Mediaboss Marketing

Article by Raksha Singla

The intrinsic characteristics of quantum computer hardware introduce a probabilistic element to their behavior. Youve likely encountered this explanation repeatedly, and it essentially signifies that when an input state undergoes processing in a quantum circuit, the resulting output state is very well defined by state evolution formula, but its measurement is probabilistic. Instead of getting access to the result of the calculation, as one would expect in classical computing, one obtain one measurement of this output state, the specific measurement following a probabilistic distribution obtained from the quantum state through what is known as the Born rule. For a software developer, this feature can be seen as a major bottleneck compared to classical computation, but on the other hand, this feature is actually giving the developer an indirect access to the huge computation space of quantum states marking a revolutionary shift in the computing landscape and enabling solving hard problems for classical computing.

A shot represents the execution of a quantum circuit and the corresponding data collected by the hardware at the output during that single run. Given the probabilistic nature of the system, conducting multiple iterations of the system (obtaining many shots) is necessary to gather data for statistical analysis of the algorithms operation.

The concept of shots is universally embraced by most quantum providers and forms an essential element in any quantum algorithm. Its implementation and management can vary among different frameworks, depending on the specific characteristics of the hardware system and the compute space.

How do we define shots at Quandela?

Our computing architecture works with linear optical elements forming the quantum circuit performing the processing action on states of photons (input source) in the Fock space (for an explanation of the Fock space see here). Detection of 1 or multiple photons by the detectors at the output define a single execution of a quantum experiment or an act of computing corresponding to the processing of data encoded into the output photons.

Our source is a single photon source emitting photons at a fixed rate into the chip implementing the circuit designed by a user. These photons may undergo absorption at various points within the hardware. Nevertheless, whenever at least one photon is detected at the optical chips output (termed as a photon-coincidence event), it marks the end of a single execution and the measured output constitutes a data sample. The count of such occurrences while running an algorithm represents our shot count.

A user may not necessarily want to sample single photon detections; they may specifically desire samples with a certain number N (>1) of photon coincidences and request these as the output. In such cases, the system may need to be run with a number of shots exceeding the requested number of samples, as multiple photon coincidences are anticipated. Recognizing this user preference, we have incorporated a tool in Perceval to estimate the necessary number of shots based on the users desired sample count. This tool conducts the estimation by considering the unitary of the implemented circuit, the input state, and the performance characteristics of the device.

How Shots will Revolutionize our Users Experience ?

Access to Quantum State Distribution:

In the light of the definition that characterizes Shots as the output detected during each circuit execution, they offer direct access to the probability distribution of a quantum state.

Predictible output rate:

In a photonic system characterized by instantaneous gate applications and a complex input state preparation timing (see detail on demultiplexing here), the time capture of shots clearly indicating the end of a single execution is exhibiting variability attributed to this input state time sequence, the actual configured circuit, and system transmittance factor. Working with shots guarantee a predictable output rate independent of these fluctuations.

Simplified User Interactions:

The incorporation of shots not only seeks to standardize user interactions with running algorithms on our Quantum Processing Unit (QPU) through our cloud services but also provides them with a more standardized parameter for understanding their resource needs. This enhancement contributes to a clearer and more consistent measure.

Predictability for Time and Cost:

Shots, being highly predictable, offer the most reliable means to estimate the time and cost of running an algorithm.

This stability in parameter counting results in fixed pricing, ensuring fairness to users and independence from the variability of the performance of the physical QPU device.

The rest is here:
Perceval Tech note: Introducing shots in a linear optic quantum computing framework - Medium