Photonic Quantum Computers

Collaborators on the German quantum computing effort “PhoQuant” are addressing system architecture, operability, and integration in their development of a photonic quantum computer. The effort involves 14 parties, spanning industry, academia, and R&D.

How to Build a Photonic Quantum Computer

A German collaboration is developing a photonic quantum computer, raising several questions. They include: How can the device be miniaturized? What material system is best-suited? And why is this so?

Expectations for quantum computers are high: They are supposed to outperform digital computers and pave the way for solutions that go far beyond the capabilities that artificial intelligence already delivers. They are predicted to crack unbreakable codes, find new materials for superconductors, and help develop medicine for the next pandemic. These are only some of the envisioned outcomes.

So far, the digital computers of today have succeeded in adding integers. These computers do leverage gates to do NAND or XOR operations, for example, but in the end, it is all about integers and their processing in a few specific types of electronic gates.

To be sure, digital computers can do complex calculations. But, at the end of the day, adding integers is what they do: One plus one equals two.

Quantum computers are fundamentally different, starting with their bits, known as qubits, which are the quantum analog to classical bits. Like classical bits, qubits can take the values “one” or “zero.” But unlike classical bits, the actual value remains uncertain until it is measured. This means that the qubit is in a state of coherent superposition of the values it can take. The probability that a qubit takes one possible value, or the other, is continuously changing.

This becomes a particularly powerful feature when qubits controllably interact with other qubits, creating entanglement and thereby exponentially increasing the dimensionality of quantum computing systems. It allows for the parallel computation of all possible outcomes. Even though the state of a qubit collapses to only a single one at the moment it is measured, dedicated quantum computing algorithms are in position to extract probability information. This makes computations using these algorithms more efficient and more powerful than classical computers achieve.

It is obvious to systems designers and engineers that we should leave behind what we know from digital computers and start from scratch when it comes to building quantum computers. Qubits can be mapped to different quantum systems: ions, atomic energy levels or photons. For this reason, different platforms are currently being proposed. One of these proposals is photonic quantum computers.

What is a quantum computer made of?

One-hundred years ago, physicist Erwin Schrödinger carved out his piece of quantum theory. Schrödinger was looking at electrons, and, more precisely, at their behavior as waves and particles. He came up with an equation to explain how electrons move through space and time. Schrödinger and Paul Dirac were later awarded the Nobel Prize for their findings.

At the time, it is doubtful that Schrödinger was thinking about quantum computing. Nevertheless, his equation for the evolution of a wave function is a mathematical way to describe how qubits behave in time and space.

Today, people create quantum states with electrons, photons, and other quantum objects; allow them to interact according to the theories of Schrödinger (and others); and measure the result. The function of a quantum computer is to prepare quantum states, controllably apply transformations/gates on them both individually and collectively, and measure the output quantum state.

A quantum computer requires a complex setup to perform such processes. Figure 1 shows a scheme in which the actual quantum device is at the lowest level of the overall system, with several layers of software and hardware above it. The top layer in this scheme is the cloud access layer. This refers to the fact that today's quantum computers are designed for remote operation, where a user feeds a specific task into it. There, software is used to translate the problem into parameters that can be sent to the quantum control unit — a hardware module that controls the input of the quantum unit. This control describes, for example, how to prepare the qubits and apply operation in single or multiple qubits, as well as the output of the quantum unit, for example, the registration of the measurement of the results. Level 1 in Figure 1 refers to hardware that is specific to the type of qubits.

Figure 1. A real quantum computer needs several layers to transform a task from a user in the cloud (Level 4) into an algorithm (Level 3). A special control system (Level 2, FPGA/ASIC) is required to manage the actual quantum system, including quantum state preparation, quantum processing (Level 1, interferometer), and measurement of the result (Level 1, detector). The result is fed back to the conventional hardware and requires additional software for interpretation.

What is a photonic computer made of?

A digital computer adds and compares numbers based on a hardware element that mainly consists of transistors and capacitors. It relies on currents and electrical charges to make computations. A photonic computer, as its name suggests, uses photons instead.

Why photons? Photons currently offer one of the viable pathways to a quantum computer at room temperature. The generation and processing of photons and photonic qubits is routinely done without excessive cooling. And, photonic integrated circuits (PICs) is happening at a fast pace, further clearing a path to scalable integrated circuits for photonic quantum computers with up to millions of qubits.

Any photonic computer consists of three major parts: a light source; a processing unit that is most often a multichannel interferometer; and a detection system (Figure 2). For a photonic quantum computer, the light source must produce quantum states or qubits. It should be noted that the qubit in this case is an information unit that propagates through the system and interacts there with other qubits. The photons are measured after such operations, and a result is calculated from these

Figure 2. Scheme of a photonic quantum computer.

The PhoQuant project

Spectrogon US - Optical Filters 2024 MR

Backed by €50 million ($54 million) in funding from the German Federal Ministry of Education and Research (BMBF), a group of German companies and research institutions have joined forces in the PhoQuant project. The effort aims to build a photonic quantum computer, made in Germany, that can be accessed worldwide via cloud services. Funding for the initiative runs through 2026.

From a long-term perspective, the project focuses on the development of photonic quantum computing chips, control components, test infrastructure, software, and novel algorithms. The computational core system is planned to work at room temperature. Currently, the detector system is a superconducting nanowire single-photon detector (SNSPD) that works in a small cryostat. All other parts of the system, the laser system, source, demultiplexer, interferometer, control unit, and data acquisition mechanism, are functional at room temperature.

While the science behind the planned system is quite clear, its components need to be optimized to meet the project’s ambitious goals. In addition, system integration toward a PIC-based solution presents its own distinct challenge.

To accomplish both targets, the project partners installed two setups: one with off-the-shelf components for system testing at the University of Paderborn, and a hybrid platform to enable step-by-step miniaturization and integration of the components into a PIC at Fraunhofer Institute for Applied Optics and Precision Engineering IOF (Fraunhofer IOF) in Jena.

The physics of the design chosen by the PhoQuant researchers differs from the qubit model explained earlier; the researchers adopted Gaussian Boson Sampling (GBS) as the main principle of the photonic QC. GBS is based on so-called qumodes, and, more specifically, on squeezed states of light. “Squeezed light” earns its name given that the uncertainty of the field amplitude is squeezed along one of the field quadratures and stretched in the orthogonal quadrature (as is to be expected from the Heisenberg uncertainty principle). Due to such properties, these non-classical states have been used in interferometric measurement, beating the shot noise limit.

The concept of squeezed light can also be expressed via a Fock state, or on a “Photon number” basis. For example, the quantum state could have zero, one, two, or N number photons, each of them with a certain probability. In the GBS scheme, N squeezed states are prepared and sent to a multi-mode interferometer. Following the interference, each output will have a different number of photons coming out. Computing the probability of each configuration becomes computational intractable as the number of photons and number of modes in the interferometer increase. This sampling problem can, for example, be mapped to problems in quantum chemistry, to simulate the vibrionic spectra of molecules, to determine an airport gate assignment, optimize a financial portfolio, or to typical pick-up/delivery problems.

Finally, the system uses a phase-stabilized dual-frequency (780/1560 nm) pulsed laser system to generate the quantum states. Its signals are fed to a second stage where squeezed states are generated in a periodically poled potassium titanyl phosphate crystal.

Processing and measuring the qumodes

For this quantum computing mode, the basic components for manipulating the input quantum states are simple: One is a beam splitter and the other a phase shifter. Qubit processing takes place in Mach-Zehnder interferometers, where a beam is first split into two replicas. Each of these passes through a phase shifter. Finally, the two beams reunite at another beamsplitter, with a controlled phase delay between the two replicas. In this way, the outcome of the interference is controlled by the electrical signals sent to the phase delays. An array of Mach-Zehnder interferometers and phase shifters can in principle perform any arbitrary unitary transformation on the input state of the interferometer.

Figure 3. Photonic integrated chip (PIC) with 4-mode-interferometers.

To put such an array on a PIC, all components must be transferred to the PIC itself. This raises the question of the right material for the photonic processor.

The PhoQuant team’s material of choice is lithium niobate on-insulator (LNOI). This material has several advantages. First, it can be processed in conventional optoelectronic fabs. Second, it has a strong electro-optic coefficient, which allows extremely fast switching (up to GHz rates) of the phase shifters. This is silicon nitride (SiN), which is often used for PIC architectures, requiring the heating of certain elements to achieve phase shift. Heating is slower than electro-optical switching and consumes more power, which also requires post-process cooling.

Obviously, the selection of material is much more complex than these considerations alone. The PhoQuant project teams considered both SiN and LNOI for the project, with extensive simulations and testing. The final decision favored LNOI because of its potential in terms of achieving low losses, high switching speeds, and excellent processability.

Currently, the group at Fraunhofer IOF is using a PIC platform (Figure 3) from industrial quantum technology and photonics solutions provider (and project partner) Q.ANT, with 4-mode interferometers. An upgrade up to 100 modes is planned. And, to measure the number of photons coming from each output of the interferometer, the team selected to SNSPDs, shown in Figure 4 as a prototype(s) with 20 channels for detection.

Figure 4. Prototype of a 20-channel-photon-number-resolving (PNR)-detector. The element uses space multiplexing of superconducting nanowire single-photon detectors (SNSPDs).

The future of photonic quantum computers

As mentioned, expectations for quantum computers are undoubtedly high. Once they achieve supremacy over digital computers, they may advance to break new ground in materials science, mathematics, or biology. However, this supremacy will be limited to very specific problems. Therefore, it seems likely that quantum computing will be part of a computing system that is comparable to other special purpose chips — such as a GPU or the new iteration of neural engines.

Under the PhoQuant initiative, the collaborators are on their way to an integrated photonics-based quantum computer. Individual efforts not only for the quantum system itself, but also for the hardware and software to control and read-out the quantum states. Future computing devices for complex computations will require not only such PICs, but also sophisticated solutions for high-speed data transfer and low power consumption — a well-known advantage of optical computing technology. This may ultimately point to a wider implementation of optoelectronic solutions, as currently developed in projects such as PhoQuant.


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What is Photonic Quantum Computing?

Photonic quantum computing is a type of quantum computing that uses photons as a representation of qubits. It consists of the ring that is used as a photons storage, along with the scattering unit. Determining whether the information they are carrying is 1 or a 0, is left to the direction of the photon travel, but it can also be both 1 and 0 as a product of the quirks of quantum superposition. The scattering unit is used as an encoder, directing the photons onto it as they will enter a cavity containing just a single atom. Interaction between the photon and atom will create a quantum state, such that any changes will affect both of them, without the restrain of the separation distance.

The main advantages are simple components, the ability to run a variety of quantum operations, and most importantly, photonic quantum computers can perform at room temperature, which reduces the size of the extreme cooling systems.

Below is the list of the 6 companies working with photonic quantum computing technology.

6 Photonic Quantum Computing Companies

1. Xanadu Quantum Technologies (Photonics Quantum Computing)

Xanadu Quantum Technologies is a Canadian technology company and a leading photonic quantum computing hardware provider.

Founded in 2016 by CEO Christian Weedbrook, Xanadu’s mission is to build quantum computers that are useful and available to people everywhere. To achieve this mission, the company has taken a full-stack approach and builds hardware, software, and pursues state-of-the-art research with select partners.

Today, enterprises and researchers can begin using Xanadu’s photonic quantum computers through the Xanadu Quantum Cloud (XQC) service and Strawberry Fields application library.

The company is also advancing the field of quantum machine learning (QML) through the development of PennyLane, an open-source project that has become a leading software library among quantum researchers and developers.

2. ORCA Computing

London-based ORCA Computing was set up by experienced scientists and entrepreneurs based on research from Professor Ian Walmsley’s Ultra-fast and Non-linear Quantum Optics Group at the University of Oxford (UK). Within the group, Ian Walmsley, Josh Nunn, and Kris Kaczmarek identified that “short-term” quantum memories could synchronize photonic operations and enable truly scalable quantum computing.

ORCA solves this redundancy problem using the ORCA quantum memory, opening up the promise of quantum photonics without the significant trade-offs of existing approaches.

Based in London, ORCA was founded in 2019 by Ian Walmsley, Richard Murray, Josh Nunn, and Cristina Escoda.

3. PsiQuantum

Made up of a team of quantum physicists, semiconductor, systems, and software engineers, system architects, and more, PsiQuantum is focused on building the world’s first useful quantum computer by taking the photonic approach as they believe in its technical advantages at the scale required for error correction. They attracted headlines with their focus on a 1m qubit quantum computer.

Founded in 2015 by Jeremy O’Brien, Terry Rudolph, Pete Shadbolt, and Mark Thompson, PsiQuantum is based in the heart of technological innovation, Silicon Valley.

4. TundraSystems Global

TundraSystems Global is a Cardiff, Wales-based Photonic Quantum Computing company founded in 2014 by Brian Antao to build from the ground up the many developments from different academic sources such as the University of Bristol, MIT, the UK Quantum Technology Hubs etc. in computational solutions in an all-optical regime using quantum mechanics’ fundamental base.

Its ultimate mission is to design and deliver new quantum technology solutions. The first phase of development is to develop Tundra Quantum Photonics Technology library. This forms part of Tundra System’s strategy, in its quest to develop a complete Quantum Photonics Microprocessor the TundraProcessor. This library should also facilitate the development of the eco-system of Photonic Integrated Circuits to enable the building of complete HPC Systems surrounding the TundraProcessor.

5. Quandela

Quandela is a startup focused on the fabrication of performing devices for research in photonics quantum computing and quantum information.

It fabricates unique solid-state sources of quantum light. These sources are used to develop a new generation of Quantum computers based on the manipulation of light.

This Paris-based photonics startup was founded in 2017 by Niccolo Somaschi, Pascale Senellart and Valerian Giesz.

6. QuiX Quantum

QuiX Quantum is a photonic quantum computing startup, located in Enschede, the Netherlands. The company was founded in 2019 by Dr. Hans van den Vlekkert, a veteran of the industry and serial entrepreneur, Dr. Jelmer Renema, an expert in photonic quantum computing, and a team of professors from the University of Twente.

Consisting of a team of quantum-photonics engineers, some with a background in integrated photonics and some in quantum technologies, QuiX’s goal is the continued disruption of quantum computing with our high-tech, scalable, future-proof, plug-and-play integrated photonic solutions.


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Building Quantum Computers With Photons

Silicon chip creates two-qubit processor

Silicon has given us the computers we have today by allowing billions of transistors to be packed onto a single chip. And it may one day lead to far more powerful computers, now that researchers have demonstrated a silicon chip that manipulates individual photons to create a quantum photonic processor.

“We made a photonic quantum processor, which creates and manipulates two qubits encoded in photons for universal two-qubit quantum computation,” says Xiaogang Qiang, a research associate at the National University of Defense Technology in Changsha, China. Xiaogang was lead author of a paper describing the work that appeared in the September issue of Nature Photonics.

Quantum computing is based on the weird rules of quantum mechanics, which give it the potential to perform computations that traditional computer designs could never achieve—such as quickly breaking cryptographic codes or simulating the big bang. Quantum computers are based on qubits, analogous to the bits in classical computing. But unlike the familiar 1s and 0s of classical computers, qubits can be in superposition, holding more than one state simultaneously and thus expanding their calculating power. They can also be entangled, so that measuring one qubit provides information about the state of another.

Companies such as IBM and Google are hard at work trying to develop devices with enough linked qubits to perform powerful calculations. But so far, they’ve achieved only a few dozen qubits. The leading contenders for qubits are superconducting wires chilled to near absolute zero and trapped ions held in place by lasers. The trouble with these is that as the number of qubits in a system grows, the more likely they are to interact with the outside world, losing their quantum state—called coherence—and becoming useless.

But photons shouldn’t have that problem, says Xiaogang, who built the chip with a team of researchers primarily based at the University of Bristol, in the United Kingdom. “Photons do not interact with [the] environment, so we do not suffer with short coherence time,” he says. Photons can also be manipulated with ultrahigh precision, he says. And, of course, they’re transmitted at light speed. On top of that, there’s the fact that a photonic chip can take advantage of the entire silicon-based infrastructure that the computer industry has built up.

The chip consists of many interferometers, which split the photons into different spatial modes. Each mode passes through a specific waveguide, so having a photon in one waveguide represents a 1, while in another it represents a 0. Knowing which path one photon is following tells you which path its entangled partner is on.

The photons are encoded using thermo-optical phase shifters, which are controlled by electrical voltages. “Different settings of the phase shifters control the photon’s transmission behaviors in the interferometers, enabling different qubit-state encoding and different quantum operations,” Xiaogang says.

To scale up the system to something truly useful, the researchers will have to figure out a way to generate many more identical, entangled photons on the chip. There’s also the engineering challenge of fitting enough phase shifters, beam splitters, and other optical components onto the chip to handle all those photons. But Xiaogang says silicon photonics has shown the capacity for cramming many devices into tight spaces and getting them all to work with high precision, “and thus it in fact is the practical way to implement the ultimate large-scale photonic quantum processor.”


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Quantum computing researchers develop an 8-photon qubit chip

A group of South Korean researchers has successfully developed an integrated quantum circuit chip using photons (light particles). It is a system capable of controlling eight photons using a photonic integrated-circuit chip. With this system, they can explore various quantum phenomena, such as multipartite entanglement resulting from the interaction of the photons.

ETRI's extensive research on silicon-photonic quantum circuits has led to the demonstration of 2-qubit and 4-qubit quantum entanglement, achieving the best performance from a 4-qubit silicon photonics chip. These achievements resulted from their collaborative effort with KAIST and the University of Trento in Italy and were published in the prestigious scientific journals, Photonics Research and APL Photonics.

As a further advancement, ETRI recently demonstrated 6-qubit entanglement using a chip designed to control 8-photonic qubits. The 6-qubit entanglement represents a record-breaking achievement in quantum states based on a silicon-photonic chip.

Quantum circuits based on photonic qubits are among the most promising technologies currently under active research for building a universal quantum computer. Several photonic qubits can be integrated into a tiny silicon chip as small as a fingernail, and a large number of these tiny chips can be connected via optical fibers to form a vast network of qubits, enabling the realization of a universal quantum computer. Photonic quantum computers offer advantages in terms of scalability through optical networking, room-temperature operation, and the low energy consumption.

A photonic qubit can be encoded using a pair of propagation paths of a photon, with one path assigned as 0 and the other as 1. For a 4-qubit circuit, 8 propagation paths are required, and for 8 qubits, 16 paths are needed. Quantum states can be manipulated on a photonic chip, which includes photon sources, optical filters and linear-optic switches, and are finally measured using highly sensitive single-photon detectors.

The 8-qubit chip includes 8 photonic sources and approximately 40 optical switches that control the propagation paths of the photons. About half of these 40 switches are specifically used as linear-optic quantum gates. The setup provides the fundamental framework for a quantum computer by measuring the final quantum states using single-photon detectors.

The research team measured the Hong-Ou-Mandel effect, a fascinating quantum phenomenon in which two different photons entering from different directions can interfere and travel together along the same path. In another notable quantum experiment, they demonstrated a 4-qubit entangled state on a 4-qubit integrated circuit (5mm x 5mm).

Recently, they have expanded their research to 8 photon experiments using an 8-qubit integrated circuit (10mm x 5mm). The researchers plan to fabricate 16-qubit chips within this year, followed by scaling up to 32-qubits as part of their ongoing research toward quantum computation.

Yoon Chun-Ju, Assistant Vice President of the Quantum Research Division of ETRI, said, "We plan to advance our quantum hardware technology for a cloud-based quantum computing service. Our main goal is to develop a lab-scale system to strengthen our research capabilities in quantum computation."

Lee Jong-Moo with ETRI's Quantum Computing Research Section, who led this achievement, stated, "Research for the practical implementation of quantum computers is highly active worldwide. However, extensive long-term research is still needed to realize practical quantum computation, especially to overcome computational errors caused by noise in the quantum processes."