Hey, um, I think I do know what I'm talking about :). You can see more of this historical side effort, which didn't pan out for a number of reasons: https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance_qua...
A 7 qubit QC NMR was implemented in 2001.
All these systems are just evolution of spin systems or other similar systems. The big difference with an NMR quantum computer is that it manipulates ensembles of spins.
The comment about universality of computing is that many physical processes can be used to compute things. I didn't say that qcs were universal computers.
Please try to read what i'm writing more carefully.
I know what NMR is. The systems are not evolutions of NMR. They're fundamentally different in their design, construction, and physics. An exchange-only silicon dot quantum computer is very different than an NMR quantum computer, which is very different than a neutral atom quantum computer.
These computers, these days, are by and large considered computers which execute programs. Perhaps in the 90s-00s that wasn't the case since physicists cared more then about the construction of a laboratory apparatus and less about turning it into a programmable device with program and data I/O.
I can't comment on what you know or don't know, but what you're writing is misleading and not accurate. The two points you've made (about what's considered computing and how modern quantum computers work) are what I refute.
I don't understand your point. Everything I said above is completely and totally technically correct from a physical point of view. You are ascribing to my statements meaning which I did not intend.
Modern trapped ion computers are doing the same underlying operations as NMR: you are using RF or other energy to perturb the energy states of underlying particles or other components, and reading out the results.
There was also a whole field called 'dna computing' which you would call biologists in a lab, but they were absolutely doing computing
I'm intently curious what sort of common-sense explanation you'd ascribe to how one programs an ordinary classical computer. Would you describe a computer programmer as one who orchestrates a delicate movement of charge through an intricate arrangement of n–p–n bipolar transistors?
Quantum computers really can be instructed to do an abstract operation on an abstract quantity, just like I can on an ordinary computer. You tell quantum computers to add and multiply, just as you do in your favorite programming language. In the programming language Quil, which a couple quantum computers of different base technologies use, one can literally write down a matrix of numbers in a program to define a new mathematical function, and later use that function to manipulate the contents of the computer's RAM[1]. On top of this, you can have loops, boolean conditions, and all that jazz a programmer expects. You don't even need a physicist to do this; a completely physics-ignorant programmer could do this.
All this business about energy levels, evolution, etc. are distractions, just as the electrodynamics of a transistor are distractions from what it means to program a computer.
[1] I'm abusing the word "RAM" here, where I truly mean the state of your quantum register(s).
> one can literally write down a matrix of numbers in a program to define a new mathematical function, and later use that function to manipulate the contents of the computer's RAM[1].
This "Quantum Computing for Computer Scientists" video https://youtu.be/F_Riqjdh2oM explains classical and quantum operators as just matrices. What are other good references?
> All this business about energy levels, evolution, etc. are distractions, just as the electrodynamics of a transistor are distractions from what it means to program a computer.
But a classical simulator - like e.g. qiskit - for a quantum circuit/experiment/function must run the experiment very^very^very many times to even probabilistically approximate a sufficient quantum system; because of the combinatorial probabilistic explosion that results from adding just one more basis state.
What are the fundamental limitations of quantum simulators? Maybe it's possible.
Your assessment of how a quantum
simulator works is not quite right. These simulators represent the entire probability distribution of basis states succinctly as an array. This array grows very large (exponentially) in the number of qubits.
A simulator only needs to run a computation once (which is multiplication of matrices in a tensor product space) and look at the resulting state. You don't need to run anything multiple times to approximate a quantum state.
The questions you're asking are the whole point of the field of quantum information science. On an ordinary quantum computer where quantum state will collapse to a basis state upon readout, indeed you might need to gather statistics to determine the answer to whatever you've asked your computer. However, "very many times" is mathematically bounded in some way for an ideal quantum computer. It's like saying "we need to do many^many^many^many comparisons to do quicksort". Well yes, but we have a relationship between the size of the input (N) and the average number of comparisons needed (N log N), which makes the algorithm feasible in practice. This is the same with quantum algorithms.
There are also different kinds of quantum algorithms. Some are more probabilistic in nature. Others are—again in purely ideal circumstances—give you the right answer in one go.
As a side note: It is very hard for me to read, understand, and respond to your comments. They seem like random buzzword soups and aren't very coherently put together, mixed with random links and references.
> Peter Shor first discovered this method of formulating a quantum error correcting code by storing the information of one qubit onto a highly entangled state of nine qubits. A quantum error correcting code protects quantum information against errors of a limited form.
> Quantum Monte Carlo encompasses a large family of computational methods whose common aim is the study of complex quantum systems. One of the major goals of these approaches is to provide a reliable solution (or an accurate approximation) of the quantum many-body problem. [...] The difficulty is however that solving the Schrödinger equation requires the knowledge of the many-body wave function in the many-body Hilbert space, which typically has an exponentially large size in the number of particles. Its solution for a reasonably large number of particles is therefore typically impossible,
What sorts of independent states can or should we map onto error-corrected qubits in an approximating system?
> Among semiconductor qubits, the electron and nuclear spins of donors in silicon play a special role for their conceptual simplicity (a 31P donor in silicon is similar to hydrogen in vacuum) and their exceptional coherence times [1] and 1-qubit gate fidelities [2]. Here I will present experimental progress on multi-qubit logic operations with donor spins, which point to several credible pathways for scalability using ion-implanted donors in MOS-compatible devices. The current state of the art is a hybrid electron-nuclear 3-qubit processor [3], where two 31P nuclear spin qubits are coupled to the same electron. The shared electron enables a geometric nuclear two-qubit CZ gate, which we perform with 99.37% average fidelity. NMR single-qubit gates reach fidelities up to 99.95%, and state preparation and measurement are performed with 98.95% fidelity. These three metrics show how close this system is to operating at fault-tolerance thresholds. Further, we entangle the two nuclei with the electron to prepare a 3-qubit GHZ state with 92.5% fidelity. Electron-nuclear entanglement unlocks the ability to connect nuclear qubits via the electrons, for instance using exchange interactions [4]. We have operated a weakly (~10 MHz) exchange-coupled 31P donor pair as a 2-qubit electron system, with native CROT gates performed by resonant microwaves. Gate fidelity benchmarks are underway and will be reported at the Meeting. On the engineering side, we have demonstrated the ability to implant single donors in silicon with confidence up to 99.85% [5]. This striking result identifies ion implantation as a scalable and accurate manufacturing strategy for spin-based quantum computers in silicon.
QoS: Quantum-on-Silicon
The survey article above just says "[Quantum] Output"? Is that different from registers? How long are those states ah coherent?
> By managing to store a qubit in a crystal (a "memory") for 20 milliseconds, a team from the University of Geneva (UNIGE) has set a world record and taken a major step towards the development of long-distance quantum telecommunications networks.
What are repeaters, and what are [quantum] prepared states in re: registers and longer-term storage for non-collapsed (or just probabilistic?) qubit outputs?
All these systems are just evolution of spin systems or other similar systems. The big difference with an NMR quantum computer is that it manipulates ensembles of spins.
The comment about universality of computing is that many physical processes can be used to compute things. I didn't say that qcs were universal computers.
Please try to read what i'm writing more carefully.