Researchers from the National Institute of Standards and Technology (NIST) and Xanadu, a young Canada-based quantum computing company, have reported developing a full-stack, photonic quantum computer able to carry out three important quantum algorithms, including one useful in quantum chemistry and another for graph similarity. Their work is published in Nature today (Quantum circuits with many photons on a programmable nanophotonic chip).

Photonic-based quantum computing has received somewhat less attention than superconducting and trapped ion-based systems. Photonic quantum computing advocates say it has advantages over other approaches, not least is the room temperature operation of photonic chips and reduced susceptibility to some of the noise vulnerabilities associated with other qubit technologies. That said the photon detectors require cryogenic temperatures although overall the apparatus is still less complicated and cumbersome than what’s required for other qubit technologies.

Xanadu has a good short video describing the tech. Briefly, there are three components to the system: 1) squeezer, 2) interferometer, and 3) photon detector. Using laser input, the squeezers (resonators) generate a special quantum state, a squeezed state, essentially forming the qubits (of superposed photons). The qubits are carried via a wave guide through a network or beam splitters and phase shifters which comprise the interferometer. Think of the interferometer as the controllable set of gates applied to the qubits. The outputs are entangled photons whose state and number is counted and interpreted.

The researchers write in their paper:

“Present day photonic quantum computers have been limited either to non-deterministic operation, low photon numbers and rates, or fixed random gate sequences. Here we introduce a full-stack hardware-software system for executing many-photon quantum circuits using integrated nanophotonics: a programmable chip, operating at room temperature and interfaced with a fully automated control system. It enables remote users to execute quantum algorithms requiring up to eight modes of strongly squeezed vacuum initialized as two- mode squeezed states in single temporal modes, a fully general and programmable four-mode interferometer, and genuine photon number-resolving readout on all outputs.

“Multi-photon detection events with photon numbers and rates exceeding any previous quantum optical demonstration are made possible by strong squeezing and high sampling rates. We verify the non-classicality of the device output, and use the platform to carry out proof-of-principle demonstrations of three quantum algorithms: Gaussian boson sampling, molecular vibronic spectra, and graph similarity.”

The figure below, taken from the Nature article, shows the experimental system setup.

The researchers note that until now, no photonic machine has been demonstrated that is simultaneously dynamically programmable, readily scalable to hundreds of modes and photons, and able to access a class of quantum circuits that could not, when the system size is scaled, be efficiently simulated by classical hardware. They write, “We report results from a new device based on a programmable nanophotonic chip which includes all of these capabilities in a single scalable and unified machine…While our device, at its current scale, can be readily simulated by a classical computer, the architecture and platform developed can potentially enable future generations of such machines to exit this regime and perform tasks that are not practically simulable by classical systems.”

The researchers ran tests around three classes of problems – Gaussian boson sampling, vibronic spectra, and graph similarity – and the results are best read directly in the paper. All three approaches show promise for being able, when run on quantum computers, to solve problems beyond the capacity of classical computers. The researchers were encouraged on all fronts but acknowledge the scale of their work now is not beyond classical computers.

The recent work is significant although as pointed out by Ulrik Andersen in a Nature news article in the same issue containing the paper, “Without doubt, the authors’ demonstration of quantum sampling on a programmable photonic chip using highly squeezed states is remarkable and represents a milestone in this field. However, the number of commercial applications that can be implemented using the current architecture is limited. Completely different platforms are required to run heftier algorithms, such as Shor’s algorithm for factoring large numbers into prime numbers, in an error-free manner. Fortunately, such platforms (also based on squeezed states) have been proposed, and their implementation constitutes the next step towards constructing a full-blown optical quantum computer.”

Scaling up is an important consideration noted by the researchers: “An important factor in assessing the viability of the platform presented is the scalability of this approach. What improvements to the platform and design are required in order to scale the system size to a level where quantum advantage could potentially be achieved? To answer this, we fix a target of 100 modes, which in our architecture would require: 50 squeezers operating with squeezing factors of r ≈ 1, a universal 50-spatial-mode interferometer, and 100 PNR detector channels. We also stipulate, as a rough estimate, that such a machine should incur no more than 3 dB of loss in the interferometer; this criterion is especially demanding, since the interferometer loss scales with the number of modes. Events with hundreds of photons would be detectable with such a machine.”

The researchers suggest a number of manufacturing improvements which would move them closer to the goal. It will be interesting to monitor Photonics-based quantum computing’s progress as several companies and working in the area.

Link to Nature paper: https://www.nature.com/articles/s41586-021-03202-1
Link to Nature news review: https://www.nature.com/articles/d41586-021-00488-z

Feature image: Xanadu’s photonic chip

Excerpt from the paper describing the apparatus details:

• A custom modulated pump laser source producing a regular pulse train (100 kHz repetition rate) of 1.5ns duration rectangular pulses.
• An electrically and optically packaged chip that synthesizes a programmable eight-mode Gaussian state with temporal mode characteristics appropriate for photon number resolving readout.
• A locking system which serves to align and stabilize the resonance wavelengths of the on-chip squeezer resonators.
• An array of digital-to-analog converters (DACs) for programming phase shifter voltages on the chip.
• An array of low-loss (off-chip) wavelength filters to sup- press unwanted light, passing only wavelengths close to the signal and idler for detection.
• A detection system, which consists of an array of eight transition-edge sensor (TES) detectors for photon number-resolving readout, and the auxiliary equipment required to operate and acquire data from them.