How powerful are photonic quantum processors?


Quantum applied sciences are paving the way in which for the way forward for computing, going past classical computation capabilities. Among the many numerous platforms accessible for quantum data processing, photonic quantum processors present promise attributable to their scalability and potential for direct deployment in quantum networks. Nevertheless, to gauge their true energy, it’s important to evaluate the efficiency of those photonic programs. On this weblog submit, we focus on a efficiency metric for quantum computer systems known as Quantum Quantity and the way it may be tailored to photonic processors. Technical particulars might be present in our current publication (arXiv preprint). 

Quantum Quantity as a Efficiency Metric

A typical quantum computing job includes performing gates on a number of qubits inside a quantum circuit (see Determine 1).

Quantum Volume as a Performance Metric
Determine 1. A quantum circuit of depth 5 with 5 enter qubits.

Present quantum processors are constructed out of a small variety of noisy qubits, which is dubbed as noisy intermediate-scale quantum (NISQ) know-how. One could surprise to what extent these noisy machines carry out faithfully, i.e., what number of extra gates we will apply (enhance circuit depth) or what number of extra qubits we will add (enhance circuit width) earlier than the quantum data is totally overwhelmed by the noise, and our quantum processor turns into a random machine. That is the essence of a metric known as Quantum Quantity (QV), which was initially proposed by IBM. Since then, it has been used to report the progress in quantum computing platforms primarily based on superconducting circuits and trapped ions. The QV is commonly reported in an exponential type as 2d, the place d represents the most important circuit dimension (i.e., width or depth) of a trustworthy processor. In what follows, we briefly evaluation photonic quantum processors and the challenges dealing with their realization; subsequent, we current the minimal optical properties required to attain a QV of two10, which is a typical worth reported for frequent quantum computing platforms akin to superconducting qubits or trapped ions. 

Photonic Quantum Processors: Benefits and Challenges

Quantum photonics emerges as a promising platform for scalable quantum data processing (presumably at room temperature). It permits the direct deployment of quantum networks (see our earlier weblog submit) by serving as a repeater for quantum error correction or as a server for distributed quantum computing sources. Photonic qubits, which might be coherent pulses or single photons, type the idea of quantum photonics. Coherent communication methods, akin to homodyne detectors, are employed for measuring qubits encoded in coherent pulses (steady variable) qubits, whereas single-photon detectors are used to measure single-photon-based (discrete variable) qubits. Nevertheless, a problem in photonics is the issue in storing qubits and making use of gates on them as they’re touring on the pace of sunshine.

Measurement-Based mostly Quantum Computing

To deal with the storage problem, photonic quantum processors usually make the most of measurement-based quantum computing (MBQC) schemes as an alternative of circuit-based quantum computing (CBQC) schemes appropriate for stationary qubits. In MBQC, photonic chips generate a stream of photonic qubits which undergo a collection of interferometers (i.e., delay traces, beam splitters, and part shifters) and are ultimately directed in direction of photonic detectors for measurement. Numerous gates are realized by totally different sequences of measurement bases. For instance, a small quantum circuit on three qubits (as proven under) might be carried out by a collection of measurements on 42 pulses within the type of 7 cases of 6 synchronous pulses touring by a 6-channel photonic chip.

Measurement-Based Quantum Computing
Determine 2. Schematic MBQC implementation of a depth-one quantum circuit of three qubits (consisting of a CNOT gate and a rotation gate as proven on high) on a photonic chip. Six waveguides (numbered 1 to six) are used to hold 6 photonic qubits (proven as touring spheres) in parallel. Waveguides 1, 3, and 5 carry the info qubits, whereas waveguides 2, 4, and 6 are auxiliary qubits for MBQC implementation. Preliminary delay traces (black circle) couple consecutive qubits on the identical waveguide, and the following beam splitters couple qubits on neighboring waveguides. The letters X, Y, and Z on the detectors point out the measurement foundation. The measurement outcomes are transmitted from the detectors to a classical pc for doable digitization and additional processing. The selection of bases and the measurement outcomes decide the ensuing gates.

Quantum Quantity Evaluation for Photonic Quantum Processors

The foremost supply of noise in photonic programs is photon loss (or optical sign attenuation) because the quantum sign travels by waveguides and is in the end measured on the detectors. In our current paper, we found a mapping from a lossy quantum photonic chip to a circuit composed of noisy gates, enabling the computation of the QV for photonic processors. Our findings point out that photonic {hardware} would require enhancements to scale back losses to roughly 10% (or 0.5 dB) and a squeezing price of 18 dB for coherent-pulse qubits to attain a QV of two10. To place numbers in perspective, the document squeezing price ever reported in optics labs around the globe is 15 dB. Additionally, most photon loss occasions happen on the waveguide interface when the sign enters the chip and is measured on the detector. A typical worth of loss price on the interface is 20% (or 1 dB). Due to this fact, our outcomes indicate that {hardware} high quality must be improved additional to fulfill the specified ranges of compacting and loss charges, though the hole just isn’t too massive. 


Photonic quantum processors provide thrilling prospects for quantum data processing. Evaluating their energy utilizing metrics like Quantum Quantity permits significant comparisons with different quantum applied sciences. Regardless of the challenges posed by photon loss, steady developments in photonic {hardware} and encoding schemes will result in enhanced efficiency. On the time of penning this weblog submit, a common photonic processor primarily based on MBQC has not been realized within the laboratory. On this regard, our QV framework is a necessary instrument for {hardware} analysis and growth efforts for use for reporting progress, figuring out bottlenecks, and designing a know-how roadmap.

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