BT Group: Quantum Key Distribution Over Live Telecom Fiber

The Problem

BT Group operates one of the UK’s largest fiber networks. That infrastructure is protected today by RSA and elliptic-curve cryptography. A fault-tolerant quantum computer running Shor’s algorithm would break both.

The timeline pressure comes from “harvest now, decrypt later” attacks: adversaries can record encrypted traffic today and decrypt it once quantum hardware matures. Long-lived secrets are already at risk.

Quantum key distribution (QKD) offers a different kind of defense. Instead of relying on mathematical hardness, it uses the physics of single photons to exchange encryption keys, with any eavesdropping attempt leaving a detectable trace. The open question for a network operator is not whether QKD works in a lab, but whether it can be sold as a commercial service over ordinary metro fiber.

What BT and Toshiba Actually Did

In October 2021, BT and Toshiba announced a commitment to build a quantum-secured metro network in London. The network went live in early April 2022, and on 26 April 2022 the two companies launched what they described as the first commercial trial of quantum-secured communication services over standard fiber optic links.

The documented facts of the trial:

• The network runs over Openreach private fiber in London, using standard fiber optic infrastructure rather than dedicated quantum-only links
• Toshiba supplies the QKD hardware and the key management software; BT operates the network and provides the end-to-end encrypted services
• Professional services firm EY signed on as the first commercial customer, using the network to connect two of its London sites, one in Canary Wharf and one near London Bridge
• The trial was planned to operate for an initial period of up to three years
• QKD-generated keys are combined with conventional public-key based ethernet security, so the encrypted links do not depend on a single mechanism

The underlying engineering collaboration took place at BT’s Adastral Park labs in Suffolk and at Toshiba’s quantum technology divisions in Cambridge and Tokyo. BT positioned the launch as a step toward the UK government’s ten-year vision of a quantum-enabled economy, set out in its 2020 strategy.

How QKD Works: BB84

BB84 is the foundational QKD protocol and a useful way to understand what systems like Toshiba’s are doing under the hood. Alice encodes random bits onto single photons using two conjugate bases. Bob measures in a randomly chosen basis. They compare bases over a classical channel and keep only the bits where bases matched (sifting). Any eavesdropper disturbs the quantum states in a detectable way, raising the quantum bit error rate (QBER) above its security threshold.

The following simulation is a simplified educational illustration of the BB84 protocol. It is not BT or Toshiba code, and real commercial QKD systems use hardened protocol variants, decoy states, and dedicated photonic hardware.

import numpy as np
from enum import Enum

class Basis(Enum):
    RECTILINEAR = 0  # |0>, |1>
    DIAGONAL = 1     # |+>, |->

def alice_prepare(n_bits: int):
    """Alice generates random bits and random basis choices."""
    bits = np.random.randint(0, 2, n_bits)
    bases = np.random.choice([Basis.RECTILINEAR, Basis.DIAGONAL], n_bits)
    return bits, bases

def bob_measure(alice_bits, alice_bases, n_bits: int):
    """Bob measures in a randomly chosen basis (simulation)."""
    bob_bases = np.random.choice([Basis.RECTILINEAR, Basis.DIAGONAL], n_bits)
    bob_bits = np.zeros(n_bits, dtype=int)

    for i in range(n_bits):
        if bob_bases[i] == alice_bases[i]:
            # Matching basis: Bob measures the correct bit
            bob_bits[i] = alice_bits[i]
        else:
            # Mismatched basis: random outcome
            bob_bits[i] = np.random.randint(0, 2)

    return bob_bits, bob_bases

def sift_key(alice_bits, alice_bases, bob_bits, bob_bases):
    """Keep only positions where both parties used the same basis."""
    matching = [i for i in range(len(alice_bases))
                if alice_bases[i] == bob_bases[i]]
    return alice_bits[matching], bob_bits[matching]

def estimate_qber(alice_key, bob_key, sample_fraction=0.1):
    """
    Quantum Bit Error Rate: fraction of sifted bits that disagree.
    QBER above ~11% indicates eavesdropping (BB84 security threshold).
    """
    n_sample = max(1, int(len(alice_key) * sample_fraction))
    indices = np.random.choice(len(alice_key), n_sample, replace=False)
    errors = np.sum(alice_key[indices] != bob_key[indices])
    return errors / n_sample

# Simulate a BB84 exchange
N = 10000
alice_bits, alice_bases = alice_prepare(N)
bob_bits, bob_bases = bob_measure(alice_bits, alice_bases, N)
alice_key, bob_key = sift_key(alice_bits, alice_bases, bob_bits, bob_bases)

print(f"Sifted key length: {len(alice_key)} (~50% expected)")
print(f"QBER:              {estimate_qber(alice_key, bob_key):.3f}")

Why a Commercial Trial Matters

Plenty of QKD experiments have run on test fiber. The significance of the BT-Toshiba launch is the word “commercial”: a paying customer (EY), a network operator running the service (BT), standard metro fiber (Openreach), and productized hardware and key management (Toshiba). That combination is what turns a physics demonstration into evidence about deployment economics, operations, and customer demand.

It is equally important to be clear about what has not been announced. BT has not published key rates, distances, or QBER figures for this trial, and there is no announced nationwide rollout. The honest status is that the trial demonstrated commercial feasibility of quantum-secured services on live London fiber, and the industry is watching what comes next.

Learn more: Quantum Networking Concepts

Sources

• BT and Toshiba launch first commercial trial of quantum secured communication services (BT Newsroom)