Scientists Store Single Photons for 180 Microseconds in a Record Quantum Memory Breakthrough

Researchers at ICFO (Institut de Ciències Fotòniques) in Barcelona have demonstrated a solid-state quantum memory that stores individual photons for up to 180 microseconds while preserving their quantum properties — a record for this type of absorptive memory using spin rephasing. The work, published in Physical Review Letters, represents a meaningful step toward the quantum repeaters that a practical quantum internet will require.

Why storing a photon is harder than it sounds

In classical computing, storing information is routine. Write bits to memory, read them back later. In quantum computing and quantum communication, storage is fundamentally harder because quantum information cannot be copied — the no-cloning theorem prohibits duplicating an unknown quantum state.

A quantum memory must store a photon's quantum state (its polarization, phase, or entanglement properties) and release it later without measuring or destroying that state. Any interaction with the environment causes decoherence — the quantum information leaks away.

For quantum communication networks, memories are essential because photons travel at the speed of light. If you need to synchronize quantum operations between distant nodes, or implement quantum repeaters that extend communication range, you need to hold photons temporarily without losing their quantum character.

What the ICFO team achieved

The experiment used a rare-earth-doped crystal combined with spin rephasing — a technique that counteracts decoherence by reversing the phase evolution that causes quantum information to degrade during storage.

Key results:

• Single-photon storage time: Up to 180 microseconds with preserved nonclassical correlations
• Signal-idler cross-correlation: 4.6 ± 0.4, well above the classical bound
• Equivalent fiber distance: Roughly 36 kilometers (the distance light travels in 180 microseconds)
• Classical light storage: Approximately 3 milliseconds in the same system, suggesting room for improvement in single-photon storage with better magnetic field control

The distinction between classical and single-photon storage times is important. Storing bright classical pulses is easier because you have many photons to work with. Storing a single photon — the unit of quantum information — while maintaining quantum correlations is the harder and more useful achievement.

Spin rephasing explained

When a quantum state is stored in a material, interactions with the atomic environment cause different parts of the state to evolve at slightly different rates. This dephasing destroys the quantum coherence that makes the stored information useful.

Spin rephasing reverses this process. By applying carefully timed electromagnetic pulses, researchers can undo the dephasing and restore the original quantum state — analogous to refocusing a blurred image.

The ICFO team's implementation in a rare-earth-doped crystal is significant because solid-state memories are more practical for deployment than alternatives requiring ultra-cold temperatures or complex vacuum systems.

Why 180 microseconds matters

180 microseconds may not sound impressive compared to classical storage measured in years. But in quantum communication, the relevant metric is what you can do with the stored photon before it decoheres.

At 180 microseconds, a stored photon could:

• Participate in synchronization protocols between quantum network nodes
• Enable quantum repeater operations that extend entanglement distribution beyond direct transmission limits
• Support quantum error correction routines that require temporary storage
• Bridge timing gaps in quantum key distribution networks

The researchers estimate that moderate improvements in magnetic field control could push single-photon storage times significantly closer to the classical light storage benchmark of 3 milliseconds — which would further expand the range of practical applications.

The quantum internet context

A quantum internet — a network for transmitting quantum information between quantum computers, sensors, and communication devices — requires several components:

1. Quantum sources that generate entangled photon pairs
2. Quantum channels (fiber optic cables) to transmit photons
3. Quantum memories to temporarily store photons at network nodes
4. Quantum repeaters that extend range beyond fiber loss limits
5. Quantum processors that use the transmitted information

Quantum memories have been the bottleneck. Without reliable storage, repeaters cannot function, and network nodes cannot synchronize operations. This experiment addresses that bottleneck directly.

The telecom-heralded nature of the experiment is also significant. The photons used are compatible with existing telecommunications fiber infrastructure — meaning a future quantum network could share physical infrastructure with the classical internet.

Comparison to previous records

Solid-state quantum memories using spin rephasing have been pursued for years, but single-photon storage times have been limited. The ICFO result advances the state of the art for absorptive memories — systems that absorb photons into the material and re-emit them later.

Alternative approaches include:

• Electromagnetically induced transparency (EIT) memories in atomic gases — longer storage times but requiring complex laboratory conditions
• Cavity-based memories — high efficiency but challenging to scale
• Diamond NV center memories — room temperature operation but shorter storage times

The solid-state, rare-earth-doped approach occupies a sweet spot of practicality and performance that makes it a leading candidate for quantum network deployment.

What comes next

The researchers themselves identify the path forward:

• Better magnetic field control to approach the 3-millisecond classical storage benchmark for single photons
• Integration with telecom fiber networks for real-world quantum communication tests
• Scaling to multi-mode memories that store multiple photons simultaneously
• Quantum repeater demonstrations using the memory as a core component

Practical quantum internet deployment remains years away. But each record — 180 microseconds today, potentially milliseconds tomorrow — compresses that timeline.

Broader significance

This result sits at the intersection of fundamental physics and engineering infrastructure. It does not make quantum computing commercially viable on its own. But quantum communication and quantum computing are complementary — a quantum internet connects quantum processors the way the classical internet connects classical computers.

For scientists and engineers tracking quantum technology progress, the ICFO result is a concrete datapoint: solid-state quantum memories are improving on a measurable trajectory, using techniques that could deploy in real network hardware.

The quantum internet is not here yet. But it now has a memory — and that memory lasts 180 microseconds longer than it did before.