoms with a discrete energy spectrum and thus perfect to be operated as a qubit.

Most examples to build qubits are microscopic, namely atoms, ions, electrons and so on. However, there is one particular system that is macroscopic: superconducting qubits.

Superconductivity and Superconducting Qubits

Superconductivity is the phenomenon by which, below a critical temperature, a charge traverses through a material without being hindered by any kind of resistance. Microscopically, it can be described as the condensation of Cooper Pairs into a Bose-Einstein condensate.

The superfluidity of superconductors enable them to be a good system for qubits. Superconducting qubits exhibit large coherence times in the order of nanoseconds and high scalability. When compared to the other physical implementations of qubits, superconducting qubits have a macroscopic dimension (size comes in orders of micrometres) and are easier to manufacture (using electron beam lithography).

An essential factor of superconducting qubits is the Josephson Junction. They consist of two superconductors separated by an insulating barrier which becomes a superconductor when cooled below a certain temperature. Most circuits that have been built for superconducting qubits consist of Josephson junctions and other components. There are different types of superconducting qubits, namely charge, flux and phase, depending on their quantized states.

That being said, superconductivity is a promising aspect of the quantum computing field. Research in superconducting quantum computing is being conducted by companies like Google, IBM, Rigetti, Intel and so on. Sycamore, Google’s quantum computer, consists of circuits of superconducting metal which entangle 53 qubits in a complex superposition state.

Advancement in Superconductivity

Recently, scientists have come up with a material made of Hydrogen, Carbon and Sulphur, that apparently conducts electricity without any kind of resistance at well above 273K. Similarly, Nickelates have been found to exhibit a kind of superconductivity that is fundamentally different from that of copper oxides. Such advancements in superconductivity help scientists, engineers and researchers envision a promising future for quantum computers.