Quantum Computers

Quantum Computer from University of California boasts separate CPU and memory

Until now, the architecture of quantum computers only allowed them to be used in single area at a time, designed and built to process one task. That took place mainly because all the programming had to be hardwired into their hardware thus limiting their potential for general use. Researchers could not implement the Von Neumann architecture - the base principle of essentially all modern day computers, which comprises a processing unit, a memory to hold data and CPU instructions and an interface to control input-output functions. Recently however, UCSB researchers led by John Martinis have finally created first programmable quantum computer using Von Neumann architecture comprising two qubit registers and two attached memory units. Additionally, the unit is boasting superconducting circuits, implements quantum principle 'Fourier transform'  and a three-qubit Toffoli-phase gate (base logic circuits which will allow to further develop this new quantum computing platform). These features allow new computer to process and solve more tasks by having active information cached and made available for the CPU unit - so the first general use quantum computer have been created.                    

                                                                                                                                                                                                                                                             Quantum Computers

How does quantum computer operate?

Just like a conventional computer, the quantum computer has its basic, irreducible unit of information called the QUBIT. However, unlike the BIT, which represents a single binary parameter 'yes or no' (by carrying or storing 'on or off' signal) the qubit is capable of representing almost infinite number of values since the value is determined by the physical angular position of the molecule; qubit's value is a continuous variable unit vector defined by the two angles. These angles are called θ and Φ; they define the so called 'superposition' or the three dimensional angle of the unit. When the angle of the qubit changes during an execution of command, the value of that qubit also changes, thereby changing the information which that qubit carries. The total computation is summarized by the combined summary of rotations, or quantum states, of that qubit.

About superconducting circuitry

Quantum Computers

There are a few reasons why superconducting circuitry was implemented for the architecture of quantum computer

First one is technological availability: superconducting circuitry is easily designed and constructed using current microfabrication manufacturing processes. The second reason is the adaptability of the new computer to conventional electronics - it's superconducting circuitry gets along well with the existing MHz - GHz radiowave structure allowing it to be integrated with conventional electronics.

Quantum Computers

Superconducting circuitry has some downsides - the stability of qubit's state. The unit tends to lose (or change) its vector value through outside environmental interactions. So far few reasons have been discovered how and why superconducting qubits lose their coherence but it has been determined that they have proven to remain stable for about 4 microseconds, which is the key parameter defining the functionality of quantum computer. The 4 microseconds window of time is sufficient to conduct several hundred operation cycles. It is the beginning; however, the number of coherent operations per 4 microseconds period of 'stability time' has to go up significantly to support a full scale quantum computer. Conventional PC industry had experienced similar problem while ago when it was discovered that RAM memory is vulnerable to the influence of cosmic particles causing computational errors and computer crashes. That problem was solved by implementing memory parity and error correcting code. Since then every byte of information coming in or out of the conventional computer random access memory gets inspected for error and replaced if found corrupted. The native 98% fidelity rate of this generation quantum computer, however, is not yet sufficient to start implementing error correcting codes which are now used in conventional machines; there is still some improvements need to take place in order to decrease the current level of computational errors in the superconducting circuitry.

Quantum Computers

The superconducting integrated circuit includes two qubits, a system bus, two cubits of memory, and a resetting register - the description of a basic computer. Computational steps take few nanoseconds - comparable to a classical computer, but the real advantage of a quantum computer is derived from its ability to perform large number of calculations simultaneously. New quantum computer architecture allows writing of information to memory, while concurrently performing other tasks.

What is Resonator?

The low-level structure of the quantum computer is called Resonator or Zero-Qubit architecture. This architecture consists of a set of superconducting qubits (currently two). Each of the superconducting qubits is capacitatively paired with a dedicated memory resonator, as well as to a common resonant quantum system bus. The bus is used to couple qubits during computational operations, while the memory resonators are used to store the current state of the qubits. When a qubit is passed into its memory resonator, it is placed in the ground state.

Quantum Computers

UCSB physicists have finally produced an integrated circuit that fully implements the Von Neumann architecture. In this architecture, a volatile but long-life random access memory is managed by a central processing unit, both located on the same chip - this providing the key components for a quantum version of a conventional computer. The UCSB hardware is built on superconducting circuits, and must be super-cooled to low temperatures in order to display quantum behavior. This architecture represents a profoundly new level in information processing, and shows that large scale quantum computer is within our reach.

Quantum Computers

Sources: UCSB, physicsworld.com

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