Amazon achieves quantum network milestone, what is quantum network

On September 12, 2022, Harvard University and Amazon Web Services (AWS) launched a strategic alliance to advance fundamental research and innovation in quantum networks. on November 3, a joint team of Harvard and AWS scientists published a research paper in the journal Science [1] discussing quantum memory that can operate at higher temperatures: this allows quantum communication networks, a fundamental component of quantum communication networks with reduced cost and increased reliability.

01Commercialization Milestone: Reducing Temperature-Related Decoherence

A joint team of Harvard and AWS scientists worked on a leading quantum memory platform: the cooperation of silicon vacancies in diamond crystals, and discovered how to reduce their temperature-dependent decoherence. For silicon vacancies in diamond, the decoherence is driven by the interaction between the state of the encoded quantum bits and phonons, which begin to appear in diamond at a temperature of 1 kelvin (-272.15°C). Using the strain in the diamond lattice, scientists were able to increase the energy of the transition between quantum bit states so that it only interacts with higher energy phonons (which appear at 20 Kelvin): ensuring that no hot phonons can drive this transition even at 4 Kelvin. Operating at 4 Kelvin was a commercially important milestone, as it allowed the team to transition from a helium dilution cooler to a more reliable and cost-effective technology.

Spin-photon entanglement at high temperature. Figure (C) shows the reconstructed density matrix using resonant electron readout at 1.5 K; (E) is the reconstructed optical-nuclear spin entanglement density matrix at 100 mK and (F) 4.3 K.

In the same paper, quantum gates between silicon vacancy electron quantum bits and silicon nuclear spin-encoded quantum bits, which are less sensitive to environmental noise, are also shown. Using these interactions was able to demonstrate high-fidelity information exchange between light, electron quantum bits and nuclear quantum bits. These results show that silicon vacancies have the ability to store and process multiple quantum bits at once: one of the key requirements for scalable quantum networks.

Interaction between photons, silicon vacancy electron quantum bits and nuclear quantum bits.(A) Quantum levels in the center of silicon vacancy in diamond. The black lines indicate the energies of the different states of the silicon vacancy. On the far left, in the absence of an external magnetic field, the silicon vacancy has only two energy levels, a "ground state" and an "excited state". If the laser hits the silicon vacancy, it will transition to the excited state, then emit a photon and jump back to the ground state. In the presence of a magnetic field (the middle part of the black line), the ground state and the excited state have different energies, depending on the direction of the magnetic field of the electron. This magnetic field is called the "spin" of the electron and encodes a quantum bit. On the far right, the effect of the nuclear quantum bit is added, which further shifts the energy up or down depending on the nuclear spin. Electrically controlled pulses, 'MW' and 'RF', can flip the spins of the nucleus and electron between up and down, allowing quantum manipulation between the quantum bit encoded on the electron and the quantum bit encoded in the nucleus.B) Scanning electron microscope used to take image of the quantum memory device. The white scale bar is 3 parts per million meters (10,000ths of an inch). The gray suspended material in the center is a thin diamond wire called a waveguide that captures and directs light along its length. The yellow area around it is a thin wire made of gold, which transmits control pulses that change the state of the quantum bits.(C) A magnified photograph of the quantum memory device. The holes in the waveguide form mirrors that trap photons near the silicon vacancies, allowing them to capture photons efficiently. The schematic below the picture shows how the interaction with the photons proceeds: depending on the quantum state of the electron, the photons are reflected in different ways, allowing quantum information to be transferred from the photon to the electron spin.

02Quantum repeaters: distributing quantum entanglement in communication to remote users

The security of quantum communication comes from a principle called the "unclonable theorem", which states that it is impossible to faithfully clone an unknown quantum state. This can be understood as follows: If an eavesdropper tries to intercept quantum information by measuring one or two quantum bits, their measurement will collapse the two bits to point in a well-defined direction and break the entanglement, in which case the measurements performed by the communicating party will sometimes not correlate in the expected way. By entangling, the attacker is also unable to entangle a third quantum bit to store information, suggesting that only two quantum bits can be entangled to the maximum extent. This allows both parties to check if the correlation is not shared with a third party by measuring the correlation of their quantum bit pairs. Two communicating parties can openly compare a random subset of their bits. If their bits are not correlated in the expected way, then the communicating parties know that someone is trying to eavesdrop on their communication.

Entanglement distribution in quantum communication. Existing systems for distributing entangled quantum bits to distant locations use lasers and nonlinear crystals to generate entangled photon pairs. This approach is hampered by photon losses as they are transmitted over long fiber links.

Most entangled distribution systems rely on photons to transmit their quantum information. Photons or individual light particles are effective quantum bits for quantum communication because they can be transmitted over long distances over fiber optic cables. Entangled photons are also easily generated by irradiating a laser with a special nonlinear optical crystal; however, photons transmitted through telecommunication fibers are eventually absorbed or scattered. This sets the maximum distance for fiber-optic-based quantum communication using only photons, which is typically about 100 km.

In classical fiber-optic networks, this loss is compensated by the introduction of amplifiers, which enhance the weak signal by generating many copies of the input photons. However, due to the unclonability theorem, the amplifier cannot perfectly replicate the quantum state. The noise it generates during amplification destroys the entanglement required for quantum communication. Instead of an amplifier, we need to build a new device, a quantum repeater, which can correct for photon losses by capturing and storing (instead of measuring) the quantum bits encoded in the photon without destroying the quantum properties of the communication information.

Quantum repeaters extend the range of quantum communication through a process known as "entanglement swapping.

Joint quantum measurement and entanglement exchange. While measuring the state of one quantum bit in an entangled pair breaks entanglement, measuring the joint state of two quantum bits preserves the entangled state. These joint quantum measurements can be used to exchange entanglement, using two entangled pairs of quantum bits to create entanglement where it did not exist before.

Quantum repeaters use joint quantum measurements to avoid breaking entanglement. By measuring collective properties such as "do the quantum bits point in the same or opposite direction?" and other collective properties. None of the quantum bits reveal any information and the entanglement is preserved. An example of this joint measurement is called a quantum parity bit measurement: if the quantum bits point in the same direction, the quantum parity bit measurement outputs +1, and if they point in the opposite direction, it outputs -1.

A quantum repeater divides a section of quantum network into two parts, as shown in the right half of the figure below. It contains two quantum memories, B and C, which capture photons and store their quantum bits without measuring them. In this way, the repeater can store each quantum bit from two separate pairs of entangled bits. Once the repeater has captured and stored a photon from both sides of the link, it performs a quantum parity bit measurement on the stored quantum bits. Using the results of the parity bit measurement and the properties of the initial entangled quantum bits, we can know the parity of the remaining two quantum bits without knowing which direction they are pointing. This "swaps" the entanglement to the end of the link without requiring a single photon to cross the full distance between them. In particular, the repeater does not have to be trusted by Alice and Bob to ensure secure communication; if an adversary tries to obtain information about the key by measuring at the repeater, Alice and Bob will see detectable changes in their measurements.

Quantum Repeaters and Quantum Networks. Quantum repeaters can extend the range of a quantum network by replacing long fiber links that may lose entangled photons with a series of shorter links. Each shorter link can attempt to distribute entanglement multiple times between the repeaters, while the successfully transmitted entanglement is stored in quantum memory. As entanglement is established on each link, the quantum repeater can perform entanglement exchange to distribute it to the end user.

Repeaters that can capture and store quantum information in photons can increase the entanglement generation rate through long quantum network links. Without a repeater, the probability of any entangled photon reaching Alice and Bob is low. By linking together repeaters that can share entanglement and perform entanglement exchange with neighboring links, quantum networks divide quantum communication links into many smaller links; if a particular link fails to establish entanglement, the remaining repeaters can continue to store the entangled bits they receive while the failed link tries to re-establish entanglement. When entanglement is established on each link, the repeater performs an entanglement exchange to pass the entangled pair to the end user. By using entanglement switching, repeater chains can reliably distribute entanglement to users over longer distances.

03Research implications: operating at higher temperatures, significant cost reduction

The most difficult part of a quantum repeater to implement is the device that can capture and store photons, the so-called quantum memory. Quantum memories are still in their infancy, and many promising quantum memories store quantum bits that degrade ("decoherence") if they do not operate at temperatures close to absolute zero. This requirement means that quantum repeaters require helium dilution chillers, which are both cumbersome and expensive: complex laser cooling systems are needed to avoid decoherence driven by thermal vibrations (called phonons).

This research advance paves the way for widely deployed, reliable quantum repeaters that will enable eavesdrop-proof communication and private access to quantum computers. This research could reduce the cost of supercooling, which is typically required to keep memories cold, and improve the performance and reliability of quantum repeaters needed to extend network distances [3]. Harvard and AWS researchers continue to work on improving the availability and quality of quantum bit host materials, improving the scalability of repeater hardware, and developing the theoretical understanding of networks and different quantum bits that will be necessary to make this technology publicly available.

Paper author David Levonian said, "Increasing the operating temperature to a point where cryogenic systems will cost 10 times less than what would otherwise be needed and be smaller, really starts to move it [quantum memory] into racks that might be in data centers." He stressed that more work needs to be done before such advances can be commercialized and before entanglement-based quantum networks using quantum repeaters can become widespread, as much of the work associated with quantum networks, especially with quantum repeaters, is still being done in the lab.

"Next, we'll put a timeline on it that will set up a network of these repeater devices to show that a QKD network can be built and implemented using off-the-shelf products."

Reference links：

[1]https://www.science.org/doi/10.1126/science.add9771

[2]https://www.nature.com/articles/s41586-020-2103-5

[3]https://www.insidequantumtechnology.com/news-archive/aws-claims-major-quantum-networking-research-advancement/