Quantum Computing is based on physical materials that deal with very low temperature close to absolute zero, today, in order to increase abilities of Quantum Computers and to make them more convenient, the most important question is to handle is temperature problem. Semiconducting materials are the best choices to solve low temperature problem by approaching room temperature conditions. Yet, since many semiconducting materials can have many quantum
degrees of freedom, the qubits may interact and dechore quickly. Thanks to growing atomic engineering and advanced semiconductor fabrication technologies, these effects are reducing day by day. Hence, in this presentation, I’m going to talk about semiconductor roles in Q.C. and how to people solve (their approach to solve) the interaction problem with qubits.

Today, compared to classical computing (e.g. classical computers), quantum computing is the most effective way to store and manipulate information. For instance, instead of capacitors in classical computers where we store information such as empty ones (0’s) and filled ones (1’s), in quantum computing we are using quantum states (quantum bits – qubits) with quantum mechanical properties. Hence, we don’t only use zeros and ones as binary states from classical computers but we also use quantum states that represents zeros and ones at the same time.

(a) Quantum Mechanical Properties of a Qubit

In quantum computing, we owe the quantum mechanical properties that provides the best ability to store and manipulate information such as; -superposition-, -entanglement-, interference- .

✦ Quantum superposition: If we add two or more quantum states, their result will also be a quantum state.

✦ Quantum entanglement: If you have two identical particles and if you separate them (very far distance), a situation effects also effects the very distant one.

✦ Quantum interference: A particle can’t be more than one place at the same time, but sometimes it crosses its own trajectory and interfere its own path.

(b) Structure of Solid State Q.C.

Credit: IBMQ

✦ (1) & (5) are the amplifiers that capture and process read out signals.
✦ (2) & (3 ) transmits the input and output qubits respectively .
✦ (4) enables qubits signals to go forward while preventing noise from compromising qubit quality.
✦ (6) the quantum processor sits inside a shield that protects it from electromagnetic radiation.
✦ (7) provides the necessary cooling power.

(c) Heat in Solid State Q.C.

As I mentioned in part (b), we are dealing with very low temperatures close to absolute zero (~ 0°K). Since dealing with qubits, we don’t want them to dechore quickly. While the temperature is getting lower, the degrees of freedom is getting reduced. If we consider %75 of a Q.C. as a refrigerator, it seems impossible to operate Q.C. in room temperature before qubit decoherence occurs.

How to Run a Q.C. in Room Temperature Conditions ?

In 2013, Canadian researches stored a qubit in room temperature for 39 minutes by stored quantum information in the nuclear spins of phosphorous-31 atoms in a silicon-28 crystal. Since, phosphorous atoms in silicon at room temperature tend to give up their electrons and become positive ions, at first they cooled its crystal to 4.2 °K and used laser and radio frequency (RF) pulses to put neutral phosphorous atoms into specific quantum states. A laser pulse then ionized the atoms before the crystal was warmed up to room temperature (~ 298°K). [Original article is placed to bottom]

As a result, RF pulses were used to perform a “spin echo” (refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation) measurement of the coherence time, which was found to be 39 minutes. Thus, imagine that, what if we reduce %75 cooler part of the quantum computer and optimize for our daily life ? With that much computational power, our classical computers would be like todays calculators. Imagine that simulations that takes months with performed by classical computers would be done in hours at your home. Therefore, it’s the future obviously. However, except the advantages of using nuclear spin qubits, there are also disadvantages.

Challenges of Using Semiconductor Qubits

In semiconductors many quantum degrees of freedom are present, and all tend to interact with each other. Thus, semiconductor qubits may decohere rapidly and in order to store and manipulate information quantum logic operations must be performed on a qubit before decoherence occurs. In order to avoid decoherence, devices must be engineered at or near the atomic level with respect to spin-orbit interaction.

Most effective semiconductor fabrication techniques, to avoid spin-orbit interaction problems, are SRT-Embedded Heterostructures and Quantum Dot Arrays. Heterostructures are basically semiconductor structures where chemical composition changes with respect to position. In our case, it’s beneficial to use SRT (Spin Resonance Transistors) Embedded Heterostructures, since it enables quantum entanglement between qubits. Hence, we can use them for quantum logic gates (e.g. CNOT gates) on the surface of the semiconductor heterostructure. The other remarkable technique is the Quantum Dot Arrays where we can use them to lower electron tunneling barrier when two qubits couple, placed on top of a semiconductor heterostructure. Therefore, we can use them as entanglement switches such as, when the electrical field is turned off, the quantum dot qubits entangle.

To sum up, Quantum Computing is the most efficient way for computational operations today and also future. However, since it operates low temperatures close to absolute zero and to provide that temperature conditions, its %75 of the structure is consistent of cooler mechanisms and its cost due to that needs don’t make them the number one choice. However, in 2013, the Canadian researchers stored a quantum state (a.k.a qubit) in room temperature conditions and show that we can optimize a solid state quantum computer by solving the low temperature problem. If we solve the problem, quantum computers would be smaller (like classical computers) and will be inevitable to operate that much computational power at our homes. Yet, since optimizing the computer structure and dealing with spin-orbit interactions, semiconductor structure must be fabricated at atomic level. The most effective fabrication processes are SRT- Embedded Heterostructures and Quantum Dot Arrays to provide quantum mechanical properties of qubits and computational needs (store-manipulate) of the computer. Thus, in the future we would be using our quantum computers, since researches are accelerated that much.

O.S. Tapsin

Resources:

Semiconductor Qubits for Quantum Computation, presentation by Matthias Fehr (TU Munich) JASS (2005) St.Petersburg/Russia — Semiconductor devices for Quantum Computing, presentation by Bruce Kane(University of Maryland) (2004) — http://www.ibm.com/quantum-computing/http://www.semiengineering.com/quantum-computing-becoming-real –www.eetimes.com/purdue-builds-quantum-computing-semiconductor-chip/# — Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28. Saeedi, Simmons, Salvail, et al. Science 342 (6160): 830-833 (2013)