The current state of the art is trying to prove that the thing did something quantum. Programmability is at best swapping some wires around to change that something.
Moreover, it's remarkably easy to see that the machines—universal gate-based machines—are doing something quantum. What's not easy to see is if they'll ever break away from their scaling challenges and become useful, large scale machines.
This is not the same as universal, gate-based computation, which does take an actual program containing instructions, and executes those instructions more-or-less sequentially, just as any computer programmer would expect. The state of the system begins with what is essentially equivalent to a large array of 0's, and you manipulate it accordingly, without resorting to evolution as your primary computational means of marching forward. In fact, left alone, these computers are designed to remain static, like an ordinary computer. (However, they still couple with the environment and decohere, which is one of the major challenges we, as humanity, face in building a useful quantum computer.) This is what Rigetti, IBM, Google, HRL, Amazon, IonQ, ColdQuanta, etc. are doing. They use superconducting transmon qubits, ion qubits, neutral atom qubits, or silicon quantum dot qubits. (There are other companies and qubit technologies still.)
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.
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.
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
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).
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?
Quantum state: https://en.wikipedia.org/wiki/Quantum_state
Quantum logic; quantum logical operators: https://en.wikipedia.org/wiki/Quantum_logic
> 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.
Quantum simulator: https://en.wikipedia.org/wiki/Quantum_simulator
- [ ] Maybe Twistor theory has insight into a classical geometrical formulation that could be run on a non-QC?
Amplituhedron: https://en.wikipedia.org/wiki/Amplituhedron
[Photon] wave-particle constructive superpositions approximate which operators, which may form a neat topology like this:
- [ ] > A research question for a new school year:
> The classical logical operators form a neat topology. Should we expect there to be such symmetry and structure amongst the quantum operators as well? https://commons.m.wikimedia.org/wiki/File:Logical_connective...
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.
How to best quantize reals into matrices (~= tensors)? https://en.wikipedia.org/wiki/Quantization_(signal_processin...
> 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.
Here's "Quantum Algorithm Zoo" by Microsoft Quantum: https://quantumalgorithmzoo.org/
And "Timeline of quantum computing and communication" https://en.wikipedia.org/wiki/Timeline_of_quantum_computing_...
I have a hard time with the idea that the outcome of the ultimate quantum simulation is a collapsed float.
Quantum Monte Carlo: https://en.wikipedia.org/wiki/Quantum_Monte_Carlo :
> 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?
Propagation of Uncertainty ... Numerical stability ... Chaotic convergence, ultimately, apparently: https://en.wikipedia.org/wiki/Propagation_of_uncertainty
"Multi-qubit quantum logic operations with ion-implanted donor spins in silicon" (2022) https://scholar.google.com/citations?view_op=view_citation&h... https://meetings.aps.org/Meeting/MAR22/Session/G39.1 (Veritasium video)
> 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?
"Researchers store a quantum bit for a record-breaking 20 milliseconds" (2022) https://phys.org/news/2022-03-quantum-bit-fora-record-breaki...
> 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?
Q: "Ask HN: What's the Equivalent of 'Hello, World' for a Quantum Computer?" https://news.ycombinator.com/item?id=22707580 [ IBM Qiskit, Microsoft Q#, Google TFQ TensorFlow Quantum, Google Cirq ([NumFOCUS,] SymPy) ]
- https://www.tensorflow.org/quantum/tutorials/hello_many_worl...
A: Set a register to zero and see how many times it reads as zero: A) in the local software simulator; and B) with just one modern day qubit register.
As far as I know, nobody in the industry is making absurd claims about their current offerings. That said, some claims of projected growth I've seen appear to be batshit.
And, I believe that you're wrong: Google's quantum supremacy result appears to be genuine, and IBM's refutation was, essentially, "we can use a ginormous supercomputer to match that" -- not your ordinary desktop.