To connect the countless devices across the globe, the internet uses hardware components. Similarly, a quantum network requires quantum hardware to enable communication and transmission of quantum information between interconnected quantum nodes (computers).

The main components of a quantum network

A quantum network depends on two main components:

• a quantum processor node, and
• an interface to link two or more nodes.

A quantum processor node can transmit and receive information. The function of a quantum processor node ranges from performing simple operations, like measuring and preparing a single qubit to operate as a large-scale quantum computer.

There are different implementations for quantum processor nodes. Among these implementations are nitrogen-vacancy (NV) centers in diamond. These NV centers are a defect in the diamond crystal, where a carbon atom is replaced by a nitrogen atom and an adjacent empty site - a vacancy. An NV centre can trap an electron whose spin can be used as a qubit. Additionally, the electron spin can also interact with light, more specifically with photons - single light particles. Light emitted by NV centers can be used as the interface to link two or more nodes.

The advantage of NV-centre quantum processor nodes with a photon-based interface is that the NV-centre remains stationary as the photon is transmitted over a fibre connection or through the air, allowing interaction between distant nodes. Two NV centres and their emitted photons form an elementary link in a quantum network.

Two or more elementary links can be entangled, with appropriate measurements performed on the photons, emitted by each NV centre. Entanglement is essential in quantum communication since it forms the cornerstone of several techniques – from quantum key distribution (QKD) or teleportation through to quantum repeaters.

How does it work?

The researchers at QuTech’s Quantum Internet Division are creating an isolated and stable quantum system on a chip by using NV centres in diamond as quantum processor nodes. A diamond containing an NV-centre defect is placed on a chip, which is then mounted in a cryostat and cooled down to about 4 Kelvin. The NV centre traps an electron, whose spin behaves like a tiny magnet. This spin comprises the communication qubit, and it can be controlled and manipulated using magnetic fields. The qubit’s state is initialised and read out using laser beams. Under the right conditions, the laser illumination causes the NV centre to emit light that correlates to the spin state of the qubit. By detecting the emitted light using single-photon detectors, the state of the qubit is determined.

From qubits to entanglement

To create entanglement between two quantum nodes, both nodes must be initialised and synchronised. Qubits hosted in the quantum nodes are initialised in a superposition state (0 and 1) using optical (laser) and microwave (magnetic) controls. A bright laser pulse causes the qubit to emit a photon only if it is in state 0, thereby creating entanglement between the photon and the qubit. This photon is carefully collected and transmitted to a shared location between the two nodes, using an optical fibre. Since the two nodes are synchronised, the photons from each node arrive simultaneously at a beam splitter at a shared location. By monitoring clicks using a pair of single-photon detectors placed behind the beam splitter, successful entanglement between the two qubits can be confirmed when a photon is measured by one of the two detectors. The resulting entangled communication qubits can then be used for further network operations.

The challenges ahead for quantum networks

Compatible co-existence

The entanglement procedure is inherently probabilistic, due to the odds of losing a photon before it reaches the detectors. Processes leading to photon losses must therefore be minimised to increase the chances of success. To scale up the network, for example where the nodes are in different cities, light emitted by the NV centres must be compatible with existing telecom infrastructure. Quantum frequency conversion modules can be used to convert the emitted light into the telecom band.

Timing is everything

Synchronisation is a crucial aspect of successful entanglement. Timing requirements for emitted photons are very strict – the time difference between NV centres needs to be less than one nanosecond (a billionth of a second). Maintaining synchronisation between nodes in a lab environment is challenging and over deployed fibre connections is even more complicated. Many technical solutions are needed to bring a large-scale quantum network, i.e., a quantum internet closer to reality, addressing both synchronisation and compatibility issues.

Bridging the distance

Another curve in the road comes from the fact that fundamentally single photons entangled with NV centres cannot be amplified. This limits the realisable entanglement rate – a lost photon cannot be recovered, instead, the process of generating entangled photons needs to be restarted. As the distance between the nodes increases, losing photons becomes more likely.

To enable the transmission of qubits over long distances, researchers at QuTech are working on developing quantum repeaters. The main concept of a quantum repeater is to break up the long entanglement distance into smaller segments. Demonstrating the quantum repeater principle is a crucial milestone for paving the way to large-scale quantum networks.